# CME Evolution in the Structured Heliosphere and Effects at Earth and Mars
During Solar Minimum

###### Abstract

The activity of the Sun alternates between a solar minimum and a solar
maximum, the former corresponding to a period of “quieter” status of the
heliosphere. During solar minimum, it is in principle more straightforward to
follow eruptive events and solar wind structures from their birth at the Sun
throughout their interplanetary journey. In this paper, we report analysis of
the origin, evolution, and heliospheric impact of a series of solar transient
events that took place during the second half of August 2018, i.e. in the
midst of the late declining phase of Solar Cycle 24. In particular, we focus
on two successive coronal mass ejections (CMEs) and a following high-speed
stream (HSS) on their way towards Earth and Mars. We find that the first CME
impacted both planets, whilst the second caused a strong magnetic storm at
Earth and went on to miss Mars, which nevertheless experienced space weather
effects from the stream interacting region (SIR) preceding the HSS. Analysis
of remote-sensing and in-situ data supported by heliospheric modelling
suggests that CME–HSS interaction resulted in the second CME rotating and
deflecting in interplanetary space, highlighting that accurately reproducing
the ambient solar wind is crucial even during “simpler” solar minimum periods.
Lastly, we discuss the upstream solar wind conditions and transient structures
responsible for driving space weather effects at Earth and Mars.

Space Weather

Predictive Science Inc., San Diego, CA, USA Space Sciences Laboratory,
University of California–Berkeley, Berkeley, CA, USA CPAESS, University
Corporation for Atmospheric Research, Boulder, CO, USA Department of
Astronomy, University of Maryland, College Park, MD, USA Heliospheric Physics
Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA nextSource
Inc., New York, NY, USA Solar System Exploration Division, NASA Goddard Space
Flight Center, Greenbelt, MD, USA Lockheed Martin Solar and Astrophysics
Laboratory, Palo Alto, CA, USA Department of Physics and Astronomy, University
of Iowa, Iowa City, IA, USA Leidos Innovations Corp., Houston, TX, USA Swedish
Institute of Space Physics, Kiruna, Sweden Space Sciences Department, The
Aerospace Corporation, Los Angeles, CA, USA Department of Earth Sciences,
Millersville University, Millersville, PA, USA

E<EMAIL_ADDRESS>
We analyse the eruption and propagation of two CMEs from the Sun up to Earth
and Mars during August 2018

Both CMEs were observed at Earth, but the second largely missed Mars, possibly
due to interaction with a following high-speed solar wind stream

The sequence of events observed resulted in a strong magnetic storm at Earth,
but only moderate disturbances at Mars

The Sun is characterised by a 11-year periodicity of its levels of activity,
resulting in a solar minimum and a solar maximum alternating approximately
every 5.5 years. During solar minimum, the Sun and its whole environment are
in their simplest configuration, and eruptive events are significantly less
frequent. It follows that periods of lower activity are generally considered
optimal for tracking solar phenomena from their origin at the Sun throughout
their journey in interplanetary space. In this paper, we analyse a series of
solar eruptions that took place during the second half of August 2018 and
follow them until their arrival at Earth and Mars, taking into account their
associated effects on the two planets. We find that, even during solar
minimum, the large-scale structure of the solar and interplanetary environment
can have more or less dramatic impacts on the evolution of eruptions as they
travel away from the Sun. Additionally, we suggest that the same event can
cause diverse levels of disturbances at different planets, depending on the
particular structure and properties of the impacting solar wind.

##  1 Introduction

The solar activity is characterised by numerous short- and long-term
periodicities, the most renowed of which is the 11-year solar magnetic
activity cycle (Hathaway, 2015). Over the duration of a full cycle, the number
of sunspots as well as the fraction of solar surface covered by them rise
until reaching a maximum and then fall again. This trend is also followed by
the occurrence of solar eruptions, including flares (e.g., Benz, 2017) and
coronal mass ejections (CMES; e.g., Webb and Howard, 2012), which tend to peak
close to solar maximum and decrease drastically around solar minimum. Due to
the general lack of active regions, CMEs during solar minimum tend to be of
the slow, streamer-blowout kind (e.g., Vourlidas and Webb, 2018), although
some major active-region eruptions may still occur (e.g., Nitta, 2011). Solar
minimum periods are characterised by a simpler configuration of the solar
magnetic field, a generally slower and less variable solar wind, and a less
energised space environment (Riley et al., 2001, 2022). These aspects make
minima excellent times for tracing solar phenomena “from start to finish” and
for defining the baseline heliophysical system, giving rise to large,
coordinated initiatives such as the Whole Sun Month (WSM; Galvin and Kohl,
1999), which took place during the cycle 22–23 minimum, and the Whole
Heliosphere Interval (WHI; Thompson et al., 2011), which took place during the
cycle 23–24 minimum. CME occurrence usually drops to about one per week,
meaning that CME–CME interactions tend to be significantly less likely and
that it is possible to follow the evolution of single CMEs and their
interaction with solar wind structures, including the slow wind, high-speed
streams (HSSs), and slow–fast stream interaction regions (SIRs; e.g.,
Richardson, 2018).

During the cycle 24–25 minimum, the availability of space- and ground-based
assets at various planets or scattered throughout the heliosphere, together
with the growing consensus about the importance of multi-point studies within
the heliosphere as a whole, have led to the establishment of an even more
comprehensive follow-up effort to WSM and WHI, i.e. the Whole Heliosphere and
Planetary Interactions (WHPI; https://whpi.hao.ucar.edu/) initiative, which
aims to study the interconnected solar–heliospheric–planetary system. In
particular, a relatively large fleet of instruments was and is still
operational at Mars, providing us with the opportunity to follow solar
transients from the Sun to Earth and/or Mars and to characterise their space
weather response at the two planets. Occasionally, the same transient may even
encounter both Earth and Mars and elicit dissimilar impacts at each planet.
Earth is characterised by a strong intrinsic quasi-dipolar magnetic field that
is able to sustain a full-fledged magnetosphere (e.g., Pulkkinen, 2007). Since
Earth’s field is roughly directed towards the North in the equatorial plane at
the magnetopause, the most geoeffective solar wind structures are those
containing southward magnetic field (Zhang et al., 2007) as well as high speed
and ram pressure (Gonzalez et al., 1994). Mars, on the other hand, lacks a
global magnetic dipole and interaction between the solar wind and the Martian
ionosphere generates a so-called induced magnetosphere (e.g., Bertucci et al.,
2011), which however differs from a Venus- or comet-like one due to the
presence of localised crustal magnetic fields (Acuna et al., 1998), thus
leading to a “hybrid” magnetosphere (e.g., DiBraccio et al., 2018). As a
result, the main parameters that regulate the arieffectiveness—from the Greek
name for Mars, ${{}^{\prime}}\\!\\!A\rho\eta\varsigma$ or Áris—of a solar wind
transient are the dynamic pressure (Opgenoorth et al., 2013), usually enhanced
in CMEs and SIRs, and the topology of the interplanetary magnetic field
(Jakosky et al., 2015a).

