# CME Magnetic Structure and IMF Preconditioning Affecting SEP Transport

###### Abstract

Coronal mass ejections (CMEs) and solar energetic particles (SEPs) are two
phenomena that can cause severe space weather effects throughout the
heliosphere. The evolution of CMEs, especially in terms of their magnetic
structure, and the configuration of the interplanetary magnetic field (IMF)
that influences the transport of SEPs are currently areas of active research.
These two aspects are not necessarily independent of each other, especially
during solar maximum when multiple eruptive events can occur close in time.
Accordingly, we present the analysis of a CME that erupted on 2012 May 11
(SOL2012-05-11) and an SEP event following an eruption that took place on 2012
May 17 (SOL2012-05-17). After observing the May 11 CME using remote-sensing
data from three viewpoints, we evaluate its propagation through interplanetary
space using several models. Then, we analyse in-situ measurements from five
predicted impact locations (Venus, Earth, the Spitzer Space Telescope, the
Mars Science Laboratory en route to Mars, and Mars) in order to search for CME
signatures. We find that all in-situ locations detect signatures of an SEP
event, which we trace back to the May 17 eruption. These findings suggest that
the May 11 CME provided a direct magnetic connectivity for the efficient
transport of SEPs. We discuss the space weather implications of CME evolution,
regarding in particular its magnetic structure, and CME-driven IMF
preconditioning that facilitates SEP transport. Finally, this work remarks the
importance of using data from multiple spacecraft, even those that do not
include space weather research as their primary objective.

Space Weather

Department of Physics, University of Helsinki, Helsinki, Finland Space
Sciences Laboratory, University of California–Berkeley, Berkeley, CA, USA
CPAESS, University Corporation for Atmospheric Research, Boulder, CO, USA
ESTEC, European Space Agency, Noordwijk, Netherlands STFC RAL Space,
Rutherford Appleton Laboratory, Harwell Campus, Oxfordshire, UK School of
Physics and Astronomy, University of Leicester, Leicester, UK Space Research
Institute, Austrian Academy of Sciences, Graz, Austria Institute of Physics,
University of Graz, Graz, Austria Institute of Geodesy, Graz University of
Technology, Graz, Austria Heliophysics Science Division, NASA Goddard Space
Flight Center, Greenbelt, MD, USA Solar–Terrestrial Centre of Excellence—SIDC,
Royal Observatory of Belgium, Brussels, Belgium Institute of Geodynamics of
the Romanian Academy, Bucharest, Romania Skobeltsyn Institute of Nuclear
Physics, Moscow State University, Moscow, Russia School of Earth and Space
Sciences, University of Science and Technology of China, Hefei, China CAS
Center for Excellence in Comparative Planetology, University of Science and
Technology of China, Hefei, China IPAC–Spitzer, California Institute of
Technology, Pasadena, CA, USA Rays of Space Oy, Vantaa, Finland Swedish
Institute of Space Physics, Kiruna, Sweden

Erika<EMAIL_ADDRESS>
We analyse the 2012 May 11 CME and the 2012 May 17 SEP event through the inner
heliosphere

The May 11 CME appeared to rotate rapidly upon eruption and then considerably
during its interplanetary propagation

The May 11 CME provided direct magnetic connectivity for the efficient
transport of SEPs from the May 17 event

##  1 Introduction

Solar energetic particle (SEP; e.g., Reames, 2015; Vainio et al., 2009) events
are increases in high-energy particle fluxes with energies in the keV–GeV
range lasting from hours to days and important drivers of space weather
effects (e.g., Koskinen et al., 2017). They are intrinsically related to two
major classes of eruptions from the Sun, namely flares (e.g., Benz, 2017) and
coronal mass ejections (CMEs; e.g., Webb and Howard, 2012). These phenomena
can take place either together or separately in the solar atmosphere, and both
can contribute to the production of SEPs, although in different but yet
connected ways. Explosive magnetic reconnection during solar flares can
accelerate particles, which then travel along the interplanetary magnetic
field (IMF; e.g., Owens and Forsyth, 2013) lines connecting to an observer.
CMEs, on the other hand, consist of copious amounts of plasma and magnetic
field that are released into interplanetary space and that can drive shock
waves, thus acting as efficient particle accelerators. Contrarily to flares
that accelerate particles only on the solar surface, CME-driven shocks may
accelerate particles locally in the low corona and also at large distances in
the heliosphere, with acceleration sites that typically extend much wider.
These two forms of SEP production have traditionally resulted in a clear
distinction of particle acceleration processes (e.g., Cane et al., 1986;
Reames, 2013; Vlahos et al., 2019) into flare-accelerated (often with an
impulsive profile; e.g., Reames, 1990) and shock-accelerated (more likely to
show a gradual time evolution; e.g., Desai and Giacalone, 2016). This ‘dual
nature’ of SEPs, however, is not fully representative of the complex nature
and interplay of processes that result in particle injection and acceleration,
given that different mechanisms can contribute to a single event (e.g.,
Anastasiadis et al., 2019; Cane et al., 2010).

Since SEPs are accelerated and can propagate more efficiently along magnetic
field lines, their spatial distribution is, at least in principle, supposed to
be limited to the heliolongitudes (and latitudes) that are magnetically
connected to the acceleration site(s) (e.g., Reames, 1999). As a result,
impulsive SEP events are expected to be observed predominantly within a
narrower region compared to gradual ones (e.g., Reames, 2002, 2013).
Nevertheless, surprisingly wide (i.e., significantly larger than a flare site
or a CME-driven shock front) longitudinal distributions of SEPs have been
reported for both impulsive (e.g., Lario et al., 2017; Wibberenz and Cane,
2006; Wiedenbeck et al., 2013) and gradual (e.g., Dresing et al., 2012;
Richardson et al., 2014; Rouillard et al., 2012) events. Possible reasons for
wide-spread SEP distributions are cross-field transport in the interplanetary
medium and/or an extended source region at the Sun injecting particles over a
broad region (e.g., Dresing et al., 2014). Accordingly, from a space weather
perspective, current SEP research focuses not only on how intense an event
could be, but also on which heliolongitudes it could extend to (see, e.g., the
recent reviews by Klein and Dalla, 2017; Malandraki and Crosby, 2018).

Being large-scale magnetic disturbances, CMEs profoundly affect the structure
of the IMF during their journey away from the Sun (e.g., Witasse et al.,
2017). As a result, the passage of a CME may provide a temporary magnetic
connection between two regions in the heliosphere that would otherwise not be
linked. This may result in energetic particles observed in situ ‘inside’ an
interplanetary CME (or ICME; e.g., Kilpua et al., 2017) that preceded the SEP
event (e.g., Dresing et al., 2016; Larson et al., 1997; Masson et al., 2012;
Rodriguez et al., 2008). In these efforts, knowledge of the magnetic structure
of CMEs in interplanetary space is crucial. Regardless of their pre-eruptive
magnetic configuration, it is generally agreed that all CMEs lift off from the
Sun as helical magnetic structures called flux ropes (e.g., Forbes, 2000;
Green et al., 2018; Vourlidas et al., 2013), which consist of bundles of
magnetic field lines that wind about a common axis. From a space weather
perspective, there are several factors to take into account after a CME has
left the Sun, i.e. its size and propagation direction, which determine whether
a CME will impact a certain location (e.g., Mays et al., 2015; Möstl et al.,
2017; Rodriguez et al., 2011), its propagation speed, which determines the
arrival time (e.g., Verbeke et al., 2019; Zhao and Dryer, 2014), and its
magnetic structure, which is important in determining the resulting space
weather response (e.g., Kilpua et al., 2019a; Savani et al., 2015). A review
summarising the current status of space weather forecasting of CMEs has been
recently published by Vourlidas et al. (2019). Whilst hit/miss and arrival
time predictions presently lie around a hit rate of ${\sim}0.5$ and an
accuracy of ${\pm}10$ hours (e.g., Riley et al., 2018; Wold et al., 2018),
current models can only reproduce, rather than effectively forecast, the
magnetic structure of CMEs. This is because, even if the magnetic
configuration of flux ropes during eruption can be indirectly estimated from
remote-sensing observations (e.g., Gopalswamy et al., 2018; Palmerio et al.,
2017), it can differ significantly when measured in situ (e.g., Palmerio et
al., 2018; Yurchyshyn, 2008). Parameters that can influence the evolution of
CMEs as they travel away from the Sun are e.g. deflections (e.g., Kay et al.,
2015; Wang et al., 2004; Zhuang et al., 2017), rotations (e.g., Isavnin et
al., 2013; Kliem et al., 2012; Vourlidas et al., 2011), deformations (e.g.,
Manchester et al., 2004; Owens, 2008; Savani et al., 2010), and interactions
with the ambient solar wind (e.g., Rouillard et al., 2010; Rodriguez et al.,
2016; Winslow et al., 2016), stream interaction regions (e.g., Heinemann et
al., 2019; Liu et al., 2019; Prise et al., 2015), and/or with other CMEs
(e.g., Kilpua et al., 2019b; Lugaz and Farrugia, 2014; Scolini et al., 2020).
Comprehensive reviews on the interplanetary evolution of CMEs have been
recently published by Lugaz et al. (2017), Luhmann et al. (2020), and
Manchester et al. (2017).

In this work, we explore the large-scale preconditioning of the IMF resulting
from the passage of an ICME and its subsequent effects on the transport of
SEPs. We study the eruption and interplanetary evolution of a CME that erupted
on 2012 May 11, with a particular focus on its magnetic structure as it
travels away from the Sun. We evaluate the propagation of the CME across the
inner heliosphere and how its orientation changed, indicating that the flux
rope rotated and was distorted significantly after erupting. Furthermore, we
analyse the SEP event following the 2012 May 17 eruption, which was observed
in situ at eight well-separated locations in the inner heliosphere, including
all four planets therein: Mercury, Venus, Earth and Mars. We show that five of
the eight available observers recorded a nearly simultaneous SEP event
characterised by an impulsive profile, whilst the remaining three observed a
more gradual event. These findings are consistent with the IMF being affected
by the passage of an ICME, since impulsive SEP profiles were observed at those
locations that were predicted to be encountered by the May 11 CME. We suggest
that the ICME provided the required ‘direct’ magnetic connectivity for SEPs to
spread rapidly over a broad region (the observing locations engulf over
${\sim}0.9$ AU in radial distance and ${\sim}70^{\circ}$ in longitude,
extending up to ${\sim}150^{\circ}$ in longitude away from the flaring site).
The 2012 May 17 SEP event was studied extensively in the literature,
especially between the Sun and Earth, but, to our knowledge, this is the first
report of its observations at eight different locations, five of which were
impacted by a preceding ICME.

This article is organised as follows. In Section 2, we enumerate the space-
and ground-based instruments that are employed in this study. In Section 3, we
describe the two eruptive events under analysis (2012 May 11 and May 17) from
a remote-sensing observational perspective. In Section 4, we estimate the
propagation of the May 11 CME and its impact at different locations using
several models. In Section 5, we present the in-situ signatures of the May 11
CME and May 17 SEP event across the inner heliosphere. In Section 6, we
discuss various aspects of the evolution of the May 11 CME in terms of its
propagation, its magnetic structure, and its role in the transport of SEPs, by
combining observational data with modelling outputs. Finally, in Section 7, we
conclude by summarising our results.

##  2 Spacecraft and Ground-based Data

In this section, we list the fleet of instruments that are involved in this
study, in the order in which they are introduced in this article. We use a
synthesis of remote-sensing and in-situ data in order to follow and observe
signatures of the 2012 May 11 CME and the 2012 May 17 SEP event at different
locations throughout the inner heliosphere.

Solar observations from Earth’s viewpoint are made with the Solar Dynamics
Observatory (SDO; Pesnell et al., 2012). The extreme ultra-violet (EUV) images
and line-of-sight magnetograms we use are taken with the Atmospheric Imaging
Assembly (AIA; Lemen et al., 2012) and the Helioseismic and Magnetic Imager
(HMI; Scherrer et al., 2012), respectively. Solar observations from other
locations at ${\sim}1$ AU are made with the Sun Earth Connection Coronal and
Heliospheric Investigation (SECCHI; Howard et al., 2008) Extreme UltraViolet
Imager (EUVI) onboard the Solar Terrestrial Relations Observatory (STEREO;
Kaiser et al., 2008). STEREO consists of twin spacecraft that orbit the Sun,
one ahead of Earth in its orbit (STEREO-A) and the other one trailing behind
(STEREO-B, which has been out of contact since October 2014). Furthermore, we
use data from the X-ray Sensor (XRS) onboard the Geostationary Operational
Environmental Satellites (GOES) 15 satellite to study the solar X-ray flux.

Coronagraph observations are also made from three vantage points. The view
from Earth is provided by the Large Angle and Spectrometric Coronagraph
(LASCO; Brueckner et al., 1995) C2 (2.2–6 $R_{\odot}$) and C3 (3.5–30
$R_{\odot}$) instruments onboard the Solar and Heliospheric Observatory (SOHO:
Domingo et al., 1995). The views from STEREO-A and STEREO-B are provided by
the SECCHI/COR1 (1.5–4 $R_{\odot}$) and COR2 (2.5 – 15 $R_{\odot}$)
coronagraphs.

Heliospheric observations are made with the Heliospheric Imagers (HI; Eyles et
al., 2009) onboard the twin STEREO spacecraft. Each HI instrument comprises
two cameras, HI1 (4–24∘) and HI2 (18–88∘), that image the space between the
Sun and Earth (the degrees measure the elongation in helioprojective radial
coordinates).