Despite the recent increasing interest in space weather at Mars (e.g., Geyer
et al., 2021; Huang et al., 2021; Lee et al., 2017; Luhmann et al., 2017; Zhao
et al., 2021), most studies have focused on the analysis of the consequences
of large and clear solar events (e.g., Crider et al., 2005; Jakosky et al.,
2015a; Lee et al., 2018), whilst less is known about minor events observed
during periods of low solar activity (e.g., Kajdič et al., 2021; Sánchez-Cano
et al., 2017). In order to tackle this issue, we present in this article a
detailed analysis of a sequence of solar transients during the second half of
August 2018 (i.e. during the late declining phase of Solar Cycle 24) between
Earth and Mars, which were separated by ${\sim}8^{\circ}$ in longitude,
${\sim}2^{\circ}$ in latitude, and ${\sim}0.4$ AU in radial distance. Some
aspects of these events were addressed by a number of studies (e.g., Abunin et
al., 2020; Akala et al., 2021; Chen et al., 2019; Cherniak and Zakharenkova,
2022; Gopalswamy et al., 2022; Mishra and Srivastava, 2019; Moro et al., 2022;
Nitta et al., 2021; Piersanti et al., 2020; Thampi et al., 2021; Younas et
al., 2020; Zhang et al., 2020), and our goal here is to provide a holistic
Sun-to-Mars investigation of their evolution from the Sun through the inner
heliosphere and their effects at the two planets. Accordingly, we first
present remote-sensing observations of the eruptive events. Then, we summarise
the heliospheric context necessary to interpret in-situ observations at Earth
and Mars. Then, we show solar wind and interplanetary magnetic field
measurements at the two planets, with particular attention to the space
weather responses to the interplanetary disturbances. Finally, we discuss
these observations within the larger context of terrestrial and martian space
weather during solar minimum periods.

##  2 Remote-Sensing Observations

The 20 August 2018 eruptive events were observed in remote-sensing imagery
from two vantage points, namely Earth and the Solar Terrestrial Relations
Observatory Ahead (STEREO-A; Kaiser et al., 2008) spacecraft, located
${\sim}110^{\circ}$ east of Earth close to 1 AU. Here, we provide an overview
of the sequence of events from their origin at the Sun (Section 2.1) through
their evolution across the solar corona (Section 2.2) and inner heliosphere
(Section 2.3).

###  2.1 Solar Disc

The sequence of eruptive events analysed in this study commenced on 20 August
2018 from an extended quiet-Sun region in the northern hemisphere located
close to the central meridian from Earth’s perspective. Figure 1(a) shows the
pre-eruptive configuration in extreme ultra-violet (EUV) at 193 Å as seen by
the Atmospheric Imaging Assembly (AIA; Lemen et al., 2012) onboard the Solar
Dynamics Observatory (SDO; Pesnell et al., 2012) in orbit around Earth. The
most striking features that were present on the Earth-facing Sun are an
extended filament channel (marked as ‘F’) and two coronal holes (CHs), a large
one to the southwest of the filament (marked as ‘CH1’) and a smaller one to
the northeast (marked as ‘CH2’). Additionally, we note an S-shaped feature to
the north of the filament (marked as ‘S’), reminiscent of a sigmoid (e.g.,
Green et al., 2007)—this structure is visible in several SDO/AIA channels,
including 94 Å and 131 Å, which are known to respond to hotter plasma (O’Dwyer
et al., 2010).

Figure 1: Overview of the 2018 August 20 eruptions from remote-sensing solar
disc imagery. (a) Pre-eruptive configuration on the solar disc. The erupting
S-shaped structure (‘S’) and filament (‘F’), as well as the two nearby coronal
holes (‘CH1’ and ‘CH2’) are labelled. (b) and (d) Zoomed-in images of the
erupting sigmoid (CME1) and filament (CME2), respectively. (c) and (e) Base-
difference images of the sigmoid and filament eruptions, respectively, with
magnetogram data saturated to ${\pm}100$ G overlaid (red = positive, blue =
negative). The approximate eruption footpoints are indicated with green
circles in both panels.

Both structures (i.e., the S-shaped feature and the filament) began erupting
essentially at the same time, around 07:30 UT on 20 August, possibly in a
sympathetic fashion (e.g., Török et al., 2011; Lynch and Edmondson, 2013).
Whilst the S-shaped structure (Figure 1(b)) left the Sun rather rapidly (no
signatures of the event were visible after a few hours), the filament (Figure
1(d)) was involved in a much slower eruption, with post-eruption arcades still
developing many hours after the onset and into the following day. Therefore,
in this work we will define the S-shaped feature eruption as CME1, and the
filament eruption as CME2. Around 18:30 UT on 20 August, a small portion of
filament lying between the source regions of CME1 and CME2 erupted in a jet-
like fashion (see Mishra and Srivastava, 2019, for details)—this may be
considered as a “second part” of a two-step filament eruption, as suggested by
Abunin et al. (2020). The full sequence of events observed by SDO is shown in
EUV at 211 Å in Movie S1.

Analysis of the pre-eruptive magnetic configuration of the two CMEs (see
Palmerio et al., 2017, and references therein) suggests that CME1 was
characterised by right-handed chirality (note its forward-S shape), whilst
CME2 was left-handed (note its reverse S-shape). The approximate eruption
footpoints (estimated via the locations of coronal dimmings; e.g., Reinard and
Biesecker, 2009; Thompson et al., 2000) and the magnetic field polarities in
which they are rooted are shown in Figure 1(c) for CME1 and Figure 1(e) for
CME2. Given that CME2 originated from a decayed active region, it is evident
that its northeastern (southwestern) footpoint is rooted in a patch of
positive (negative) polarity. Thus, its flux rope magnetic configuration upon
eruption should be west–south–east (WSE), following the convention of Bothmer
and Schwenn (1998) and Mulligan et al. (1998). CME1, on the other hand,
originated from a region of much weaker, quiet-Sun magnetic field, making
determination of its magnetic structure and orientation more difficult.
Nevertheless, the easternmost footpoint seems to be rooted in a patch of
positive polarity, which for a right-handed flux rope would result somewhere
between a west–north–east (WNE) and a south–west–north (SWN) type.

###  2.2 Solar Corona

At the time of the 20 August 2018 eruptive events, white-light imagery of the
solar corona was available from the Large Angle and Spectrometric Coronagraph
(LASCO; Brueckner et al., 1995) onboard the Solar and Heliospheric Observatory
(SOHO; Domingo et al., 1995), located at Earth’s Lagrange L1 point, as well as
the coronagraphs forming part of the Sun Earth Connection Coronal and
Heliospheric Investigation (SECCHI; Howard et al., 2008) suite onboard
STEREO-A. Given that both CME1 and CME2 originated close to the central
meridian as seen from Earth, we expect SOHO to have observed the eruptions
approximately along their propagation direction, whilst STEREO-A had a near-
quadrature view of the events. Such observations are summarised in Figure 2,
and the full sequence of imagery from STEREO/SECCHI/COR2-A and SOHO/LASCO/C2
is provided in Movie S2 and Movie S3, respectively.

Figure 2: Overview of the 2018 August 20 eruptions from remote-sensing
coronagraph imagery (shown here in running difference with $\Delta t=1$ hr).
The first appearances of the preceding outflow, CME1, and CME2 in the field of
view each observing instrument are indicated in the respective panels. The
rightmost column shows coronagraph images with the Graduated Cylindrical Shell
wireframes overlaid, showing the reconstruction of CME1 in panels (d) and (h)
and CME2 in panels (l) and (p).

First of all, we note the presence of coronal outflows preceding the
eruption(s), indicated in Figure 2(a,f) and possibly associated with blobs
originating from the cusp of the helmet streamer belt (e.g., Lynch, 2020; Wang
et al., 2000). CME1 first appeared in the COR2-A field of view around 10:24 UT
on 20 August (Figure 2(a–c)), featuring a rather irregular morphology (it
could be classified as a ‘jet’ according to the definition of Vourlidas et
al., 2013, 2017). Furthermore, it appeared very faintly in STEREO-A imagery
(and is more visible in Movie S2 than in the still images shown in Figure
2(a–c)), likely partially due to the presence of the preceding outflow along
the line of sight, and was not clearly discernible in SOHO data (Figure
2(e–g)). We note that CME events in the solar corona that appear faint/jet-
like from one viewpoint and are not visible at all in another have been
reported in previous studies, and may cause moderate geomagnetic disturbances
(e.g., Palmerio et al., 2019).

CME2 first appeared in the COR2-A field of view around 13:24 UT on 20 August
(Figure 2(i–k)) and in the C2 field of view around 21:48 UT on the same day
(Figure 2(m–o)). This event was significantly more evident than CME1 in images
from both perspectives, with a flux rope-like morphology observed by STEREO-A
and a full halo (albeit very faint) detected by SOHO. In particular, COR2-A
imagery reveals an initially asymmetric structure, with its southern leg ahead
of the northern one, that slowly swells before finally accelerating away from
the Sun in a streamer-blowout fashion (e.g., Vourlidas and Webb, 2018). As the
CME travelled through the solar corona, its front became progressively less
asymmetric, possibly indicating that either the northern leg caught up with
the southern one, or that the whole structure rotated during its early
propagation.

In order to estimate the geometric and kinematic properties of CME1 and CME2
through the solar corona, we fit both eruptions as they appeared in
coronagraph imagery with the Graduated Cylindrical Shell (GCS; Thernisien et
al., 2006, 2009) model. This will also serve to determine the CME input
parameters needed for heliospheric modelling of their propagation through the
structured solar wind (see Section 3). The GCS model consists of a
parametrised shell (with six free parameters) intended to reproduce the flux
rope morphology of CMEs, which can be applied to one or more nearly-
simultaneous images and visually adjusted until its projection onto each field
of view best matches the observations. Examples of GCS results are shown in
the rightmost column of Figure 2, for both CME1 (Figure 2(d,h)) and CME2
(Figure 2(l,p)). Both eruptions are estimated to have a low inclination to the
solar equatorial plane in the outer solar corona, and the CME2 results
indicate that it is larger (extending ${\sim}60^{\circ}$ versus
${\sim}40^{\circ}$ along the axis) but slower (${\sim}300$ km$\cdot$s-1 versus
${\sim}500$ km$\cdot$s-1) than CME1. We remark, however, that the GCS model is
applied to single-point (STEREO-A) measurements for CME1 (Figure 2(h) simply
shows the projection of the shell in Figure 2(d) onto the C2 field of view),
inevitably resulting in larger uncertainties. Considering the flux rope types
estimated at the Sun (see Section 2.1) and the low inclination of both
eruptions through the solar corona, CME1 is expected to have an SWN
configuration, whilst CME2 would reach a north–west–south (NWS) orientation
via a counterclockwise rotation of its axis (as expected for left-handed flux
ropes, e.g., Green et al., 2007; Lynch et al., 2009).

###  2.3 Inner Heliosphere

After leaving the COR2-A field of view, the eruptions under analysis were
observed by the Heliospheric Imager (HI; Eyles et al., 2009) cameras onboard
STEREO-A. Figure 3 shows an overview of the observations of CME1 and CME2
through the STEREO/SECCHI/HI1-A camera, whilst the complete set of images is
provided in Movie S4.

Figure 3: Overview of the 2018 August 20 eruptions from remote-sensing inner
heliospheric imagery (shown here in running difference with $\Delta t=40$
min). The locations of the preceding outflow, CME1, and CME2 are indicated in
each panel, where appropriate. The vertical band visible throughout the image
sequence is caused by Mercury.

CME1 first appeared in the HI1-A field of view at 15:29 UT on 20 August,
whilst CME2 emerged at 06:09 UT on 21 August. CME1 maintained a rather
irregular morphology, similar to that observed in COR2-A imagery (see Figure
2(a–c) and Movie S2). CME2, on the other hand, displayed a significantly
flatter front than the one featured in COR2-A observations (see Figure 2(e–g)
and Movie S2), likely due to pancaking, i.e. flattening of the CME cross-
section as it propagates through a latitudinally-structured ambient wind
(e.g., Owens, 2006; Riley and Crooker, 2004). Furthermore, the front of CME1
appeared well ahead of CME2 throughout the sequence of frames in which both
eruptions are visible, hence we do not observe signatures of CME–CME
interaction in HI1-A imagery.