In-situ measurements around Venus are taken with Venus Express (VEX; Svedhem
et al., 2007). We use data from the Magnetometer (MAG; Zhang et al., 2006) and
the Analyser of Space Plasmas and Energetic Atoms (ASPERA-4; Barabash et al.,
2007). The sensors that we use from ASPERA-4 are the Ion Mass Analyser (IMA)
and the Electron Spectrometer (ELS), both performing local charged particle
measurements. VEX, which ended its operations in 2015, had a 24-hour highly
elliptical and quasi-polar orbit, and spent a couple of hours each day inside
the induced magnetosphere of Venus. ASPERA-4 was operational for several hours
close to periapsis and apoapsis only, whilst the magnetometer ran
continuously.

In-situ measurements from near Earth are mainly taken with the Wind satellite,
which is operational at Earth’s Lagrange L1 point. We use data from the
Magnetic Fields Investigation (MFI; Lepping et al., 1995) and Solar Wind
Experiment (SWE; Ogilvie et al., 1995) instruments, which measure magnetic
field and plasma (including solar wind electron distributions) continuously.
Additionally, we use proton flux data from the Electron, Proton, and Alpha
Detector (EPEAD) instrument onboard GOES-13. We also study variations in
cosmic rays and SEPs on the ground using count rate data from the Neutron
Monitor Database (NMDB), and in particular from the South Pole (SOPO) neutron
monitor.

Measurements around 1 AU are also taken with the Spitzer Space Telescope
(Werner et al., 2004), which orbits the Sun on an Earth-trailing orbit. In
order to evaluate the impact of space weather events at Spitzer, we count the
radiation hits on the Infrared Array Camera (IRAC; Fazio et al., 2004).
Spitzer was deactivated in 2020.

We also analyse data recorded by the Mars Science Laboratory (MSL; Grotzinger
et al., 2012) spacecraft that was en route to Mars at the time of this study.
We use data from the Radiation Assessment Detector (RAD; Hassler et al., 2012)
instrument onboard the Curiosity rover, and in particular from the plastic
scintillator ‘E’, which measures the radiation dose rate contributed by all
particles reaching the detector from all directions and provides the best
statistics of background cosmic ray fluxes.

In-situ measurements from Mars are taken with two different spacecraft. The
first is Mars Express (MEX; Chicarro et al., 2004). We use data from the Mars
Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS; Picardi et
al., 2004) and the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3;
Barabash et al., 2006). MARSIS is a high-frequency sounding radar dedicated to
probe the Martian subsurface, surface, and ionosphere. ASPERA-3 is identical
to the ASPERA-4 instrument onboard VEX, and the sensors that we use are again
IMA and ELS. MEX has a 7-hour elliptical orbit, with a periapsis distance of
${\sim}300$ km and an apoapsis distance of ${\sim}10000$ km from the planet’s
surface. MARSIS takes measurements at periapsis only, whilst ASPERA-3 was
operational for about half of the orbit during the time of the events under
study. The second satellite is 2001 Mars Odyssey (MOdy; Saunders et al.,
2004), from which we use data taken with the High Energy Neutron Detector
(HEND), which is part of the Gamma Ray Spectrometer (GRS; Boynton et al.,
2004) suite.

Finally, we inspect SEP measurements made at the twin STEREO spacecraft and
Mercury. At STEREO, we use data from the High Energy Telescope (HET; von
Rosenvinge et al., 2008), which is part of the In situ Measurements of
Particles And CME Transients (IMPACT; Luhmann et al., 2008) investigation. At
Mercury, we use data from the Mercury Surface, Space Environment,
Geochemistry, and Ranging (MESSENGER; Solomon et al., 2007) spacecraft, which
orbited the innermost planet between 2011 and 2015. We examine measurements
taken with the Neutron Spectrometer (NS) sensor, part of the Gamma-Ray and
Neutron Spectrometer (GRNS; Goldsten et al., 2007) instrument and dedicated to
measuring the flux of ejected neutrons.

##  3 Remote-sensing Observations

In this section, we describe the eruptive events of 2012 May 11 (Section 3.1)
and May 17 (Section 3.2) from a remote-sensing observational perspective. The
positions of the inner planets and the spacecraft employed in this study,
together with the eruptions’ source region locations at the Sun, are shown in
Figure 1 for (a) May 11 and (b) May 17.

Figure 1: Position of various planets and spacecraft in the inner solar system
(i.e., up to Mars’ orbit) on (a) 2012 May 11 and (b) 2012 May 17, projected
onto the solar equatorial plane. The projected source locations of the May 11
and May 17 eruptions are indicated by a star symbol on the surface of the Sun
in panels (a) and (b), respectively. The planets are marked with circles,
whilst the spacecraft are marked with squares (STA = STEREO-A; STB = STEREO-B;
SST = Spitzer Space Telescope; MSL = Mars Science Laboratory). The orbits of
all planets are also indicated.

###  3.1 The 2012 May 11 Eruption

The first eruptive event that we focus on in this work initiated from a small
active region containing a filament on 2012 May 11 around 23:00 UT. In this
section, we follow the eruption and subsequent propagation of the associated
large-scale CME using remote-sensing observations of the solar disc, the solar
corona, and the heliosphere.

####  3.1.1 Solar Disc Observations

The active region from which the 2012 May 11 CME originated appeared on the
eastern limb of the Earth-facing Sun on 2012 May 6, on the southern
hemisphere. It was not attributed a National Oceanic and Atmospheric
Administration (NOAA) classification number, but it had Space-weather HMI
Active Region Patch (SHARP; Bobra et al., 2014) number 1642. An S-shaped
filament was clearly visible within the active region throughout its rotation
towards the central meridian. The filament erupted on 2012 May 11, around
23:00 UT, from approximately S13E13. The top panels of Figure 2 show the
evolution of the eruption process in remote-sensing data from SDO’s
perspective. For the complete set of observations of the erupting filament
from three viewpoints (STEREO-A, SDO, and STEREO-B), see Movie S1.

Figure 2: The 2012 May 11 eruption from solar disc data. (a–c) The erupting
filament as seen by SDO/AIA in the 304 Å channel, with the rotating filament
indicated by an arrow in each panel. (d) Base-difference image taken with
SDO/AIA in the 211 Å channel and overlaid with SDO/HMI magnetogram contours
(blue = negative polarity, red = positive polarity). The dimming regions
(signatures of the flux rope footpoints) have been circled in green. (e–h) The
erupting filament triangulated with the tie-pointing technique using SDO/AIA
and STEREO/SECCHI/EUVI-B data in the 304 Å channel. The same reconstruction is
shown from four different longitudinal perspectives (indicated in each panel
in Stonyhurst coordinates; Thompson, 2006) to facilitate its 3D visualisation.
Each colour represents one different timestamp within the same panel.

The magnetic structure of the flux rope associated with the erupting CME, or
intrinsic flux rope type, can be estimated using a combination of several
indirect proxies at different wavelengths in remote-sensing data of the solar
disc (for a summary of the proxies that can be used to determine chirality,
tilt, and field direction at the axis of a flux rope, see Palmerio et al.,
2017). In the particular case under study, we note that the forward S-shape of
the erupting filament (Figure 2a) is a sign of right-handed chirality (e.g.,
Martin et al., 2012). Furthermore, the forward J-shape of the flare ribbons
associated with the eruption (bright features in Figure 2b–c) confirms the
right-handedness of the flux rope (e.g., Démoulin et al., 1996). We infer the
tilt of the flux rope axis by taking the average inclination between the
polarity inversion line (PIL) and post-eruption arcades (PEAs). We determine
the tilt of the flux rope to be ${\sim}65^{\circ}$ with respect to the solar
equator. Finally, we determine the magnetic field direction at the flux rope
axis from coronal dimmings, which are believed to map to the CME footpoints
(e.g., Thompson et al., 2000). The dimming regions marked in Figure 2d
indicate that the axial field was directed roughly towards the south at the
time of the eruption. Thus, we determine the intrinsic flux rope type of the
CME under study to be right-handed with a mostly-southward axial field, or
east–south–west (ESW) according to the convention of Bothmer and Schwenn
(1998) and Mulligan et al. (1998).

Upon eruption, the filament spine was seen to rotate clearly clockwise (Figure
2a–c), as expected for a right-handed flux rope (e.g., Fan and Gibson, 2004;
Green et al., 2007; Lynch et al., 2009). Its southern leg (i.e., the eastern
leg upon rotation) could no longer be seen in SDO images shortly after the
eruption, whilst its northern leg (i.e., the western leg upon rotation) could
be observed for a couple of hours after the eruption onset. STEREO-B
observations (see Movie S1), taken from an almost-quadrature view with respect
to SDO, show that the southeastern leg of the filament appeared to disconnect
from the Sun approximately at the time of its disappearance in SDO imagery.
Such asymmetric filament eruptions have been observed in previous studies
(e.g., Liu et al., 2009; Tripathi et al., 2006). According to the definition
of Liu et al. (2009), the filament eruption studied here can be classified as
whipping-like, where the filament “active” leg whips upwards and the
“anchored” leg remains fixed to the photosphere. In such a scenario, the mass
in the active leg could either fall back towards the Sun or fail to follow the
motion of the filament spine, thus showing an apparent detachment of the leg
from the solar surface, to which the filament field may however still be
connected. Another possibility is that the active leg undergoes interchange
reconnection with a nearby coronal hole open field (e.g., Baker et al., 2009;
Zhu et al., 2014), hence opening one end of the filament to interplanetary
space. In the event studied here, the motion of the flare ribbon brightenings
to the south of the CME source region (visible in Figure 2b–c and in Movie
S1), corresponding to the extent of a small region of open field (visible in
SDO/AIA 193 Å imagery, data not shown), supports this latter scenario.

The bottom panels of Figure 2 show 3D reconstructions of the filament during
the early phase of its eruption using the tie-pointing triangulation
technique. The method was first employed by Thompson (2009) to triangulate a
Sun-grazing comet, but it has been used since then also to evaluate the 3D
rotation of erupting filaments (e.g., Bemporad et al., 2011; Thompson et al.,
2012). The reconstructions presented here are obtained using SDO/AIA and
STEREO/SECCHI/EUVI-B data, both in the 304 Å channel. At the fourth and last
triangulation (23:36 UT), the southeastern (active) leg of the filament had
completely disappeared, hence we could only triangulate the northwestern
(anchored) one. It should be emphasised that erupting filaments are thick in
EUV images and the resulting triangulated structures are expected to be
accompanied by significant uncertainties. Consequently, the resulting
structures shown in Figure 2e–h should be considered only as approximate
indicators of the global 3D shape of the erupting filament. Nevertheless,
regardless of the connection of the active filament leg to the Sun, the
observations reported here suggest that the filament changed its orientation
from a roughly perpendicular to a roughly parallel one with respect to the
solar equator, rotating by ${\sim}85^{\circ}$ clockwise. Whether the
associated flux rope experienced the same evolution, however, is unclear, as
the spatial relationship between erupting filaments and the overlying flux
rope is an open question (e.g., Gibson et al., 2006; Vourlidas et al., 2013).
If we assume that the orientation of a flux rope follows that of its
corresponding filament, we can deduce that the flux rope type changed from ESW
to north–east–south (NES) due to such rotation.

####  3.1.2 Coronagraph Observations

After erupting, the 2012 May 11 CME was visible in coronagraphs from three
well-separated viewpoints (SOHO, STEREO-A, and STEREO-B) as it propagated
through the solar corona (see Movie S2). The CME is seen pointing towards the
west in STEREO-B and towards the east in STEREO-A imagery, indicating an
Earth-directed eruption. However, we note from SOHO images that the CME
propagated slightly eastwards rather that along the Sun–Earth line, indicating
that the eruption was directed roughly towards Mars (see the positions of
different planets and spacecraft in Figure 1a).

Figure 3: (a–h) Evolution of the erupting filament and the overlying CME
(marked by arrows in some of the panels) through the low corona in composite
images from STEREO/SECCHI/EUVI in the 304 Å channel and STEREO/SECCHI/COR1
using (a–d) STEREO-A and (e–h) STEREO-B data. The COR1 data are shown as base-
difference images, with the background taken at 23:00 UT. (i–l) 3D
reconstruction of the erupting filament and the overlying large-scale CME
performed at 23:45 UT. The filament (shown in red) is triangulated with the
tie-pointing technique using STEREO/SECCHI/EUVI in the 304 Å channel and COR1
composite data from both STEREO spacecraft. The overlying CME (shown as a blue
wireframe) is reconstructed with the GCS technique using STEREO/SECCHI/COR1
data from both STEREO spacecraft. The same reconstruction is shown from
different perspectives (indicated in each panel in Stonyhurst coordinates) to
facilitate its 3D visualisation.

The top two rows of Figure 3 show composite images using EUVI and COR1 data
from both STEREO-A (panels a–d) and STEREO-B (panels e–h). In particular,
STEREO-B imagery shows that the filament leg that disappeared first as seen
from SDO (i.e., the active leg, Figure 2a–c) rotated significantly from a
downward-facing hook (Figure 3e) to an upward-facing hook (Figure 3g)
morphology. The western leg appeared to be still attached to the Sun
throughout the rotation phase. The corresponding large-scale CME, in turn,
featured a single-front structure from the STEREO-A perspective (marked by an
arrow in Figure 3b–d and visible in Movie S2) and a double-bump structured
front from the STEREO-B and SOHO viewpoints (marked by arrows in Figure 3f–g
and visible in Movie S2). We note that asymmetric white-light CMEs associated
with whipping-like filament eruptions were reported in previous studies, e.g.
by Zhu et al. (2014).