##  3 Heliospheric Context

In order to explore the propagation of CME1 and CME2 within their
corresponding inner heliospheric context, we employ the Wang–Sheeley–Arge
(WSA; Arge et al., 2004) coronal model coupled with the Enlil (Odstrcil, 2003)
heliospheric model, hereafter WSA–Enlil. WSA is used to generate the solar
wind background from synoptic magnetogram maps (in this case, from the Global
Oscillation Network Group or GONG; Harvey et al., 1996), which Enlil takes as
input at its inner boundary (set at $21.5\,R_{\odot}$ or 0.1 AU). Enlil then
models the heliospheric conditions outwards to an outer boundary (in this case
set at 2 AU) by solving the magnetohydrodynamic (MHD) equations. CMEs are
inserted in WSA–Enlil at the heliospheric inner boundary as hydrodynamic
structures, i.e. lacking an internal magnetic field. We derive the input
parameters for CME1 and CME2 (shown in Table 1) entirely from the GCS
reconstructions outlined in Section 2.2 and Figure 2. The latitudes
($\theta$), longitudes ($\phi$), and tilts ($\gamma$) are taken directly from
their values at the last GCS reconstructions before the CMEs left the COR2-A
field of view (at the times shown in the rightmost column of Figure 2). Both
CMEs are inserted with an elliptical cross-section, and their semi-major
($\psi_{1}$) and semi-minor ($\psi_{2}$) angular extents are obtained by
“cutting a slice” out of the GCS shell (see Thernisien, 2011, for details).
Finally, the CME insertion speeds ($V_{0}$) are calculated from the CME apex
height at the time of the last GCS reconstruction (again, shown in the
rightmost column of Figure 2) and the height obtained at the reconstruction
performed one hour earlier, and the insertion times ($t_{0}$) are estimated by
propagating the “final” CME apex until $21.5\,R_{\odot}$ assuming constant
speed $V_{0}$.

Table 1: CME Input Parameters for the WSA–Enlil Simulation Run.

CME | $t_{0}$ | $\theta$ | $\phi$ | $\gamma$ | $\psi_{1}$ | $\psi_{2}$ | $V_{0}$
---|---|---|---|---|---|---|---
# | [UT] | [∘] | [∘] | [∘] | [∘] | [∘] | [km$\cdot$s-1]
1 | 2018-08-20T18:16 | $12$ | $2$ | $10$ | $21$ | $12$ | $483$
2 | 2018-08-21T10:56 | $5$ | $10$ | $9$ | $30$ | $16$ | $290$

Note. The table shows, from left to right: CME number, time ($t_{0}$) of
insertion of the CME at the Enlil inner boundary of $21.5\,R_{\odot}$ or 0.1
AU, latitude ($\theta$) and longitude ($\phi$) of the CME apex in Stonyhurst
coordinates, tilt ($\gamma$) of the CME axis with respect to the solar equator
(defined positive for counterclockwise rotations), semi-major ($\psi_{1}$) and
semi-minor ($\psi_{2}$) axes of the CME cross-section, and CME speed at
$21.5\,R_{\odot}$ ($V_{0}$).

An overview of the WSA–Enlil simulation results is shown in Figure 4 and a
full animation is shown in Movie S5. The top panels show snapshots of the
solar wind radial speed ($V_{r}$) on the ecliptic plane, from which it is
evident that both CME1 and CME2 are “sandwiched” between two fast streams
marked as ‘HSS1’ and ‘HSS2’, which we attribute to CH1 and CH2, respectively
(see Section 2.1 and Figure 1). ‘ICME1’ and ‘ICME2’ refer to the
interplanetary CMEs (ICMEs; e.g., Kilpua et al., 2017) counterparts of CME1
and CME2, respectively. Note that here we refer to ICME as the interplanetary
structure as a whole, often composed of a shock, a sheath, and an ejecta.
Hence, the ICME arrival time considered here corresponds to the interplanetary
shock arrival. By complementing the overall simulation results on the ecliptic
plane with the synthetic solar wind speed ($V$) measurements at Earth and Mars
shown in the bottom panels of Figure 4, it is clear that ICME1 and ICME2 would
be expected to impact Earth as successive, largely separate structures, whilst
the two would have merged by the time they reach Mars. In the context of the
events considered here, there are two possible reasons as to why the slower
CME2 would catch up with the initially faster CME1: (1) Due to solar wind
preconditioning, resulting in a rarefied ambient medium after the passage of
CME1 and, thus, diminished drag (e.g., Temmer and Nitta, 2015), as well as (2)
due to the presence of HSS2 trailing CME2, resulting in less deceleration or
even acceleration (e.g., Winslow et al., 2021). We approximately estimate
ICME1 to hit Earth on 2018-08-24T02:25, ICME2 to hit Earth on
2018-08-24T20:45, and the combined ICME1+ICME2 structure to hit Mars on
2018-08-25T09:45.

Figure 4: Overview of the WSA–Enlil simulation results at Earth and Mars. Top:
Snapshots of the simulation results for the radial speed $V_{r}$ on the
ecliptic plane around the arrival times at (left) Earth and (right) Mars.
Bottom: Results for the solar wind speed $V$ at Earth and Mars.

These evaluations and estimates will be used as support to interpret the in-
situ measurements at both Earth and Mars, presented in Section 4.
Nevertheless, there are a couple of caveats to consider when analysing these
results. For example, the lack of an internal magnetic field in the modelled
CMEs inevitably results in unrealistic CME–CME interaction outcomes, since the
resulting structure simply corresponds to the superposition of two
hydrodynamic pulses. In fact, the ICME ejecta boundaries identified in the
simulation (black contours in Figure 4) are estimated in the model via the so-
called ‘cloud tracer’ parameter, which tracks the injected mass based on the
density enhancement with respect to the background solar wind, i.e. they can
be considered merely an approximation of the spatial extent of a fully-
magnetised ejecta.

##  4 In-Situ Observations

In this section, we show and analyse in-situ observations following the solar
events described in Section 2 at both Earth (Section 4.1) and Mars (Section
4.2). Then, in Section 5, these measurements will be synthesised and discussed
in relation to the heliospheric context presented in Section 3.

###  4.1 Measurements at Earth

In-situ measurements at Earth are shown in Figure 5. They include: magnetic
field data from the Magnetic Field Investigation (MFI; Lepping et al., 1995),
plasma data from the Solar Wind Experiment (SWE; Ogilvie et al., 1995), and
electron pitch angle distribution (PAD) data from the Three-Dimensional Plasma
and Energetic Particle Investigation (3DP; Lin et al., 1995) instruments
onboard the Wind (Ogilvie and Desch, 1997) spacecraft at the Sun–Earth L1
point; suprathermal and energetic ion data from the Electron, Proton, and
Alpha Monitor (EPAM; Gold et al., 1998) onboard the Advanced Composition
Explorer (ACE; Stone et al., 1998) also at the Sun–Earth L1 point; space-based
galactic cosmic ray (GCR) estimates from the Cosmic Ray Telescope for the
Effects of Radiation (CRaTER; Spence et al., 2010) onboard the Lunar
Reconnaissance Orbiter (LRO; Chin et al., 2007) orbiting Luna; ground-based
GCR estimates from the Thule, Nain, and South Pole stations part of the
Neutron Monitor Database (NMDB; Mavromichalaki et al., 2011); as well as Kp
index from the National Geophysical Data Center (NGDC) and Dst index from the
World Data Center (WDC) for Geomagnetism, Kyoto.

Figure 5: In-situ measurements at Earth, showing (a) magnetic field magnitude,
(b) magnetic field components in GSE coordinates, (c) $\theta$ and (d) $\phi$
components of the magnetic field, solar wind (e) speed, (f) density, (g)
dynamic pressure, and (h) temperature, (i) plasma $\beta$, (j) electron pitch
angle distribution, (k) ion intensities, galactic cosmic ray variation (l) in
space and (m) on ground, and (n) Kp and Dst indices, quantifying
geoeffectiveness. The ejecta intervals show flux rope fitting results with the
Elliptic-Cylindrical model superposed on the magnetic field data.

The sequence of events observed at Earth commenced with a weak (i.e.,
characterised by small downstream-to-upstream ratios in speed, magnetic field
magnitude, and plasma density) interplanetary shock (solid pink line in Figure
5) followed by a weak flux-rope-like ejecta (shaded pink region in Figure 5),
which we attribute to ICME1, i.e. the interplanetary counterpart of CME1. We
fit the ejecta with the Elliptic-Cylindrical (EC; Nieves-Chinchilla et al.,
2018) analytical flux rope model (shown in blue superposed on the Wind/MFI
data in Figure 5), resulting in a moderately inclined structure with axis
orientation ($\Theta$, $\Phi$) = ($36^{\circ}$, $230^{\circ}$) in Geocentric
Solar Ecliptic (GSE) coordinates and right-handed chirality, which roughly
corresponds to a SWN-type flux rope. About two-thirds into the ICME1 flux
rope, a second weak interplanetary shock (solid grey line in Figure 5) was
detected, displaying classic shock-in-ejecta signatures (e.g., Lugaz et al.,
2015): Proton temperature and plasma beta maintained lower-than-ambient values
(typical ICME ejecta indicators; e.g., Zurbuchen and Richardson, 2006) and all
the magnetic field components did not display changes in the clock angle after
the shock passage. We attribute this shock to ICME2, i.e. the interplanetary
counterpart of CME2, suggesting that the two eruptions were at the initial
stages of their CME–CME interaction process at the time they travelled past
Earth. We note no significant particle enhancements in ACE/EPAM data, no
Forbush decreases (e.g., Forbush, 1937) in the GCR intensity, and no major
geomagnetic effects associated with ICME1 in terms of the Kp and Dst indices.
Nevertheless, although the relatively low values of speed, dynamic pressure,
and magnetic field did not result in a geomagnetic storm, some substorm
activity was observed, with the AE index peaking at ${\gtrsim}700$ nT (not
shown), likely in response to the period of weakly negative $B_{Z}$ in the
leading portion of the ICME ejecta.

After a brief period of high density and dynamic pressure measured by Wind/SWE
as well as a rather weak but turbulent magnetic field measured by Wind/MFI,
which has been interpreted as a signature of interaction between two
successive CMEs (Lugaz et al., 2005, 2017; Wang et al., 2003) and evidently
part of the ICME2 sheath, a clear flux-rope ejecta (shaded grey region in
Figure 5) was detected. Fitting results with the EC model (shown in blue below
the Wind/MFI data in Figure 5) yield a left-handed flux rope with a rather
high inclination and central axis orientation ($\Theta$, $\Phi$) =
($-70^{\circ}$, $145^{\circ}$) in GSE coordinates, corresponding roughly to a
WSE-type rope. Contrary to ICME1, ICME2 featured clear counterstreaming
electron signatured in Wind/3DP data and was accompanied by a weak (${\sim}2$%
drop) Forbush decrease detected both in space by LRO and on ground by
different neutron monitors, albeit with slightly different profiles—the
LRO/CRaTER data are closest to the profile measured on ground at the Thule
station. Most strikingly, ICME2 was particularly geoeffective, with a maximum
Kp of 7+ (corresponding to a G3 storm, see https://www.swpc.noaa.gov/noaa-
scales-explanation) and a minimum Dst of $-175$ nT (well exceeding the usual
threshold for “strong” storms of Dst${}_{\mathrm{min}}\leq-100$ nT; e.g.,
Zhang et al., 2007), while the AE index peaked at ${\gtrsim}2200$ nT (not
shown). These factors make ICME2 the driver of a so-called “problem
geomagnetic storm” (Nitta et al., 2021), not because its source was stealthy
or elusive (see Nitta and Mulligan, 2017, for a discussion on CMEs without
appreciable low-coronal signatures on the Sun), but because its effects at
Earth were largely unexpected given the slow and “unimpressive” nature of CME2
at the Sun. In fact, the solar wind speed remained around values of
${\sim}400$ km$\cdot$s-1 throughout the ICME passage, indicating that the
storm was mostly driven by the sustained southward $B_{Z}$ magnetic field
component within the ejecta, which peaked at $-16$ nT. Other factors
contributing to the intensification of the geomagnetic storm may have been the
interaction with the preceding ICME1, leading to compression at the front of
the ejecta, as well as the presence of a nearby CH (in this case CH2, see
Figure 1) back at the Sun, resulting in CME–HSS interaction in interplanetary
space and compression in the trailing part of the ejecta (e.g., Nitta et al.,
2021).