The bottom row of Figure 3 shows 3D reconstructions of the filament, performed
using the tie-pointing technique on composite EUVI and COR1 images from both
STEREO spacecraft, and the overlying large-scale CME, performed using the
graduated cylindrical shell (GCS; Thernisien et al., 2006, 2009)
reconstruction technique on COR1 images from both STEREO spacecraft. The
morphology of the GCS model is that of a hollow wireframe, with six free
parameters that can be manually adjusted until they best match the data. For
the reconstruction shown here, we only include the “main” structure seen in
STEREO-B (i.e., that indicated by the top arrow in Figure 3g). According to
the resulting 3D images shown in Figure 3i–l, the western leg of the filament
(i.e., the anchored leg discussed in Section 3.1.1) follows the western leg of
the larger-scale CME quite closely, and the detachment and slight rotation
northwards of the active filament leg are also evident. To the thick nature of
filaments discussed in Section 3.1.1, we add that in this case the tie-
pointing reconstruction was applied to simultaneous EUV and white-light
images, i.e. using instruments that measure at different wavelengths,
resulting in large uncertainties. Nevertheless, considering how well the
resulting 3D thread fits within the larger CME structure, these results can be
considered a good approximation of the general morphology and relative
orientation of the two features. The tilt of the CME inferred from the GCS
reconstruction is in this case close to $0^{\circ}$, meaning that the eruption
was seen basically edge-on from both STEREO spacecraft. This also implies that
it is not possible to determine whether the eastern CME leg, which corresponds
to the active filament leg, was still attached to the Sun at this point,
either entirely or at least partially. Nevertheless, the low tilt angle
inferred from the GCS reconstruction is consistent with the filament rotation
(see Section 3.1.1 and Figure 2), suggesting that the flux rope axis roughly
followed the orientation of the underlying filament and had a NES orientation
in the low corona.

Finally, in order to define the global geometric and kinematic parameters of
the 2012 May 11 CME during its journey through the solar corona, we further
apply the GCS technique to coronagraph images from all three available
spacecraft (i.e., SOHO and the twin STEREO). An example of the resulting GCS
reconstructions is shown in Figure 4 (for the full CME kinematics, see Figure
S1). Throughout its passage through the coronagraphic fields of view, the CME
did not seem to experience significant deflections, featuring a propagation
direction of $-10^{\circ}$ in latitude and $-30^{\circ}$ in longitude in
Stonyhurst coordinates. On the other hand, the CME seemed to continue rotating
as it travelled away from the Sun, with its axis inclination resulting from
our reconstructions evolving gradually from $-10^{\circ}$ to $-65^{\circ}$ (a
positive tilt value is defined for counterclockwise rotations) with respect to
the solar equatorial plane. Finally, its propagation speed was ${\sim}1000$
km$\cdot$s-1 (calculated between successive reconstructions from the CME apex
height) throughout the COR2 field of view. However, consisting of a “hollow
croissant”, the GCS model cannot provide information on the internal magnetic
field. Hence, although the handedness of the CME is known from solar disc
observations (see Section 3.1.1), the direction of the corresponding flux
rope’s axial field is characterised by a $180^{\circ}$ ambiguity. If we assume
that the flux rope followed the same rotation pattern as its associated
filament, then it can be concluded that the flux rope axis rotated
${\sim}130^{\circ}$ from its pre-eruptive configuration to altitudes of a
couple tens solar radii. According to these assumptions and minding that the
CME was right-handed, it would follow that the flux rope left the outer corona
close to a west–north–east (WNE) type, whilst a $180^{\circ}$ reversal of the
axis would yield a east–south–west (ESW) type. We remark that, as shown in
Figure 3, Figure 4, and Movie S2, the CME under study appeared highly
asymmetric, thus the GCS results should be considered as an approximation of
the global orientation of a significantly distorted structure. Furthermore, we
note that for this event it is particularly difficult to clearly discern the
boundaries between the CME itself and the white-light shock driven by it.
Hence, being the 2012 May 11 CME far from a “textbook” case, we expect GCS
reconstructions to not be able to unequivocally derive the 3D structure of the
CME in white light.

Figure 4: Example of the GCS reconstruction technique applied to the 2012 May
11 CME at the last available time before the CME left the COR2-B field of view
(2012 May 12, 01:54 UT). (a–c) Base-difference images taken from
STEREO/SECCHI/COR2-A, SOHO/LASCO/C3, and STEREO/SECCHI/COR2-B. (d–f) Same
images as in (a–c), with the GCS wireframe overlaid.

####  3.1.3 HI Observations

After leaving the STEREO/SECCHI/COR2 field of view, the 2012 May 11 CME
appeared in the HI cameras onboard both STEREO spacecraft (see Movie S3).
Figure 5a–b shows a snapshot of the CME as seen in the HI1 field of view. At
both spacecraft, observations of the CME in HI1 began at approximately 02:00
UT on 2012 May 12. The event was clearly visible from both STEREO viewpoints,
and the CME apex was observed at progressively larger elongation angles in
STEREO-B imagery compared to STEREO-A (note the position of the apex in Figure
5 and in Movie S3). This suggests that the CME was closer to quadrature-view
from the STEREO-B perspective, consistently with the coronagraph analysis
reported in Section 3.1.2 (see Figure 1a for the position of the STEREO
spacecraft).

Figure 5: The 2012 May 11 CME as seen in images from the STEREO/SECCHI/HI
cameras. (a–b) Snapshots of the CME in running-difference images taken with
the (a) HI1-A and (b) HI1-B cameras. The projected locations of Venus and Mars
are marked in panel (a). (c–d) Time–elongation maps from (c) STEREO-A and (d)
STEREO-B. The CME tracks are marked in red. The maps are constructed along
position angles of $90^{\circ}$ for STEREO-A and $264^{\circ}$ for STEREO-B,
i.e. close to the ecliptic plane (the position angle of Earth was
${\sim}89.9^{\circ}$ from the STEREO-A perspective and ${\sim}264.4^{\circ}$
from the STEREO-B perspective).

In order to follow the propagation of the CME under study through both HI
cameras, we use time–elongation maps (e.g. Sheeley et al., 2008; Davies et
al., 2009) produced from running-difference images. Within these
time–elongation maps, a propagating structure such as a CME appears as a
bright front followed by a dark front. This is due to the increase and
subsequent decrease in density and allows features to be tracked in elongation
as a function of time. This is performed on the CME under investigation here,
which we track for over 2.5 days in HI-A to an elongation of
${\sim}30^{\circ}$ and for over 1.5 days in HI-B to an elongation of
${\sim}35^{\circ}$. The resulting time–elongation maps, with the corresponding
CME tracks, are shown in Figure 5c–d.

Furthermore, the CME under study is listed in the HELiospheric Cataloguing,
Analysis and Techniques Service (HELCATS) catalogues. The HELCATS project ran
from 2014 to 2017 and aimed, amongst other goals, to catalogue and analyse
solar transients (such as CMEs) detected in the STEREO/SECCHI/HI cameras. This
event is included in the HICAT catalogue (Harrison et al., 2018), which was
generated through visual inspection of background-subtracted and difference
HI1 images, and in the HIGeoCAT catalogue (Barnes et al., 2019), which was
generated using time–elongation maps and applying single-spacecraft fitting
techniques to derive CME kinematic properties. In both catalogues, the CME is
labelled as HCME_A__20120511_01 for STEREO-A and HCME_B__20120512_01 for
STEREO-B. We remark that, in both HICAT and HIGeoCAT, CMEs are identified
using single-spacecraft data, hence the STEREO-A and STEREO-B observations are
reported separately. Of the fitting techniques used in HIGeoCAT, we report
here the results obtained with the Self-Similar Expansion (SSE; Davies et al.,
2012; Möstl and Davies, 2013) fitting technique with a fixed half-width of
$30^{\circ}$ (hereafter, SSEF30) applied to time–elongation single-spacecraft
data. In the SSE model, CMEs are assumed to have a circular front and to
propagate radially with a constant speed and half-width. We note that the
SSEF30 results obtained using STEREO-B data are consistent with the GCS
results reported in Section 3.1.2, i.e., the CME propagates in direction
$(\theta,\phi)=(-9^{\circ},-19^{\circ})$ and with a speed of $869$
km$\cdot$s-1. SSEF30 results based on STEREO-A data, however, are
significantly different, reporting a propagation direction of
$(\theta,\phi)=(-4^{\circ},-54^{\circ})$ and a speed of $2008$ km$\cdot$s-1.
Since, as stated above, the 2012 May 11 CME was closer to quadrature view
(i.e., with less projection effects) from STEREO-B than from STEREO-A, we
expect the fitting results retrieved from STEREO-B data to be more accurate.

###  3.2 The 2012 May 17 Eruption

The second eruptive event that we focus on in this work initiated from AR
11476 on 2012 May 17 around 01:00 UT. Since in this case we are mostly
interested in the release of energetic particles, rather than the CME eruption
and evolution itself, we provide here a brief overview of the event. The
source region was located close to the western limb from Earth’s perspective
(N11W76) and on disc (on the north-eastern quadrant) from STEREO-A’s
perspective, whilst it was fully back-sided from STEREO-B’s viewpoint (see
Figure 1b). The large-scale CME associated with this eruption was fast, with a
speed above 1500 km$\cdot$s-1, and large, as it appeared as a halo from all
three viewpoints. An intense “snowstorm”, caused by high-energy protons
striking the cameras, was seen in both SOHO/LASCO coronagraphs (C2 and C3)
starting around 02:00 UT. Furthermore, the GOES/XRS instrument reported an
M5.1 flare associated with this event, starting at 01:25 UT, peaking at 01:47
UT, and ending at 02:14 UT. Figure 6 provides images of the CME’s source
region as seen by SDO (Figure 6a) and STEREO-A (Figure 6b), together with the
soft X-ray flux measured by GOES-15 (Figure 6c).

The 2012 May 17 eruption has been studied, e.g., by Gopalswamy et al. (2013),
Li et al. (2013), Shen et al. (2013), and Rouillard et al. (2016), hence the
reader is referred to these articles for additional information and images on
the eruptive event and the CME’s propagation through the solar corona. In
particular, we note that Gopalswamy et al. (2013) estimated the shock
formation and SEP release heights for the May 17 CME to be 1.38 $R_{\odot}$
(at 01:32 UT) and 2.32 $R_{\odot}$ (at 01:40 UT) from the solar centre,
respectively. Together with the occurrence of the M5.1 flare, this suggests
that the 2012 May 17 SEP event had contributions from both flare-accelerated
and shock-accelerated particles, which is what Cane et al. (2010) concluded to
be the most likely scenario for large SEP events. This conclusion was, in
fact, reached by Li et al. (2013), who estimated that electrons were
accelerated by the flare at 01:29 UT and protons were accelerated by the CME-
driven shock at 01:39 UT from an altitude of 3.07 $R_{\odot}$. Furthermore,
Shen et al. (2013) suggested that the 2012 May 17 CME was composed of two
nearly-simultaneous ejecta that erupted from the same active region and
neutral line, leading to a CME–CME shock interaction scenario. Finally,
previous studies reported also the presence of a so-called EUV wave (e.g.,
Thompson et al., 1998; Zhukov and Auchère, 2004), visible from both SDO’s and
STEREO-A’s viewpoints.

Figure 6: Overview of the 2012 May 17 eruption. (a) SDO/AIA image of the
source region (AR 11476) in the 171 Å channel shortly after the flare onset
time. (b) STEREO/SECCHI/EUVI-A image of the source region in the 195 Å channel
taken at the same time as (a). (c) GOES-15/XRS soft X-ray flux, showing the
occurrence of an M5.1 flare.

##  4 CME Propagation Modelling

In this section, we propagate the 2012 May 11 CME using different techniques
and evaluate its impact at different planets and spacecraft scattered
throughout the inner heliosphere. The results of the propagation models that
we consider in this work are summarised in Table 1.

Table 1: CME arrival times from the different modelling techniques presented
in Section 4.

Model | Venus | Earth | Spitzer | MSL | Mars
---|---|---|---|---|---
SSEF30–A | — | — | 05/12 20:18 | 05/13 07:55 | 05/13 11:51
SSEF30–B | 05/13 09:11 | 05/14 04:40 | — | 05/15 03:18 | 05/15 13:52
SSSE | 05/14 19:15 | 05/17 18:51 | 05/15 15:01 | 05/17 00:13 | 05/17 14:57
DBM | 05/14 00:08 | 05/15 00:20 | 05/14 23:42 | 05/16 14:42 | 05/17 05:49
Enlil (S) | 05/13 19:33 | 05/14 21:30 | 05/14 20:03 | 05/16 05:54 | 05/16 21:58
Enlil (E) | 05/14 01:00 | 05/15 17:52 | 05/15 21:07 | 05/17 03:06 | 05/17 23:27

Note. Dates are shown in the format MM/DD HH:MM. The notations ‘(S)’ and ‘(E)’
denote shock and ejecta, respectively.

###  4.1 (S)SSE Propagation

We first evaluate the impact of the 2012 May 11 CME at various locations in
interplanetary space using HI data (see Section 3.1.3). Considering again the
HELCATS products, we initially search for the CME under study in the ARRCAT
catalogue (Möstl et al., 2017), which was generated from the list of events in
HIGeoCAT by predicting their impact throughout the heliosphere. The
predictions were made using the SSEF30 model introduced in Section 3.1.3. We
note that, in ARRCAT, HCME_A__20120511_01 is reported to arrive at MSL and
Mars, whilst HCME_B__20120512_01 is predicted to impact Venus and Earth as
well. The exact arrival times predicted by ARRCAT (SSEF30-A and SSEF30-B) are
reported in Table 1. We emphasise that Spitzer is not included as a posible
target in ARRCAT. Nevertheless, we evaluate arrival estimates at the
spacecraft using the CME parameters listed in HIGeoCAT and the equations
reported in Möstl and Davies (2013). This results in an impact according to
SSEF30-A and a close miss according to SSEF30-B.

Since the CME under study was well-visible in both STEREO spacecraft, we also
use the two-spacecraft version of the SSE model, i.e. the Stereoscopic Self-
Similar Expansion (SSSE; Davies et al., 2013) model, to triangulate the CME
position over the ${\sim}1.5$ days in which the event was observed by both
spacecraft using time–elongation data. This model also assumes that the CME
possesses a circular cross-section in the plane in which the CME is observed
(in our case, this is the ecliptic) and a constant half-width. To estimate the
half-width to use in this case, we use the formulas in Rodriguez et al. (2011)
to calculate the maximum angular extent of a CME in both latitude and
longitude using the GCS parameters as input. This results in an half-angular
extent of $58^{\circ}$ in latitude and $54^{\circ}$ in longitude. Since the
SSSE is a 2D model, we are only interested in the longitudinal extent of the
CME, hence we set a half-width of $54^{\circ}$. In order to extrapolate the
position of the CME beyond the time it was last observed, we fit a second-
order polynomial to the distance of the CME apex as a function of time. We
also assume that the CME continues to propagate in a constant direction beyond
its last observed value. As a result of this extrapolation we expect the CME
front to pass over Venus, Earth, Spitzer, MSL, and Mars. Table 1 reports the
arrival times at all the impacted locations, and Figure S2 shows the position
of the tracked CME front together with the resulting CME arrival times and
speeds at the three planets (Venus, Earth, and Mars).

###  4.2 DBM Propagation

As a further indication of the CME propagation and impact at different planets
and spacecraft, we the drag-based model (DBM; Vršnak et al., 2013), which
computes analytically the propagation of CMEs using aerodynamic drag equations
and with the assumption of a constant background speed and constant drag
parameter. For our run, we use an ambient solar wind speed of $450$
km$\cdot$s-1 (from measurements of the solar wind speed at the spacecraft in
the inner heliosphere a few days after the eruption time) and a drag parameter
of $1{\times}10^{-7}$ km-1 (i.e., the mean value found by Vršnak et al.,
2013). In the simplest version of the DBM, the geometry of CMEs is that of a
2D circular arc centred at the Sun and moving outwards. The CME parameters
that we introduce in the tool are entirely derived from the GCS
reconstructions presented in Section 3.1.2: height of $14.4$ $R_{\odot}$ on
2012 May 12 at 01:54 UT, speed of $1005$ km$\cdot$s-1 (which is the speed
between the reconstructions at 01:24 and 01:54 UT, when the CME reached the
height of $14.4$ $R_{\odot}$), half-width of $54^{\circ}$ (again, the full
longitudinal extent of the CME using the formulas by Rodriguez et al., 2011),
and longitude of $-30^{\circ}$ (in Stonyhurst coordinates). With the initial
set of parameters described above, impacts are estimated at Venus, Earth,
Spitzer, MSL, and Mars. The resulting arrival times at the different planets
and spacecraft are reported in Table 1, and a visual representation of the CME
propagated with the DBM is shown in Figure S3.

###  4.3 Enlil Simulation

The final CME propagation model that we employ in this study is the 3D
heliospheric magnetohydrodynamic (MHD) Enlil (Odstrcil, 2003; Odstrcil et al.,
2004) model. Enlil uses the Wang–Sheeley–Arge (WSA; Arge et al., 2004) coronal
model to simulate the background solar wind from its inner boundary (located
at $21.5$ $R_{\odot}$ or 0.1 AU) onwards. In this case, we set the outer
boundary of the simulation domain at 2 AU, i.e. including the whole inner
heliosphere. CMEs can be modelled through insertion at the inner boundary of
the heliospheric domain. In this work, we employ the WSA–Enlil+Cone model, in
which CMEs are injected as spherical hydrodynamic structures that lack an
internal magnetic field (i.e., there is no internal flux rope structure).
Again, we derive the CME initial parameters from GCS reconstructions (see
Section 3.1.2). We obtain the injection time at the inner boundary by
propagating the CME from its last GCS reconstruction (on 2012 May 12 at 01:54
UT) up to $21.5$ $R_{\odot}$ using a constant speed derived from this last GCS
reconstruction and a reconstruction made from data obtained 30 minutes earlier
(in this case, at 01:24 UT). This yields an injection time of 2012 May 12 at
03:16 UT with a speed of 1005 km$\cdot$s-1. The CME cone that we model has an
elliptical cross-section, and we derive its dimensions by ‘cutting’ an
elliptical cross-section out of the GCS shell (based on Thernisien, 2011). The
resulting values for the half-angular extent of the major and minor radii are
$46.75^{\circ}$ and $37.89^{\circ}$, respectively. Finally, the values for
latitude ($-10^{\circ}$), longitude ($-30^{\circ}$), and tilt angle
($-65^{\circ}$) are taken directly from GCS results. The resulting arrival
times at different locations throughout the heliosphere are reported in Table
1, and two screenshots from the simulation results are shown in Figure 7. The
two separate rows for Enlil reported in Table 1 refer to the shock (S) and
ejecta (E) arrival times. As was the case for the previous models, we expect
the CME to impact Venus, Earth, Spitzer, MSL, and Mars.

Figure 7: Screenshots from the WSA–Enlil+Cone simulation. The parameter shown
in the plots is the solar wind radial speed in the ecliptic plane on (a) 2012
May 15, 12:00 UT, and (b) 2012 May 17, 12:00 UT.

##  5 In-situ Measurements

Next, we analyse in-situ data from multiple locations scattered throughout the
inner heliosphere to evaluate the predicted impacts presented in Section 4.
Namely, we search for interplanetary signatures of the 2012 May 11 CME at
Venus (0.7 AU), Earth (1.0 AU), Spitzer (1.0 AU), MSL (1.4 AU), and Mars (1.6
AU). At each location, in addition to looking for ICME signatures from the
2012 May 11 CME, we search for SEP signatures from the 2012 May 17 event.

###  5.1 Measurements at Venus

The first (in terms of distance from the Sun, see Figure 1) location for which
an impact of the 2012 May 11 CME is predicted is Venus. Indeed, observations
around Venus made by the VEX spacecraft following the eruption reveal a period
of transient disturbances. Such measurements are reported in Figure 8. In
particular, an interplanetary shock was detected by VEX on 2012 May 13 at
17:10 UT. After a long-duration sheath region, flux rope-like signatures could
be identified from 2012 May 14 at 19:23 UT through 2012 May 16 at 01:30 UT. We
utilise here the terminology ‘flux rope-like’ rather than the more common
‘magnetic cloud’ because the boundaries were determined from magnetic field
data only, since plasma data are not provided by VEX continuously (as
explained in Section 2, the ASPERA-4 instrument was operational at periapsis
and apoaxis only, i.e. about twice per Earth day). The long duration of the
sheath region between the interplanetary shock and the following ejecta is
likely the result of the interaction of the May 11 CME with a small preceding
interplanetary structure. Solar observations from STEREO prior to the eruption
of the May 11 CME reveal the presence of several minor eruptions characterised
by a jet-like morphology in coronagraph images (see Vourlidas et al., 2013,
2017, for a classification of CME morphology types) that were possibly Earth-
directed. Observations following the eruption of the May 11 CME did not
feature CME events large enough to cause the interplanetary signatures shown
in Figure 8, indicating that the Sun–Venus connection of the CME is likely
correct.

Figure 8: Measurements at Venus around the expected arrival time of the 2012
May 11 CME, revealing the passage of an interplanetary disturbance. All data
are taken from VEX. The solid vertical line indicates the arrival of the
interplanetary shock, whilst the dashed vertical line marks a possible
interface between the preceding material and the following sheath driven by
the 2012 May 11 CME. The shaded area corresponds to the estimated flux rope-
like interval. The periods in which VEX is within Venus’ bow shock have been
removed from the magnetic field dataset. The parameters shown are: (a)
magnetic field magnitude, (b) magnetic field components in Venus Solar Orbital
(VSO) Cartesian coordinates, (c) $\theta$ and (d) $\phi$ angles of the
magnetic field in VSO angular coordinates, (e) solar wind speed, (f) proton
number density, (g) proton temperature, (h) plasma $\beta$, (i) proton and (j)
electron energy distribution, and (k) background counts.

A possible interpretation for the features observed before the flux rope-like
ejecta is that the interplanetary shock driven by the 2012 May 11 CME (solid
vertical line in Figure 8) propagated through the preceding structure, whilst
the following sheath material remained behind. Previous studies have shown
that a faster shock can indeed travel through a slower, preceding structure
(e.g., Kilpua et al., 2019b; Lugaz et al., 2013). A possible ‘interface’
between the preceding interplanetary structure and the following sheath region
driven by the 2012 May 11 CME is indicated by a dashed vertical line in Figure
8 (estimated via the rapid change in direction in the magnetic field
$Z$-component). The speed profile within the flux rope-like structure (shaded
region in Figure 8) appears nearly flat, suggesting that the ejecta was not
expanding as it passed Venus. We emphasise, however, that only two velocity
data points fall within the flux rope-like structure and, thus, these
conclusions may not be representative of the fine structure of the speed
profile. Visual inspection of the ejecta magnetic field shows a rotation of
the $Y$-component from west to east and a rotation of the $Z$-component from
south to north. This suggests that the corresponding flux rope is right-handed
and at an intermediate orientation between a south–west–north (SWN) and a
west–north–east (WNE) type. We also estimate the orientation of the flux rope
using two techniques. The first is the minimum variance analysis (MVA;
Sonnerup and Cahill, 1967), where the flux rope axis corresponds to the MVA
intermediate variance direction. The orientation of the flux rope axis
resulting from MVA is ($\Theta$, $\Phi$) = ($50^{\circ}$, $261^{\circ}$), thus
in the intermediate state between a SWN- and a WNE-type flux rope,
consistently with what is suggested by visual inspection of the magnetic field
data. We also fit the flux rope using the analytical Circular–Cylindrical (CC)
model described in Nieves-Chinchilla et al. (2016). According to the CC model,
the flux rope is right-handed, its axis has orientation ($\Theta$, $\Phi$) =
($24^{\circ}$, $256^{\circ}$), and the impact parameter is $y_{0}/R=-0.22$,
with $R=0.15$ AU being the radius of the cloud. The two methods yield an
almost identical $\Phi$ angle for the flux rope axis, whilst the $\Theta$
angle differs by ${\sim}25^{\circ}$. Nevertheless, given the usual
uncertainties related to all fitting techniques (e.g., Démoulin et al., 2018;
Lepping et al., 2003; Riley et al., 2004), these results can be deemed mostly
consistent, thus indicating a right-handed flux rope with a low-to-
intermediate inclination.

Finally, we note a remarkable increase in the background counts measured by
VEX/IMA (Figure 8k) during the early hours of 2012 May 17, indicating that an
SEP event had impacted Venus. The enhancement of background levels in the
ASPERA suite, in fact, corresponds to sufficiently energetic particles that
are able to penetrate the instrument (e.g., Futaana et al., 2008; Ramstad et
al., 2018). Since ASPERA-4 was operational close to periapsis and apoapsis
only, the background count enhancement was first observed at 03:22 UT after a
data gap, thus it is not possible to establish the ‘true’ onset time and peak
intensity of the SEP event at Venus. According to the May 17 eruption overview
presented in Section 3.2 and taking into account a particle propagation time
of ${\sim}10$–$15$ minutes, we would expect various locations in the inner
heliosphere to see an SEP event some time before 02:00 UT on 2012 May 17. This
is consistent with the ‘descending’ profile seen in the IMA background counts,
suggesting an earlier onset and peak. Thus, the background enhancement seen at
VEX seems to be indeed due to the May 17 eruption. Furthermore, we note that
the SEP event at Venus is not observed inside the 2012 May 11 ICME ejecta, but
${\sim}1$ day after the passage of what we defined as the trailing edge of the
flux rope-like structure. However, considering the CME propagation direction
estimated in Sections 3.1 and 4, it is likely that Venus was immersed in the
CME leg, CME wake, or some other trailing structure in the IMF at the time of
the SEP event.

###  5.2 Measurements at Earth

The next location for which we evaluate a possible impact of the 2012 May 11
CME and 2012 May 17 SEP event is Earth. Given the relatively small separation
between Venus and Earth (${\sim}0.3$ AU in radial distance and
${\sim}15^{\circ}$ in longitude, see Figure 1), we expect to observe
relatively similar ICME signatures that, with the aid of continuous plasma
measurements, could strengthen the interpretation reported in Section 5.1.
Indeed, similarly to observations around Venus, measurements of the solar wind
taken from Earth’s Lagrange L1 point during the days following the 2012 May 11
eruption reveal clear signatures of a transient period of disturbed IMF and
plasma flow. Figure 9 shows magnetic field and plasma data taken by Wind
together with GOES proton flux and neutron monitor data from the SOPO station.
Such measurements show that an interplanetary shock impacted on 2012 May 15 at
01:25 UT, followed by a long-duration sheath. Clear magnetic cloud signatures
were visible from 2012 May 16 at 15:57 UT through 2012 May 17 at 22:20 UT.