Indeed, the ICME2 ejecta was immediately followed by a small SIR (shaded green
region in Figure 5) and related HSS, most likely due to CH2 and thus indicated
in Figure 5 as HSS2. That ICME2 and HSS2 were in the process of interacting is
evident by the increasing solar wind speed and temperature detected by
Wind/SWE, as well as the particle enhancement at suprathermal energies
measured by ACE/EPAM in the trailing part of the ejecta. By the time Earth was
impacted by HSS2, the intense geomagnetic storm had essentially waned, with
the Kp index briefly reaching again values up to $6{-}$ almost a full day
later. Nevertheless, a sequence of substorms persisted through the passage of
the SIR and the initial portion of the following HSS2, with the AE index
peaking between ${\sim}1000$ and ${\sim}1500$ nT for each event (not shown).

###  4.2 Measurements at Mars

In-situ measurements at Mars are shown in Figure 6. They include: magnetic
field data from the Magnetometer (MAG; Connerney et al., 2015), plasma data
from the Solar Wind Ion Analyzer (SWIA; Halekas et al., 2015), electron PAD
data from the Solar Wind Electron Analyzer (SWEA; Mitchell et al., 2016), and
suprathermal/energetic ion data as well as space-based GCR estimates from the
Solar Energetic Particle (SEP; Larson et al., 2015) instruments onboard the
Mars Atmosphere and Volatile Evolution (MAVEN; Jakosky et al., 2015b)
spacecraft orbiting Mars; plasma data from the Analyzer of Space Plasmas and
Energetic Atoms (ASPERA-3; Barabash et al., 2006) onboard Mars Express (MEX;
Chicarro et al., 2004) also orbiting Mars; ground-based GCR estimates from the
Radiation Assessment Detector (RAD; Hassler et al., 2012) onboard the
Curiosity rover part of the Mars Science Laboratory (MSL; Grotzinger et al.,
2012); and Magnetospheric Disturbance Index (MDI; Gruesbeck et al., 2021)
data. MDI takes MAVEN/MAG measurements as input and is used to estimate the
level of the disturbance to the Martian system, or arieffectiveness.

Figure 6: In-situ measurements at Mars, showing (a) magnetic field magnitude,
(b) magnetic field components in MSO coordinates, (c) $\theta$ and (d) $\phi$
components of the magnetic field, solar wind (e) speed, (f) density, (g)
dynamic pressure, and (h) temperature, (i) plasma $\beta$, (j) electron pitch
angle distribution, (k) ion intensities, galactic cosmic ray variation (l) in
space and (m) on ground, and (n) MDI index, quantifying arieffectiveness. The
lighter portions of magnetic field data are sampled in the Martian foreshock
rather than in the solar wind.

The sequence of events at Mars commenced again with a possible weak
interplanetary shock (solid pink line in Figure 6), identified mainly via a
peak in ion fluxes at suprathermal energies detected by MAVEN/SEP because of
the frequent gaps in magnetic field and plasma data. In fact, these data gaps
are due to the MAVEN and MEX orbits, resulting in only a portion of their path
around Mars sampling the upstream solar wind. The MEX/ASPERA-3 measurements
displayed in Figure 6 consist of averages along the spacecraft’s orbit, and
the MAVEN/MAG and MAVEN/SWIA data show solar wind periods identified according
to the algorithm of Halekas et al. (2017). The MAVEN/MAG measurements shown in
lighter colours represent data that are not part of the “undisturbed” solar
wind but are still outside the Martian bow shock, i.e. they are collected in
the foreshock region. The possible interplanetary shock is followed by a weak
ejecta (shaded pink region in Figure 6), with directions of the magnetic field
vectors closely resembling those observed at Earth for ICME1 (see Figure 5).
The $\theta_{B}$ component rotates from south to north and the $\phi_{B}$ one
points to the west in Mars Solar Orbital (MSO) coordinates (i.e., the GSE
equivalent for Mars), indicating a SWN-type flux rope. Considering the ICME1
shock speed at Earth (${\sim}385$ km$\cdot$s-1) and the Earth–Mars radial
distance at the time of ${\sim}0.4$ AU, the structure would have taken
${\sim}43$ hours to propagate from one planet to the next, in agreement with
the timing of the observed candidate shock at Mars. Hence, we attribute this
structure to ICME1. Approximately in the middle of the ejecta, we identify a
possible second interplanetary shock (dashed grey line in Figure 6), again
because of a small enhancement in the MAVEN/SEP spectrum and because the solar
wind speed measured by MAVEN/SWIA displays a small increase afterwards. This
is likely the ICME2 shock, still travelling through the ICME1 ejecta—again,
the timing (${\sim}44$ hours for a shock moving at ${\sim}380$ km$\cdot$s-1
across ${\sim}0.4$ AU) is consistent between the observations at Earth and
Mars. ICME1 was characterised by some periods of bidirectional electrons
measured by MAVEN/SWEA, especially during the first half of the ejecta, no
Forbush decreases in space nor on ground, and no significant disturbances to
the Martian system.

After the passage of the weak ICME1, a period of clear enhanced magnetic field
magnitude followed (shaded green region in Figure 6). However, this structure
displays classic signatures of a SIR rather than of an ICME (e.g., Kataoka and
Miyoshi, 2006), including the absence of an organised internal magnetic field
throughout its extent, and an increase in solar wind speed and temperature as
well as a decrease in density at the stream interface. This suggests that the
ejecta of ICME2, which was rather prominent at Earth (see Figure 5), largely
missed Mars. It is unclear whether minor portions of the ejecta interacted
and/or merged with the following SIR at Mars’s heliolongitude, especially
given the numerous data gaps throughout the structure. The plasma profiles
measured by MAVEN/SWIA in the first half of the SIR resemble those observed at
Earth in the ICME2 ejecta and following SIR (see Figure 5), but the absence of
bidirectional electrons in MAVEN/SWEA data as well as the disorganised
magnetic field components seen by MAVEN/MAG suggest that, if material from
CME2 was indeed detected at Mars, the ejecta had lost its coherence and flux
rope structure (at least locally) due to interaction with the structured solar
wind. The SIR was accompanied by enhanced ion fluxes measured by MAVEN/SEP at
sub-MeV energies (see Richardson, 2004, for a review on SIR-associated
energetic particle observations), as well as by a weak (${\sim}2$% drop)
Forbush decrease measured both in space by MAVEN/SEP and on ground by MSL/RAD
(see Guo et al., 2018, for an overview of space- and ground-based observations
of CME- and SIR-driven Forbush decreases at Mars). We note that no significant
variations were detected in the MEX/ASPERA-3 Ion Mass Analyser (IMA)
background counts (not shown), which can be used as proxies for GCR intensity
(see Futaana et al., 2022, for a description of the data set). Additionally,
the SIR was rather arieffective according to MDI values, peaking at
${\sim}7$—we note that Gruesbeck et al. (2021) reported a peak MDI of
${\sim}4$ for the 8 March 2015 ICME studied by Jakosky et al. (2015a) and of
${\sim}10$ for the 13 September 2017 ICME analysed by Lee et al. (2018).
Finally, as expected, the SIR was ultimately followed by HSS2.