Figure 9: Measurements at Earth around the expected arrival time of the 2012
May 11 CME, revealing the passage of an interplanetary disturbance. Data are
taken from (a–i) Wind, (j) GOES-13, and (k) the SOPO neutron monitor. The
solid vertical line indicates the arrival of the interplanetary shock, whilst
the dashed vertical line marks a possible interface between the preceding
material and the following sheath driven by the 2012 May 11 CME. The shaded
area corresponds to the estimated magnetic cloud interval. The parameters
shown are: (a) magnetic field magnitude, (b) magnetic field components in
Geocentric Solar Ecliptic (GSE) Cartesian coordinates, (c) $\theta$ and (d)
$\phi$ angles of the magnetic field in GSE angular coordinates, (e) solar wind
speed, (f) proton number density, (g) proton temperature, (h) plasma $\beta$,
(i) electron pitch angle distribution parameter for five energy levels, (j)
energetic proton flux, and (k) neutron monitor counts per second.

The sequence of features within the ICME generally matches the measurements at
Venus (Section 5.1), the interplanetary shock (solid vertical line in Figure
9) being followed by material that appears to belong to two interacting
structures. Again, we have marked in Figure 9 (with a dashed vertical line)
the possible ‘interface’ separating the two. Interestingly, the first portion
shows significant expansion from Venus to Earth (from 8.6 hours to 19.3
hours), whilst the following portion of sheath features only minimal expansion
(from 17.6 hours to 19.2 hours). This can be explained considering the ambient
solar wind ahead of the ICME: at Venus (Figure 8e), the interplanetary shock
is preceded by the trailing portion of a fast stream and the following slow
wind, whilst at Earth (Figure 9e), the shock has travelled through the slow
stream and has caught up with the fast one. This has likely facilitated the
expansion of the small structure immediately following the shock, whilst the
remaining portion of material (which we associated to the sheath region of the
2012 May 11 CME) shows signatures of compression (with an increasing speed
profile and enhanced density) rather than expansion.

The magnetic cloud-type ICME ejecta (shaded region in Figure 9) is reported in
both the Richardson & Cane ICME list (hereafter R&C list; Cane and Richardson,
2003; Richardson and Cane, 2010) and the NASA–Wind ICME list (hereafter N-C
list; Nieves-Chinchilla et al., 2018, 2019). The R&C list reports the ejecta
measured at Earth as a clear magnetic cloud, featuring bidirectional electrons
and lacking signatures of expansion. We also note that, in the R&C list, the
2012 May 11 CME that we analysed through remote-sensing imaging in Section 3
is reported as the most probable solar counterpart of this event. The N-C list
reports the ejecta as a flux rope, with an apparent expansion velocity of $-7$
km s-1 and a distortion parameter of 0.57 (the distortion parameter is defined
as the fraction of the magnetic obstacle where 50% of the total magnetic field
magnitude is accumulated). Again, these signatures show no expansion of the
ejecta, but rather its slight compression at the back, which is consistent
with the presence of faster wind following it. Indeed, the magnetic cloud
hardly features any expansion even when considering its evolution from Venus
to Earth (its duration goes from 30.1 hours at Venus to 30.4 hours at Earth).
Figure 9i shows the electron pitch-angle-distribution (PAD) parameter (EPP)
for five energy levels. The EPP was defined by Nieves-Chinchilla et al. (2016)
and consists of the average electron intensities close to $0^{\circ}$ and
$180^{\circ}$ normalised to those close to $90^{\circ}$, thus quantifying the
bidirectionality of electrons within a magnetic obstacle from PAD data. The
profiles show signatures of bidirectionality during the first half of the flux
rope and a drop to near zero for the second half. This suggests that the
magnetic field lines at the front are still connected to the Sun, whilst in
the rear part at least one leg appears to be disconnected (PAD spectra exhibit
signatures of one-directional strahl, data not shown). Regarding the magnetic
structure of the magnetic cloud, visual inspection of the magnetic field shows
a rotation of the $Y$-component from west to east and a rotation of the
$Z$-component from south to north. The orientation of the flux rope axis from
MVA is ($\Theta$, $\Phi$) = ($46^{\circ}$, $271^{\circ}$), which is almost
identical to the orientation found at Venus using the same method. The N-C
list also provides fitting results for the flux rope using the CC model,
according to which the flux rope is right-handed, its axis has orientation
($\Theta$, $\Phi$) = ($29^{\circ}$, $229^{\circ}$), and the impact parameter
is $y_{0}/R=-0.38$, with $R=0.13$ AU being the radius of the cloud. In
addition to the uncertainties related to flux rope fitting techniques
mentioned in Section 5.1, in this case the difference in axis orientation from
the MVA and CC methods may also depend on the fact that the flux rope
boundaries do not coincide exactly (in the N-C list, the trailing edge is
marked about 4 hours later than the one considered in this work). The
importance of the boundary selection to increase the level of agreement across
different models was quantified by Al-Haddad et al. (2013). Nevertheless, both
results are consistent with a right-handed flux rope with a low-to-
intermediate inclination (i.e., somewhere between a SWN- and a WNE-type), in
agreement with measurements at Venus (Section 5.1).

Finally, Figure 9j–k shows proton flux and neutron monitor data. A Forbush
decrease (Forbush, 1937; Hess and Demmelmair, 1937) can be seen in the SOPO
time series starting around 02:30 UT on 2012 May 15, i.e. shortly after the
interplanetary shock arrival at Wind. We remark that neutron monitor data are
collected from ground-based stations, whilst Wind is located at Earth’s L1
point, thus the slight (${\sim}1$-hour) delay indicates that the features
clearly correspond to the same event. This Forbush decrease was also analysed
by Freiherr von Forstner et al. (2019), who connected 45 ICMEs from the Sun to
the MSL spacecraft using STEREO/HI data (accordingly, more information
regarding the findings of this study can be found in Section 5.4). The most
striking feature in the neutron monitor time series, however, is the
considerable peak in the count rate starting at 01:56 UT on 2012 May 17, i.e.
during the passage of the magnetic cloud at Earth, indicating the occurrence
of a ground-level enhancement (GLE; e.g., Nitta et al., 2012). This
corresponds to the SEP event registered by GOES in proton flux measurements,
with onset between 01:55 and 02:00 UT. The 2012 May 17 SEP event at Earth was
studied in detail in the literature (e.g., Battarbee et al., 2018; Ding et
al., 2016; Gopalswamy et al., 2013; Li et al., 2013; Plainaki et al., 2014;
Rouillard et al., 2016), being associated with the first GLE of solar cycle 24
(GLE71), and was even detected at the International Space Station (Berrilli et
al., 2014). Rouillard et al. (2016) suggested that the passage of the magnetic
cloud facilitated the magnetic connectivity between the May 17 eruption source
region and Earth.

###  5.3 Measurements at the Spitzer Space Telescope

Also at ${\sim}1$ AU but separated by ${\sim}70^{\circ}$ from Earth (see
Figure 1) was Spitzer. Although this telescope was dedicated to infrared
measurements of deep space, it has been shown that it can be used to
investigate space weather events (Cheng et al., 2014). Specifically, energetic
particles of both solar and extra-solar origin can impact multiple subsystems
on the spacecraft, hence Spitzer can act as a space weather monitor for SEP
events of sufficiently high energies (${\geq}100$ MeV). In this study, we
focus on the high-energy particle hits on Spitzer’s main science instrument,
IRAC, which result in saturated pixels in the infrared images taken by the
camera. Such affected pixels are flagged as ‘radhits’ and masked, in order to
ensure that they are excluded from composite images of the observations. On
average, IRAC measures about 4–6 radhits per second (depending on the phase of
the solar cycle) due to galactic cosmic rays (Lowrance et al., 2018). During
2012 May, this ‘baseline number’ was estimated to be $4\pm 0.8$ radhits per
second. Thus, a number of detected radhits higher than 4 may correspond to a
space weather event of solar origin.

The radhits measured by IRAC during 2012 May 10–20 are shown in Figure 10. As
expected, most of the data points are clustered around the value of 4. The
propagation models that we used in this work (see Section 4 and Table 1)
estimated a CME impact at Spitzer to take place around 2012 May 14–15. We do
not observe an increase in radhits during that period, suggesting that, if the
May 11 CME did indeed impact Spitzer, it was not associated with high-energy
particles (as it was the case in the study performed by Amerstorfer et al.,
2018, on a CME that erupted on 2010 November 3). This is not surprising, since
the ICME appeared rather slow at both Venus (Section 5.1) and Earth (Section
5.2). Nevertheless, we do observe an increase in counts starting on 2012 May
17 at 03:10 UT, with the measured radhits reaching a value ${>}8$. As it was
the case for particle measurements at Venus (Section 5.1), the increase
follows a several-hours data gap, hence it is not possible to establish an
exact onset time for the event. Nevertheless, the timing and descending
profile of the increase strongly hint towards an earlier onset and a ‘true
peak value’ higher than 8.2 radhits per second, consistent with the SEP event
being associated with the May 17 eruption. Since Spitzer was separated by
${\sim}150^{\circ}$ in longitude from the flaring site, it follows that there
is no possibility for the two heliolongitudes to be magnetically connected
under nominal Parker spiral conditions. Hence, it is likely that the magnetic
connectivity required for the observed impulsive feature of the SEPs was
provided by the May 11 CME, which was being crossed by Spitzer at the time of
the flare.

Figure 10: Radhits per second measured by the IRAC instrument onboard Spitzer
during the days following the 2012 May 11 CME eruption. The gaps correspond to
times in which IRAC was not taking data due to downlinking. Each data point
corresponds to exposure times of 12 s, 30 s, or 100 s, and is averaged over a
number of frames in the range 5–25. The overall data set has an average of
$3.77\pm 0.61$ radhits per second.

###  5.4 Measurements at the Mars Science Laboratory

At the time of the CME under study, the MSL spacecraft was approaching the end
of its cruise phase, before safely landing the Curiosity rover on Mars on 2012
August 6. It was at a heliocentric distance of ${\sim}1.4$ AU and its
longitude was ${\sim}35^{\circ}$ east of Earth (see Figure 1). Even though the
RAD instrument was designed to measure the particle radiation environment on
the surface of Mars, it was active during most of the cruise phase and
collected data of the interplanetary radiation environment for future crewed
missions to Mars (Zeitlin et al., 2013), detecting several Forbush decreases
and SEP events (Guo et al., 2015).

Radiation dose measurements taken by MSL/RAD during the days following the
2012 May 11 eruption are shown in Figure 11. Again, we find in the data clear
signatures of the SEP event related to the 2012 May 17 eruption (also reported
by Battarbee et al., 2018), starting at 02:04 UT and suggesting that the May
11 CME was also being crossed by MSL at that time. Therefore, we search for a
possible Forbush decrease onset before the arrival of the SEPs. We tentatively
identify such onset to take place on 2012 May 15 at 13:30 UT (marked with a
solid vertical line in Figure 11). Unfortunately, the SEP event of May 17 does
not allow us to follow the full development of the Forbush decrease, hence it
is not possible to declare with certainty whether the decrease is a
“classical” two-step one. A Forbush decrease that develops in two steps
usually indicates the arrival of an interplanetary shock, corresponding to the
first step, and its following ICME ejecta, corresponding to the second step
(Cane, 2000). Nevertheless, the Forbush decrease onset time that we identified
is consistent with the analysis performed by Freiherr von Forstner et al.
(2019), who reported an arrival time at MSL on May 15, 12:00 UT (event
20120512_01 in their study). The authors connected this Forbush decrease with
the one measured at Earth earlier on the same day (see Section 5.2) and with
the CME observed in HI1-B imagery (see Section 3.1.3), in agreement with our
analysis of the same event.

Figure 11: Radiation dose measurements taken by MSL/RAD en route to Mars.
Panel (b) is a zoomed-in version of the y–axis of panel (a). The time of the
Forbush decrease onset (used to estimate the shock arrival time) is indicated
with the solid vertical line.

###  5.5 Measurements at Mars

At 1.6 AU from the Sun and almost perfectly aligned with MSL (${<}1^{\circ}$
separation in longitude, see Figure 1) was Mars. There were no spacecraft
equipped with a magnetometer in orbit around Mars at the time of this study,
hence it is not possible to analyse the magnetic structure of the 2012 May 11
CME. Nevertheless, we take advantage of the extensive data sets that are
available from two spacecraft to estimate the arrival time of the
interplanetary shock and the boundaries of the following ICME ejecta. In
particular, we complement the solar wind and particle data collected by
MEX/ASPERA-3 and MOdy/HEND with measurements performed inside the Martian
induced magnetosphere by MEX/MARSIS. Since Mars is not protected by an
intrinsic magnetic field, the increased dynamic pressure accompanying periods
of disturbed solar wind conditions is able to push the plasma boundaries of
the system (e.g., bow shock, magnetic pileup boundary, ionopause) to lower
altitudes quite efficiently (e.g., Lee et al., 2017; Luhmann et al., 2017;
Morgan et al., 2014; Sánchez-Cano et al., 2017, 2020). Hence, the level of
compression of the Martian induced magnetosphere can help to determine whether
a solar transient has impacted the planet. In this study, we focus on the
altitude of the outbound magnetic pileup boundary crossings for successive
orbits. Since the MEX orbit precession is minimal during the period that we
investigate (11 days), we can assume that the boundary crossings should always
occur at similar altitudes in the case of a static system.

Figure 12 shows plasma and energetic particle measurements taken by MEX and
MOdy following the eruption of the CME under study. Several signatures in
these data indicate that an interplanetary shock (solid vertical line in
Figure 12) impacted Mars on 2012 May 16 around 06:00 UT. Namely, we observe
significant compression of the Martian magnetic pileup boundary between two
successive orbits (Figure 12a, showing that the altitude of the crossings
lowered from ${\sim}750$ to ${\sim}400$ km), a steep increase in the speed
profile (Figure 12b), enhancement in electron counts (Figure 12f), and the
onset of a Forbush decrease (Figure 12i). However, as it was the case for MSL
(Section 5.4), the SEP event of May 17 is seen at Mars as well, preventing us
from following the whole development of the Forbush decrease and, therefore,
possibly estimating the ejecta leading edge time. Nevertheless, the presence
of SEP signatures, seen in both MEX/IMA background counts (Figure 12g) and
MOdy/HEND count rates (Figure 12h) and starting at 02:16 UT, suggests that the
ICME ejecta was also being crossed by Mars at the time of the May 17 eruption.
If we consider the period of depressed proton temperature following the shock
arrival (one of the “classic” in-situ signatures of ejecta; e.g., Richardson
and Cane, 1995), then the ICME ejecta boundaries would fall roughly between
2012 May 17 at 02:26 UT and 2012 May 19 at 06:59 UT (shaded region in Figure
12). This interval is consistent with MEX/MARSIS measurements, since the
estimated trailing edge coincides with an increase in the crossing altitude,
suggesting that the interplanetary transient had fully travelled past Mars.
These boundaries would also place the leading edge ${\sim}1$ hour after the
May 17 flare onset and a few minutes after the arrival of energetic particles
at Mars; however, since ASPERA-3 was not operational continuously at the time
of these events, with gaps of a few hours between each observing session, the
estimated boundaries are affected by large uncertainties. Nevertheless, it is
reasonable to speculate that the SEP event onset and the passage of the ICME
ejecta leading edge happened very close in time.

Figure 12: Measurements at Mars around the expected arrival time of the 2012
May 11 CME, revealing the passage of an interplanetary disturbance. Data are
taken from (a–g) MEX and (h–i) MOdy. The solid vertical line indicates the
arrival of the interplanetary shock, whilst the shaded area corresponds to the
estimated ejecta interval. The parameters shown are: (a) altitude of the
magnetic pileup boundary (inner boundary of the magnetosheath) outbound
crossings for successive orbits, (b) solar wind speed, (c) proton density, (d)
proton temperature, (e) proton and (f) electron energy distribution, (g)
background counts, and (h–i) neutron counts per second. The three dots under
two data points in panel (a) indicate that for those orbits only an upper
limit for the crossing altitude could be estimated. Panel (i) is a zoomed-in
version of the y–axis of panel (h).

##  6 Discussion

In this section, we synthesise the multi-spacecraft observations, modelling
results, and interpretations presented in Sections 3, 4, and 5 and discuss
them in the context of three main topics: CME propagation across the inner
heliosphere, CME magnetic structure, and CME role in SEP transport from the
2012 May 17 event.

###  6.1 CME Propagation

The 2012 May 11 eruption was a case of a CME experiencing moderate deflection
very close to the Sun, with the CME source region being located at S13E13 on
the disc (Section 3.1.1) and the CME propagation direction being S10E30 from
GCS reconstructions in the corona (Section 3.1.2). This is not surprising,
since most of the deflection is expected to take place below 30 $R_{\odot}$
from the Sun (Isavnin et al., 2014). CME deflections usually occur due to
magnetic forces acting in the corona, which are dominant below 10 $R_{\odot}$
(e.g., Kay and Opher, 2015) and tend to divert CMEs towards the heliospheric
current sheet and away from coronal holes (e.g., Cremades et al., 2006; Kilpua
et al., 2009; Xie et al., 2009). In the case of the CME under study, the
observed deflection can be likely attributed to two complementary factors,
namely the global magnetic structure of the corona and the interchange
reconnection scenario described in Section 3.1.1. Specifically, the CME source
region was located at the edge between two highly inclined helmet streamers
(from potential-field reconstructions, see Figure S4), and reconnection of the
filament’s eastern leg with the nearby open field resulted in the CME
diverting towards the reconnection region (as shown in simulations by, e.g.,
Lugaz et al., 2011; Lynch and Edmondson, 2013; Török et al., 2011). We remark
that such a deflection, despite being considered moderate, has in general
important implications for space weather forecasting: if considering the
location of the source region only, then one would expect a rather frontal
encounter at Earth. Only through coronagraph observations did we estimate a
flank encounter at Earth and a more central one at Mars (see Figure 1a), which
was also the case for the 2014 January 7 CME studied by Möstl et al. (2015).
In addition, CME deflections have also implications on the structure of the
IMF, thus altering the longitudinal extent that will be available for SEP
acceleration and detection.

Multi-spacecraft coronagraph and HI observations permitted us to evaluate the
CME propagation direction and its half-angular extent in order to estimate its
impact throughout the heliosphere using different propagation models (Section
4). If we exclude modelling results from the SSEF30-A technique reported in
Table 1 (which can be considered as an outlier), then we can conclude that the
arrival locations predicted by the models employed in this study were
consistent with each other. However, despite the agreement in terms of
hit/miss, the predicted arrival times throughout the inner heliosphere
featured more substantial differences. This is mostly due to the physics and
assumptions involved in each model. In fact, we note that the spread in
arrival times increases with both heliocentric distance and angular separation
from the CME nose. In the case of approximately central encounters, the major
contribution to the spread is given by the physics that regulates the CME
radial propagation in each model (constant acceleration for SSE/SSSE, drag-
based with constant background for DBM, and MHD-based with variable background
for Enlil). In the case of flank encounters, the major contribution to the
spread is instead given by the geometry of the CME front assumed in each model
(spherical cross-section for SSE/SSSE, circular arc for DBM, and evolving
hydrodynamical structure for Enlil). Interestingly, we note that in the case
of the SSSE model the CME is predicted to reach Mars before it reaches Earth,
which is due to the perfectly circular shape of the CME front. In such cases,
employing a larger CME half-width may solve the issue (a detailed study of how
the SSSE model performs with respect to different CME half-widths is shown in
Barnes et al., 2020). Furthermore, we remark that Palmerio et al. (2019)
applied the (S)SSE model to two CMEs and noted that adding $15^{\circ}$ to the
GCS-derived half-width improved arrival time predictions in both cases.
Nevertheless, employing several models with different assumptions can provide
an overall context useful to interpret in-situ observations, as was
demonstrated in this study.

A comparison of different modelling results with the in-situ observations
presented in Section 5 is shown in Table 2. When considering differences
between the predicted and measured arrival times, it is important to remark
that all the forecasts reported in Section 4 were initiated using only remote-
sensing data as input, and no adjustments were made to match in-situ
observations. The propagation models were used as a guide to search for in-
situ signatures, rather than to reproduce the observed arrivals. Most
important, we remind the reader that we approximated the highly asymmetric and
distorted 2012 May 11 CME with idealised, symmetrical structures in both
coronagraph reconstructions (Section 3.1.2 and Figure 4) and propagation
models (Section 4). Furthermore, apart from the well-known uncertainties in
modelling CME arrival times that are estimated to be of the order of $\pm 10$
hours at 1 AU regardless of the model used (e.g., Riley et al., 2018;
Vourlidas et al., 2019; Wold et al., 2018), additional complications may arise
from the pre-existing solar wind conditions at the observing spacecraft. For
example, the arrival times at Venus and Earth were likely affected by the
preceding interplanetary structure (see Figures 8 and 9) that we attributed to
an earlier, narrow eruption and that likely slowed down the 2012 May 11
ejecta. Furthermore, all the models predicted the impacts at MSL and Mars to
take place significantly later than observed, which is possibly due to the
faster wind preceding the CME at those heliolongitudes (see Figure 12). Taking
all the aforementioned factors into account, these results suggest that, even
in the case of particularly complex events, models that simplify their
geometry could be used to satisfactorily estimate at least the impact
location(s), albeit with sometimes significant errors in arrival times.

Table 2: CME arrival times from three modelling techniques presented in
Section 4 compared with the in-situ observations presented in Section 5.

Model | Venus | Earth | Spitzer | MSL | Mars
---|---|---|---|---|---
SSSE | 05/14 19:15 | 05/17 18:51 | 05/15 15:01 | 05/17 00:13 | 05/17 14:57
DBM | 05/14 00:08 | 05/15 00:20 | 05/14 23:42 | 05/16 14:42 | 05/17 05:49
Enlil (S) | 05/13 19:33 | 05/14 21:30 | 05/14 20:03 | 05/16 05:54 | 05/16 21:58
Enlil (E) | 05/14 01:00 | 05/15 17:52 | 05/15 21:07 | 05/17 03:06 | 05/17 23:27
Observed (S) | 05/13 17:10 | 05/15 01:28 | ? | 05/15 13:30 | 05/16 06:00
Observed (E) | 05/14 19:23 | 05/16 15:57 | ? | ? | 05/17 02:26

Note. Dates are shown in the format MM/DD HH:MM. The notations ‘(S)’ and ‘(E)’
denote shock and ejecta, respectively. The question marks indicate that it is
not possible to determine whether there has been an impact, and/or at what
time.

###  6.2 CME Magnetic Structure

The magnetic structure of the 2012 May 11 CME was inferred at several
locations: at the Sun, through the solar corona, at Venus, and at Earth. The
corresponding flux rope type was found to change dramatically across the
different observation points. As shown in Section 3.1.1, the eruption of the
2012 May 11 CME involved the presence of a filament that disconnected
asymmetrically from the Sun. The western leg stayed anchored to the
photosphere for longer than the eastern one, which detached rapidly and
resulted in a significant clockwise rotation of the filament. Based on these
observations and on 3D reconstructions of the CME in the low corona (Figure
3), we estimated that the corresponding flux rope erupted as an ESW type, but
was a NES type close to the Sun and a WNE type in the outer corona. The
scenario of a filament eruption where one leg disconnects from the Sun early
in the process whilst the other follows later was also observed by Vourlidas
et al. (2011). The CME analysed in their work erupted on 2010 June 16,
featured negligible rotation below 3 $R_{\odot}$, but was observed in
coronagraph imagery to rotate at an exceptionally fast rate, i.e. 60∘/day. In
contrast, the 2012 May 11 CME studied here appeared to rotate significantly
(${\sim}65^{\circ}$) already in the low corona. Both of these cases, however,
feature the same outcome, i.e. that the resulting magnetic configuration in
the outer corona is significantly different from that at the Sun. These
rapidly rotating events are particularly challenging for space weather
forecasting of CME magnetic fields (e.g., Kilpua et al., 2019a), since
information on the intrinsic flux rope type (inferred at the Sun) becomes
practically obsolete.

Furthermore, the observed disconnection of one filament leg during the
eruption raised questions about the evolution of the connectivity of the
large-scale CME and whether the corresponding flux rope leg remained attached
to the Sun. The relationship between the structure and evolution of erupting
filaments and their overlying flux ropes is not always straightforward, and
hence is an active area of research (e.g., Gibson and Fan, 2006; Howard et
al., 2017; Schmieder et al., 2002). In the case of the event presented in this
work, two main outcomes are possible: 1) the flux rope undergoes interchange
reconnection together with the filament, hence completely detaching its
western leg from the Sun, or 2) the large-scale flux rope (partially or in its
entirety) maintains its field lines connected to the Sun, with the filament
dynamics occurring at its periphery. We could not determine the connectivity
of the flux rope from remote-sensing images alone, but measurements at Earth
(Section 5.2) indicated the presence of bidirectional electrons during the
first half of the ICME ejecta passage, suggesting that the front portion of
the corresponding flux rope was still attached to the Sun at both ends, whilst
in the rear part at least one leg was disconnected. A similar PAD profile was
reported by Nieves-Chinchilla et al. (2020), who analysed a streamer-blowout
flux rope that was observed by the Parker Solar Probe spacecraft on 2018
November 11–12. The authors provided three possible explanations for such a
scenario: 1) a single (albeit distorted) flux rope transient, 2) a double flux
rope, or 3) a combination of a flux rope and open magnetic field lines. Given
the strong evidence for interchange reconnection presented in this work, known
to be a common CME process (e.g., Crooker et al., 2002), we conclude that the
latter option seems the most likely.