##  5 Discussion

The eruptive events that took place during the second half of August 2018 were
typical of solar minimum conditions: They did not originate from active
regions nor did they display an “explosive” nature, they were slow, and they
were rather “isolated” (i.e., there was no other activity elsewhere on the Sun
during the same period). Nevertheless, such a seemingly simple picture
resulted in a number of unexpected outcomes, including a major problem storm
at Earth and a missed CME impact at Mars, despite the small (${<}10^{\circ}$)
longitudinal separation between the two planets. Here, we synthesise the solar
and heliospheric observations discussed in the previous sections together with
the WSA–Enlil modelling results, and elaborate on the possible evolutionary
scenario that is consistent with measurements at both planets.

First of all, we note that the CME arrival times estimated by WSA–Enlil (see
Figure 4) are between ${\sim}4$ hours (ICME1 at Earth) to ${\sim}12$ hours
(ICME1 at Mars) too early compared to observations. This is likely due to HSS2
impacting Earth approximately 1 day too early and Mars approximately 2 days
too early in the simulation, resulting in a diminished deceleration of the
CMEs ahead due to solar wind drag. Nevertheless, Gressl et al. (2014) found
that predicted HSS arrival times in MHD models have uncertainties of the order
of about one day, suggesting that the period under analysis was not
particularly challenging in terms of characterising the solar wind background,
since it was affected by typical errors. Arrival times aside, the sequence of
events at Earth was well reproduced in the simulation (see Movie S5 for a more
complete view of the modelling results): ICME2 closely followed ICME1 (and the
eruptions had just started to interact by the time they crossed Earth) and was
immediately trailed by HSS2. What was impossible to forecast from the
WSA–Enlil simulation was the impressive magnitude of the geomagnetic storm
driven by ICME2.

In fact, the factors that led to the unexpected geoeffectiveness of ICME2 at
Earth may also explain why the same structure missed Mars. This is illustrated
in Figure 7, which shows a comparison of the CME flux rope orientations
retrieved from solar observations (i.e., the GCS reconstructions in Figure 2)
and in-situ measurements at Earth (i.e., the EC fits in Figure 5). It is
evident that CME1/ICME1 maintained a low inclination to the equatorial plane
throughout its heliospheric evolution, resulting in a flux rope type that was
estimated to be SWN in the solar corona, at Earth, and later at Mars. CME2, on
the other hand, left the Sun with a WSE configuration but was later observed
in the solar corona to have assumed a lowly-inclined orientation (possibly
NWS, see especially its morphology in SOHO/LASCO data in Figure 2(m–p)). At
Earth, however, the retrieved flux rope type of ICME2 was again WSE, leading
to the strong southward field that mainly drove the observed geomagnetic
storm. We speculate that interaction with the following HSS2 resulted in both
CME rotation towards a high-inclination configuration and CME deflection
towards western heliolongitudes. The latter effect is marginally visible in
the WSA–Enlil simulation (see Movie S5), whilst the former could not be
reproduced with the current set up since the two CMEs have been modelled
without an internal magnetic field, leading to unrealistic CME–CME and CME–HSS
interaction processes. The rotation and deflection of the CME2 flux rope
ejecta in interplanetary space may account for the fact that only the ICME2
shock seems to have impacted Mars. Of course, these considerations imply that
CME2 was indeed characterised by a low inclination with respect to the solar
equator throughout the solar corona—since the GCS reconstructions were
performed using the only two available viewpoints of Earth (halo event) and
STEREO-A (limb event), the resulting morphology may have been under-
constrained and thus associated with rather large uncertainties.

Figure 7: Comparison of the GCS-derived CME orientation in the solar corona
and EC-derived ICME ejecta orientation at Earth for (a) CME1/ICME1, in pink,
and (b) CME2/ICME2, in grey. The croissant-like features show the CME
morphology obtained via the GCS reconstructions shown in Figure 2, whilst the
cylinders represent the corresponding ICME structures retrieved via the EC
fits displayed in Figure 5.

Another interesting result is that, whilst Mars was not impacted by the ICME2
ejecta, it was still affected by significant large-scale disturbances during
the period under study, namely due to the SIR ahead of HSS2. At Earth, the SIR
was not particularly geoeffective, suggested by the fact that the Dst index
did not display a second dip after the passage of ICME2 (although it does
appear to have interrupted the recovery of the ongoing storm)—it is worth
noting, however, that the presence of ICME2 at Earth likely impeded the
formation of a proper slow–fast stream interaction region, and that what we
identified as a small SIR displays characteristics of a CME–HSS interaction
region. At Mars, the SIR passage lasted for over a day and was fundamentally
responsible for the observed arieffectiveness. We remark, nevertheless, that
the plasma signatures in the first half of the SIR at Mars displayed
similarities with those observed in the ICME2 ejecta at Earth, possibly
indicating that some material from CME2 may have become part of the SIR due to
interaction with the structured solar wind. Whilst it is not known whether
similar effects would have been experienced at Mars had the ICME2 ejecta
impacted the planet as a well-defined flux rope structure, we can speculate
that, during solar minimum conditions, when CMEs are on average less
energetic, we may generally expect higher arieffectiveness from SIRs than from
ICMEs (contrarily to Earth, where the most geoeffective structures are
associated with CMEs across the whole solar cycle; e.g., Richardson et al.,
2001). In fact, whilst CMEs tend to expand (and thus experience a decrease in
speed, density, and dynamic pressure) until ${\sim}10$–15 AU, where they reach
pressure balance with the ambient wind (e.g., von Steiger and Richardson,
2006), SIRs are known to increase the compression of the fast stream against
the slow stream with heliocentric distance until a few AUs (e.g., Forsyth and
Marsch, 1999)—for example, Jian et al. (2008) found a significant increase in
SIR-associated shock detections from Venus to Earth, and Geyer et al. (2021)
found that the occurrence of SIR-driven fast-forward shocks is three times
higher at Mars than at Earth. Additionally, Edberg et al. (2010) found that
most solar wind pressure pulse events—which lead to a 2.5-fold increase in
atmospheric escape—correspond to SIR drivers, whilst only a few are driven by
CMEs.