As we remarked in Section 3.1.2, the 2012 May 11 CME appeared significantly
asymmetric and distorted in coronagraph imagery, especially from the SOHO and
STEREO-B perspectives. Even though we approximated the CME morphology with a
perfectly symmetrical GCS shell to derive its geometric and kinetic parameters
(see Figure 4), it is important to keep in mind that the underlying flux rope
may be considerably warped and that such deformations may be preserved or even
enhanced in interplanetary space. Examples of deformed CME (and shock) fronts
were reported by Farrugia et al. (2011), who found distortions and rotations
in a magnetic cloud measured during 2007 November 19–21 by three spacecraft at
1 AU covering $40^{\circ}$ in longitude, and Möstl et al. (2012), who found
inconsistent flux rope inclinations with respect to the ecliptic plane in a
series of CMEs launched on 2010 August 1 and measured at various locations in
the inner heliosphere covering a $120^{\circ}$ longitudinal span. From a space
weather forecasting perspective, this means that knowledge of the global CME
orientation at the Sun may have little to no correlation with the portion of
CME that will be encountered in situ. More generally, Möstl et al. (2012)
suggested that the orientation of a flux rope may be viewed as a local
parameter, rather than a global one. It follows that it is especially
difficult to distinguish between global rotations and local deformations of a
flux rope, as was pointed out by Palmerio et al. (2018) who compared the
orientations at the Sun with those at Earth for 20 CME events.

In interplanetary space, the magnetic structure of the 2012 May 11 CME could
be evaluated at Venus (Section 5.1) and Earth (Section 5.2). The flux rope
type and axis orientation resulted quite compatible when determined separately
at the two locations but, in light of the aspects considered above, it is
useful to evaluate whether the same holds true when regarding the ejecta as a
coherent, rigid structure. In order to investigate this, we use the 3D Coronal
Rope Ejection (3DCORE; Möstl et al., 2018; Weiss et al., 2021) modelling
technique. 3DCORE is a forward simulation model that describes the structure
of a CME using a torus-like geometry that is attached to the Sun and expands
self-similarly as it propagates throughout the heliosphere. The expanding
structure contains an embedded magnetic field that is based on an approximate
analytical solution for torii (Vandas and Romashets, 2017) that is similar to
a Gold–Hoyle (Gold and Hoyle, 1960; Farrugia et al., 1999) field. The fitting
is performed using an approximate Bayesian computation sequential Monte Carlo
algorithm, the implementation of which is described in detail in Weiss et al.
(2021), that generates an ensemble of solutions. One significant advantage of
this approach is that it is possible to estimate the errors on our parameters
even if only using a single measurement. We initially proceed by applying the
3DCORE fitting algorithm to Wind measurements from Earth’s L1 point, using
boundary conditions that are very similar to those shown in Figure 9. The fit
is evaluated on the interval spanning 2012 May 17, 00:00 UT to 18:00 UT, using
seven equidistant fitting points and a root-mean-square error metric. The
overall time period in which ensemble solutions are accepted is set to 2012
May 16, 16:00 UT, until 2012 May 17, 22:30 UT. Figure 13a shows the reproduced
flux rope signatures from the ensemble at Earth and the corresponding
2-$\sigma$ spread in the magnetic field generated by the underlying
uncertainties. We then back-propagate the obtained ensemble solution to Venus,
allowing cross-verification of the Wind fit with the VEX measurements. This is
achieved by simply re-running the simulations with the 3DCORE solution
ensemble and moving the observer to Venus. The resulting magnetic field
measurements from the ensemble solution at Venus, together with the propagated
uncertainties, are shown in Figure 13b. Finally, we also show the 3D model
structure of a representative ensemble sample from two different viewing
angles (Figure 13c–d).

Figure 13: Modelling results from the 3DCORE analysis of the 2012 May 11 CME.
(a) 3DCORE ensemble fit of the ICME ejecta at Earth using measurements from
Wind. (b) The fit shown in panel (a) back-propagated to Venus and superposed
on VEX measurements. (c–d) Visualisation of the CME structure resulting from
3DCORE for a representative ensemble sample.

The results of the 3DCORE analysis suggest that the flux rope was oriented
with a high inclination, up to $60^{\circ}\pm 10^{\circ}$, which is slightly
larger than the result from the previous analysis (Sections 5.1 and 5.2), but
nevertheless consistent with a WNE-to-SWN flux rope. Furthermore, the
propagation direction of the CME is inferred to be on the opposite side of
Earth when compared to the CME propagation results shown in Section 4 (see,
e.g., Figure 7 for the Enlil simulation). In particular, these results predict
a close miss at Mars, which is most likely not the case as there are in-situ
measurements from MSL and the spacecraft orbiting Mars that strongly hint
towards the contrary. These types of disagreements, however, are not out of
the ordinary, as we have only fitted the in-situ magnetic field measurements
at Earth’s L1 point and have not added any additional constraints from other
positions. Furthermore, these discrepancies may be related to CME distortion
during transit and to how coherent the internal MHD structure of CMEs remains
beyond ${\sim}0.3$ AU from the Sun (Owens et al., 2017).

Figure 13a shows that we are able to largely reconstruct the measured magnetic
field profile using the 3DCORE model. The only larger discrepancy can be found
in the $B_{X}$ component towards the end of the flux rope. Further assessment
on the quality of this fit can be obtained by cross-verifying our results with
measurements at Venus. In Figure 13b, for simplicity, we only show the mean
back-propagated ensemble of our solution at the position of the VEX
spacecraft. We can conclude that, in general, the back-propagated and measured
flux ropes are more or less in qualitative agreement. There is a big
difference with respect to the total magnetic field strength, which can be
attributed to the scaling relations that are implemented in the model. When
compensating for this, the only large remaining discrepancy is again the
$B_{X}$ component. As the VSO coordinate system is essentially the GSE
equivalent for Venus, and at the time point of interest Venus and Earth are
almost radially aligned, this can be expected from the same $B_{X}$
discrepancy at Earth. This shows that, at least to the first order, the
measurements at Venus and Earth are consistent with each other and do not show
any drastic evolution, clearly demonstrating that the same ICME flux rope was
observed at both planets.

In conclusion, the direction of the 2012 May 11 flux rope axis was found to
rotate by at least ${\sim}180^{\circ}$ clockwise between the Sun and Venus,
highlighting the difficulties for space weather forecasting of the $B_{Z}$
component for rapidly rotating events. In particular, at least
${\sim}55^{\circ}$ of this rotation seems to have taken place between the last
coronagraph observation and Venus, in agreement with Isavnin et al. (2014) who
reported that a significant amount of CME deflection and rotation can still
happen between 30 $R_{\odot}$ and 1 AU. However, we remark that, in light of
the discussion above, CME rotation might have been extreme at the longitudes
of Venus and Earth but not at other locations, e.g. at Mars, where the flux
rope type could not be determined. In this regard, it is worth mentioning that
the rotating CME analysed in the solar corona by Vourlidas et al. (2011) was
then analysed from the Sun to Earth by Nieves-Chinchilla et al. (2012),
passing by Mercury that was separated by ${\sim}20^{\circ}$ from the Sun–Earth
line. The authors found significant discrepancies in the flux rope tilt
between observations in the solar corona and those at Mercury (by
${\sim}100^{\circ}$) and then from Mercury to Earth (between $20^{\circ}$ and
$50^{\circ}$ depending on the chosen flux rope boundaries). The results
presented in Nieves-Chinchilla et al. (2012) and in this work show the
importance of having magnetic field measurements of the same ICME at widely
longitudinally separated spacecraft, in order to discern between global
rotations and local distortions.

###  6.3 CME Role in SEP Transport

One key aim of this work was to explore the effect of the 2012 May 11 CME on
the observed profiles of the SEP event that originated from the 2012 May 17
eruption. SEP signatures were indeed observed at all the locations that were
predicted to be encountered by the May 11 CME (Section 4), i.e. Venus, Earth,
Spitzer, MSL, and Mars. These findings suggest that the required magnetic
connectivity for the impulsive characteristics of the in-situ SEPs was
provided by the May 11 ICME, which was likely crossing all the five in-situ
locations (Section 5) where SEP signatures were detected. This was also the
conclusion drawn by Rouillard et al. (2016) based on in-situ measurements at
Earth only. In order to further explore this hypothesis, we first check
whether SEP signatures were observed at other locations in the inner
heliosphere, i.e., Mercury and the two STEREO spacecraft (see Figure 1b).
Measurements from these three points were analysed by Battarbee et al. (2018,
see also Figure 14), who reported the presence of a gradual event at all
observers, hence confirming that fast-spreading, rapidly rising SEP profiles
were detected only at those locations that were being encompassed by the May
11 ICME.

Figure 14: The 2012 May 17 SEP event throughout the inner heliosphere. (a–b)
Schematic to illustrate the scenario of the 2012 May 11 CME and the 2012 May
17 SEP event in the inner heliosphere. The flaring site (N11W76) is indicated
with a star symbol on the edge of the circle representing the Sun. The
configuration of the various planets and spacecraft is the same as in Figure
1b. (a) Hypothetical SEP transport scenario in the absence of the May 11 CME.
All planets and spacecraft in the inner heliosphere have been connected to the
Sun through the nominal Parker spiral for a solar wind speed of 400
km$\cdot$s-1. The same spiral is streaming from the flare location and is
represented with a thicker line. (b) The SEP transport scenario in the
presence of the May 11 CME (shaded in blue) and the May 17 CME (shaded in
red). SEPs propagate along the field lines within the magnetic ejecta of the
May 11 CME. Connectivity to Mercury and the twin STEREO spacecraft is not
shown. (c) SEP profiles (in linear scale) at all the in-situ locations shown
in panels (a–b). All measurements are normalised and shown in arbitrary units,
apart from STEREO-B measurements that are normalised to the STEREO-A ones. The
approximate minimum energies that each shown data set is sensitive to are: 15
MeV at MESSENGER, 14 MeV at STEREO, 30 MeV at GOES, 100 MeV at Spitzer (Cheng
et al., 2014), 35 MeV at MSL (Zeitlin et al., 2013), and 0.2 MeV at MOdy
(Sánchez-Cano et al., 2018). The minimum energy range for VEX is unknown, but
it can be approximated to be similar to that of MEX (20 MeV for protons),
which was derived by Ramstad et al. (2018) via cross-calibration with
measurements from the Mars Atmosphere and Volatile Evolution (MAVEN)
spacecraft.

Figure 14a–b illustrates the 2012 May 17 SEP transport scenario with and
without the presence of the 2012 May 11 CME. The flare site and all planets
and spacecraft in the inner heliosphere have been connected through Parker
spiral (Parker, 1958) field lines for a solar wind speed of 400 km$\cdot$s-1
in Figure 14a. As this is a hypothetical situation, we have chosen the speed
of 400 km$\cdot$s-1 to be representative of typical slow solar wind conditions
close to the solar equatorial plane. It is clear from the illustration that no
spacecraft in the inner heliosphere would have been perfectly magnetically
connected to the eruption source region under these conditions. The
longitudinal separation between the flare site and the spiral line footpoints
of the locations where SEPs were observed spans from ${\sim}15^{\circ}$
(Earth) to ${\sim}85^{\circ}$ (Spitzer). The best connectivity to Earth, MSL,
and Mars would be achieved for solar wind speeds in the range 310–340
km$\cdot$s-1, whilst the best connectivity to Venus and Spitzer would be
achieved for speeds in the range 170–200 km$\cdot$s-1. Realistically speaking,
it seems highly unlikely for each of these locations to be simultaneously
connected to the eruptive region, which is the reason why more impulsive SEP
events are usually observed over a spatially narrow region compared to gradual
ones. One way to provide simultaneous connectivity to all observers would be
the passage of the ICME ejecta associated with the 2012 May 11 CME,
illustrated in Figure 14b. On 2012 May 17 at 01:30 UT, the source region of
the May 11 CME had rotated to S13W57, hence was in the vicinity (within
${\sim}20^{\circ}$ in both latitude and longitude) of the source region of the
May 17 CME (N11W76). In such a scenario, the shock driven by the May 17 CME in
the low corona (which was estimated by Gopalswamy et al., 2013, to have formed
at $1.38\,R_{\odot}$) would easily intersect the western leg of the May 11 CME
and accelerated particles would rapidly propagate throughout the magnetic
ejecta. The energisation process itself is expected to take place on particles
that are already within the May 11 CME flank, but additional SEPs could be
introduced into the leg via direct or indirect transport processes such as
magnetic reconnection or cross-field diffusion.

Figure 14c shows the SEP profiles from the 2012 May 17 eruption measured in
situ at eight different locations in the inner heliosphere. This makes it one
of the SEP events that have been most widely observed by spacecraft at
different locations. As pointed out by Battarbee et al. (2018), Mercury and
STEREO-A observed a slowly rising SEP profile. Both locations did also measure
the passage of an ICME: the one at Mercury (between 12:10 and 15:39 UT on May
17) was reported by Winslow et al. (2015), whilst the one at STEREO-A (between
May 18 at 12:43 UT and May 19 at 09:12 UT) can be found in the STEREO ICME
list (Jian et al., 2018). The peak speed of 840 km$\cdot$s-1 measured at
STEREO-A is consistent with the fast May 17 CME. The gradual SEP profile at
Mercury and STEREO-A is followed by an additional population of energetic
storm particles (ESPs) upon the arrival of the ICME-driven shock, suggesting
that particles were locally accelerated at the shock as it propagated through
interplanetary space. Furthermore, STEREO-B observed a weak increase in proton
flux. Such increase may be related to the May 17 CME or, alternatively, may
stem from other mechanisms such as drift motion, corotation, cross-field
diffusion, and turbulence (as suggested by Battarbee et al., 2018). We point
out that the large extent and high speed of the 2012 May 17 CME, together with
the other possible mechanisms enumerated above, do not rule out the fact that
SEPs may have been measured at all eight locations without the presence of the
May 11 CME, but it is likely that at least some of them would have experienced
more slowly rising profiles such as those seen at Mercury and STEREO-A. The
impulsive SEP profiles observed at Venus, Earth, Spitzer, MSL, and Mars
suggest that an ‘instantaneous’ connectivity was established through the May
11 CME, which does not exclude that a connectivity may have been established
via other means. For instance, a later connection to the wide-extent and
outward propagating shock of the May 17 event could also be possible for
observers to the west of the event source (e.g., Lario et al., 2017).

One interesting aspect to consider in the framework of IMF connectivity is the
path length travelled by energetic particles up to the locations encompassed
by the May 11 CME. If SEPs propagate through the preceding ejecta rather than
the nominal Parker spiral, then such lengths can be expected to present
somewhat higher values than ‘normal’. In the case under study, this analysis
cannot be performed for Venus and Spitzer, where SEPs were first detected
after 03:00 UT on 2012 May 17 due to data gaps. Continuous measurements at
Earth, MSL, and Mars, on the other hand, enabled us to estimate precise onset
times for the SEP event (01:56 UT, 02:04 UT, and 02:16 UT, respectively).
These times were estimated using the Poisson–CUSUM (Lucas, 1985) method, which
has been widely employed in the case of SEP events (e.g., Huttunen-Heikinmaa
et al., 2005; Xu et al., 2020). Assuming 1 GeV protons (which can be
considered an upper energy limit for this event; e.g., Kühl et al., 2015), a
particle release time of 01:39 UT (consistent with the results in Gopalswamy
et al., 2013; Li et al., 2013), and uniform acceleration efficiency, the SEP
onset times at the three locations would yield path lengths of 1.79 AU at
Earth, 2.63 AU at MSL, and 3.89 AU at Mars. These values are all significantly
higher than the corresponding Parker spiral lengths shown in Figure 14a (1.18
AU at Earth, 1.91 AU at MSL, and 2.25 AU at Mars), thus suggesting that
connectivity was not achieved through the nominal IMF but instead via the May
11 CME ejecta. Furthermore, the path length at Earth (1.79 AU) is consistent
with the one derived by Rouillard et al. (2016), i.e. 1.89 AU. The large
difference between the path lengths at MSL (2.63 AU) and Mars (3.89 AU), when
the two locations where separated by only ${\sim}0.2$ AU, may be explained by
the almost simultaneous arrival at Mars of the May 17 SEP event and the May 11
ejecta leading edge, as was shown in Section 5.5. On one hand, it is possible
that optimal connectivity was reached slightly later at Mars; on the other
hand, Mars might have been magnetically connected to the outer (helical) flux
rope fields of the May 11 CME, which in Lunquist-like (e.g., Lundquist, 1950;
Lepping et al., 1990) models present the highest degree of twist, hence
resulting in a longer path length.

It is not unheard that SEPs can arrive at an observer from source regions that
would be considered poorly to not connected. Apart from Rouillard et al.
(2016) who analysed the same event described here, Masson et al. (2012)
studied ten GLE events between 2000 and 2006 and concluded that only three of
them are consistent with particle propagation under nominal Parker spiral
conditions. Five of these events occurred during the passage of CME-related
disturbances past the spacecraft observing the SEPs, i.e. sheath regions,
magnetic clouds, or ejecta rear regions, suggesting that regions on the Sun
that would not otherwise be magnetically connected to Earth may be temporarily
connected through transient magnetic structures. Furthermore, Dresing et al.
(2016) reported observations of an impulsive electron event on 2013 November 7
at both STEREO spacecraft, which were separated by $68^{\circ}$ in longitude.
Whilst STEREO-A was well connected with respect to the flaring region,
connectivity at STEREO-B was shown to be due to the passage of a CME that
erupted three days earlier from the same source region. Indeed, as mentioned
in the Introduction, observations of impulsive SEPs over large longitudinal
ranges are widely documented in the literature but, to our knowledge, this is
the first time that SEP transport inside a preceding ICME has been observed at
five well-separated locations, distributed over $70^{\circ}$ in longitude and
at four heliocentric distances between 0.7 and 1.6 AU. It is important to
consider the space weather implications of such a scenario: firstly,
Gopalswamy et al. (2013) noted that the 2012 May 17 flare size (M5.1) was
rather small for having resulted in a GLE event. One possible interpretation
for the unexpected acceleration of SEPs to high energies is the twin-shock
and/or twin-acceleration scenario discussed by Shen et al. (2013) and Ding et
al. (2016). However, the observations herein discussed are also consistent
with the findings of Lario and Karelitz (2014), who demonstrated that SEP
events observed within preceding ICMEs tend to show higher peak intensities
than those observed in the undisturbed solar wind. Secondly, the 2012 May 11
CME was observed at Earth as a rather slow event with almost no negative
$B_{Z}$, hence its geoeffectiveness was very modest (it was associated with a
$\mathrm{Dst}_{\mathrm{min}}=-43$ nT). Yet, its passage enabled observations
of the first GLE event of solar cycle 24, and could have possibly represented
the first GLE measured on the surface of two planets had Curiosity already
landed on Mars (the “record” went later to the 2017 September 10 event studied
by, e.g., Guo et al., 2018; Hassler et al., 2018; Lee et al., 2018; Zeitlin et
al., 2018).

##  7 Conclusions

In this work, we have followed the eruption and evolution of the 2012 May 11
CME and its role in spreading SEPs that originated from a later eruption on
2012 May 17. After analysing the 2012 May 11 event using remote-sensing
imagery of the solar disc, corona, and inner heliosphere, we have estimated
its impact throughout interplanetary space using several propagation models.
Then, we have searched for signatures of the CME passage at each of the five
predicted impact locations (i.e., Venus, Earth, Spitzer, MSL, and Mars). Where
possible, we have studied well-known properties and phenomena usually
associated with ICMEs, such as the magnetic field configuration and the
associated Forbush decrease. After finding nearly-simultaneous SEP signatures
at all five in-situ locations, we suggested that energetic particles
accelerated by the 2012 May 17 eruption could spread over a large range of
heliolongitudes due to the magnetic connectivity provided by the May 11 CME.

This work highlights the importance of using data from multiple viewpoints,
both from a remote-sensing and an in-situ perspective, to characterise the
complex evolution of CMEs. The event under study here appeared to be
significantly distorted and rapidly rotating, which may explain why the
magnetic configuration of the corresponding flux rope was not consistent
across all the observation points. Some of the spacecraft employed in this
study are no longer operational (e.g., MESSENGER, VEX, Spitzer, and STEREO-B),
but several new missions are currently available (e.g., Parker Solar Probe,
Solar Orbiter, BepiColombo, and MAVEN). Coordinated observations that employ
multiple spacecraft and ground facilities will be beneficial to our
understanding of CME physics, from their eruption through their interplanetary
journey. International efforts such as the Whole Heliosphere and Planetary
Interactions (WHPI; https://whpi.hao.ucar.edu) initiative, which aims to
coordinate observations and modelling of the solar–heliospheric–planetary
system during solar minimum, or the planned coordinated campaigns between
Parker Solar Probe and Solar Orbiter (Velli et al., 2020), are the needed step
forward towards a better understanding of these complex processes.

Finally, this work showed how the interplanetary impact of the SEPs associated
with the 2012 May 17 eruptive flare was tremendously influenced by the
presence of the preceding May 11 ICME in the inner heliosphere. The IMF
connectivity to the flare/shock acceleration site is a critical component for
estimating the severity and potential impact of impulsive SEPs. While these
events tend to be relatively narrow in longitudinal extent, we have shown the
2012 May 17 event was seen nearly simultaneously at five separate locations
(three of which were planets) separated by up to ${\sim}150^{\circ}$ in
heliographic longitude from the flaring region. Therefore, an instantaneous
extrapolation of the magnetic field configuration of the flare/eruption site
may not be sufficient to estimate the actual spatial extent of impulsive SEP
events—the temporal history of the coronal and heliospheric field structure
and evolution can also play an important role in the preconditioning of the
IMF necessary for extreme events.

## Sources of Data

The HELCATS catalogues are available at https://www.helcats-fp7.eu. Images and
additional information on the 2012 May 12 CME are available at
https://www.helcats-fp7.eu/catalogues/event_page.html?id=HCME_A__20120511_01
(STEREO-A viewpoint) and https://www.helcats-
fp7.eu/catalogues/event_page.html?id=HCME_B__20120512_01 (STEREO-B viewpoint).
The WSA–Enlil+Cone simulation results have been provided by the Community
Coordinated Modeling Center (CCMC) at NASA Goddard Space Flight Center through
their public Runs on Request system (http://ccmc.gsfc.nasa.gov). The full
simulation results are available at
https://ccmc.gsfc.nasa.gov/database_SH/Erika_Palmerio_093020_SH_1.php. The
Richardson & Cane ICME list is available at
http://www.srl.caltech.edu/ACE/ASC/DATA/level3/icmetable2.htm, whilst the
NASA–Wind ICME list can be found at https://wind.nasa.gov/ICMEindex.php. Solar
disc and coronagraph data from SDO, SOHO, and STEREO are openly available at
the Virtual Solar Observatory (VSO; https://sdac.virtualsolar.org/). These
data were processed and analysed through SunPy (SunPy Community et al., 2015,
2020), IDL SolarSoft (Bentely and Freeland, 1998), and the ESA JHelioviewer
software (Müller et al., 2017). Level-2 processed STEREO/HI data were obtained
from the UK Solar System Data Centre (UKSSDC;
https://www.ukssdc.ac.uk/solar/stereo/data.html). GOES/XRS data were retrieved
from https://sohoftp.nascom.nasa.gov. VEX and MEX data are openly available at
ESA’s Planetary Science Archive (https://archives.esac.esa.int/psa). These
data were processed and analysed with the aid of the irfpy library
(https://irfpy.irf.se/irfpy/index.html). Wind data are publicly available at
NASA’s Coordinated Data Analysis Web (CDAWeb) database
(https://cdaweb.sci.gsfc.nasa.gov/index.html/). Energetic particle data from
GOES can be accessed at https://www.ngdc.noaa.gov/stp/satellite/goes/. NMDB
data are publicly available at http://www.nmdb.eu and Dst data can be found at
http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html. Spitzer data are available at the
NASA/IPAC Infrared Science Archive (https://irsa.ipac.caltech.edu/). MSL data
are openly available at the Planetary Plasma Interactions (PPI) Node of NASA’s
Planetary Data System (PDS), accessible at https://pds-ppi.igpp.ucla.edu. MOdy
and MESSENGER data are available at the Geosciences Node of the PDS,
accessible at https://pds-geosciences.wustl.edu/. STEREO/HET data were
accessed at http://www.srl.caltech.edu/STEREO/Public/HET_public.html. The
STEREO ICME list can be found at https://stereo-
ssc.nascom.nasa.gov/pub/ins_data/impact/level3/STEREO_Level3_ICME.pdf.

###### Acknowledgements.

E. P. acknowledges the Doctoral Programme in Particle Physics and Universe
Sciences (PAPU) at the University of Helsinki, the Emil Aaltonen Foundation,
and the NASA Living With a Star Jack Eddy Postdoctoral Fellowship Program,
administered by UCAR’s Cooperative Programs for the Advancement of Earth
System Science (CPAESS) under award no. NNX16AK22G. E. K. acknowledges the
SolMAG project (ERC-COG 724391) funded by the European Research Council (ERC)
in the framework of the Horizon 2020 Research and Innovation Programme,
Academy of Finland project SMASH 310445, and the Finnish Centre of Excellence
in Research of Sustainable Space (Academy of Finland grant no. 312390). B.
S.-C. acknowledges support through UK-STFC grant ST/S000429/1. A. W. and C. M.
thank the Austrian Science Fund (FWF): P31521-N27. M. M, A. Z., and L. R.
thank the European Space Agency (ESA) and the Belgian Federal Science Policy
Office (BELSPO) for their support in the framework of the PRODEX Programme. J.
G. thanks the Strategic Priority Program of the Chinese Academy of Sciences
(grant no. XDB41000000 and XDA15017300), and the CNSA pre-research Project on
Civil Aerospace Technologies (grant no. D020104). The work of L. T. is
supported by the Academy of Finland (grant no. 322544). We thank two anonymous
reviewers, whose comments and suggestions have significantly improved this
article. We acknowledge support from the European Union FP7-SPACE-2013-1
programme for the HELCATS project (grant no. 606692). The HI instruments on
STEREO were developed by a consortium that comprised the Rutherford Appleton
Laboratory (UK), the University of Birmingham (UK), Centre Spatial de Liège
(CSL, Belgium) and the Naval Research Laboratory (NRL, USA). The STEREO/SECCHI
project, of which HI is a part, is an international consortium led by NRL. We
recognise the support of the UK Space Agency for funding STEREO/HI operations
in the UK. The WSA model was developed by C. N. Arge (currently at NASA/GSFC),
and the Enlil model was developed by D. Odstrcil (currently at GMU). We thank
the model developers, M. L. Mays, R. Colaninno, and the CCMC staff. We
acknowledge the NMDB, founded under the European Union’s FP7 programme
(contract no. 213007), for providing neutron monitor data. We thank the WDC
for Geomagnetism, Kyoto, and the geomagnetic observatories for their
cooperation to make the final Dst indices available. This work is based (in
part) on archival data obtained with the Spitzer Space Telescope, which was
operated by the Jet Propulsion Laboratory, California Institute of Technology,
under a contract with NASA. Support for this work was provided by an award
issued by JPL/Caltech. Finally, we thank the instrument teams of all the
spacecraft involved in this study.

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