##  6 Concluding Remarks

In this work, we have analysed the eruption, evolution, and impact at Earth
and Mars of a series of solar transients during the second half of August
2018, i.e. close to the activity minimum between solar cycles 24–25. In
particular, we have tracked the eruption (on 20 August 2018) and propagation
of two CMEs (CME1 and CME2) throughout their journey in the inner heliosphere
as they were followed by a fast solar wind stream (HSS2). We found that both
planets experienced space weather effects, but due to different drivers: at
Earth, an ICME ejecta (ICME2) with a strong southward component of its
magnetic field was the main driver of a major geomagnetic storm, whilst at
Mars, the main structure responsible for the observed arieffectiveness was a
SIR. The first event in the sequence of transients, i.e. a preceding, smaller
ICME (ICME1), was detected at both planets but had no significant effects on
their space environments.

We also compared in-situ measurements at Earth and Mars with simulation
results using the MHD WSA–Enlil model. First of all, we noted that the
observed geomagnetic storm could not have been predicted with the used set up,
since the CMEs were modelled without an internal magnetic field. Heliospheric
models that include a magnetised CME are expected to provide more realistic
insights into the dynamics of CME–CME and CME–HSS interaction (e.g., Asvestari
et al., 2022; Scolini et al., 2020). The hydrodynamic nature of the modelled
CMEs may also explain why CME2 was predicted to merge with CME1 beyond 1 AU
and ultimately impact Mars, whilst it likely rotated and drifted towards
western heliolongitudes due to interactions with the following HSS2.

At Mars, the SIR-induced effects would have been predicted more successfully
with the modelling set up used in this study, although the HSS of interest
(HSS2) was simulated to arrive earlier than observed at both planets. Riley et
al. (2012) found that global MHD models can diverge more or less significantly
from observations in capturing the overall structure of the ambient solar wind
even for solar minimum periods, when conditions are relatively steady. Jian et
al. (2011) reported that SIR timing predictions can have temporal offsets of
up to two days at 1 AU and up to four days at 5 AU. Large-scale efforts
dedicated to benchmarking solar wind models (e.g., Reiss et al., 2022) will
likely lead to improved predictions of solar wind structures (including SIRs
and CMEs) and their effects at different planets. In fact, a well-constrained
and well-reproduced solar wind background is of great importance for
simulating solar minimum CMEs, which tend to alter their structure and
orientation during propagation largely due to interactions with the steady
wind (see, e.g., the May 1997 event; Cohen et al., 2010; Odstrcil et al.,
2004; Titov et al., 2008).

Finally, we comment on two aspects that may be of interest to martian space
weather research. First, although most major geomagnetic storm are associated
with CMEs, it has been reported that SIRs can occasionally drive strong
responses at Earth (Richardson et al., 2006). Given that SIRs are expected to
strengthen beyond 1 AU, an interesting exercise would be to follow one such
strongly geoeffective structure from Earth to Mars and evaluate its
arieffectiveness—as space weather at Mars is a relatively novel area of
research, it is currently unclear which solar wind transients are generally
associated with the most severe disturbances. Additionally, the different
types of magnetospheres may lead to different solar wind driving conditions at
Earth and Mars. Of course, analysis of the solar wind drivers of martian
storms would strongly benefit from continuous space weather monitoring,
perhaps via a satellite placed at the Sun–Mars L1 point as is the case for
Earth (e.g., Sánchez-Cano et al., 2021).

## Data Availability Statement

Solar disc and coronagraph data from SDO, SOHO, and STEREO are openly
available at the Virtual Solar Observatory (VSO;
https://sdac.virtualsolar.org/), whilst STEREO/HI Level-2 data can be
retrieved from the UK Solar System Data Centre (UKSSDC;
https://www.ukssdc.ac.uk). These data were visualised, processed, and analysed
trough SunPy (SunPy Community et al., 2020), IDL SolarSoft (Bentely and
Freeland, 1998), and the ESA JHelioviewer software (Müller et al., 2017).
Enlil simulation results have been provided by the Community Coordinated
Modeling Center at 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/results/viewrun.php?domain=SH&runnumber=Erika_Palmerio_050922_SH_1
(run id: Erika_Palmerio_050922_SH_1). Wind and ACE data are publicly available
at NASA’s Coordinated Data Analysis Web (CDAWeb) database
(https://cdaweb.gsfc.nasa.gov/index.html/). NMDB data are publicly available
at http://www.nmdb.eu. LRO, MAVEN, and MSL data can be accessed at the
Planetary Plasma Interactions (PPI) Node of NASA’s Planetary Data System (PDS)
database (https://pds-ppi.igpp.ucla.edu). MEX data are openly available at
ESA’s Planetary Science Archive (PSA) database
(https://archives.esac.esa.int/psa).

###### Acknowledgements.

E. Palmerio acknowledges the NASA Living With a Star (LWS) Jack Eddy
Postdoctoral Fellowship Program, administered by UCAR’s Cooperative Programs
for the Advancement of Earth System Science (CPAESS) under award no.
NNX16AK22G, as well as NASA’s HTMS Program (grant no. 80NSSC20K1274) and NSF’s
PREEVENTS Program (grant no. ICER-1854790). C. O. Lee, J. R. Gruesbeck, J. S.
Halekas, and S. Xu acknowledge support from the MAVEN mission, which is
supported by NASA through the Mars Exploration Program. C. O. Lee, S. Xu, B.
J. Lynch, and J. G. Luhmann acknowledge NASA HGI (no. 80NSSC21K0731) and LWS
(no. 80NSSC21K1325). I. G. Richardson acknowledges support from NASA LWS
Program NNH19ZDA001N-LWS. 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 and the CCMC staff. E. Palmerio, I. G.
Richardson, N. V. Nitta, T. Mulligan, and B. J. Lynch thank the International
Space Science Institute (ISSI) for their support of International Team no.
415, “Understanding the Origins of Problem Geomagnetic Storms”
(https://www.issibern.ch/teams/geomagstorm/).

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