# Investigating Remote-sensing Techniques
to Reveal Stealth Coronal Mass Ejections

Erika Palmerio 1,2,∗, Nariaki V. Nitta 3, Tamitha Mulligan4, Marilena Mierla
5,6, Jennifer O’Kane 7, Ian G. Richardson 8,9, Suvadip Sinha 10, Nandita
Srivastava 11,10, Stephanie L. Yardley 7, and Andrei N. Zhukov 5,12

###### Abstract

## 1

Eruptions of coronal mass ejections (CMEs) from the Sun are usually associated
with a number of signatures that can be identified in solar disc imagery.
However, there are cases in which a CME that is well observed in coronagraph
data is missing a clear low-coronal counterpart. These events have received
attention during recent years, mainly as a result of the increased
availability of multi-point observations, and are now known as ‘stealth CMEs’.
In this work, we analyse examples of stealth CMEs featuring various levels of
ambiguity. All the selected case studies produced a large-scale CME detected
by coronagraphs and were observed from at least one secondary viewpoint,
enabling _a priori_ knowledge of their approximate source region. To each
event, we apply several image processing and geometric techniques with the aim
to evaluate whether such methods can provide additional information compared
to the study of “normal” intensity images. We are able to identify at least
weak eruptive signatures for all events upon careful investigation of remote-
sensing data, noting that differently processed images may be needed to
properly interpret and analyse elusive observations. We also find that the
effectiveness of geometric techniques strongly depends on the CME propagation
direction with respect to the observers and the relative spacecraft
separation. Being able to observe and therefore forecast stealth CMEs is of
great importance in the context of space weather, since such events are
occasionally the solar counterparts of so-called ‘problem geomagnetic storms’.

##  2 Keywords:

Sun, coronal mass ejection (CME), stealth CME, solar corona, space weather,
remote-sensing observations

##  3 Introduction

Coronal mass ejections (CMEs) are powerful solar eruptions containing large
amounts of plasma and magnetic field that are regularly expelled from the Sun
into the heliosphere. They were first identified in white light in the early
1970’s (Tousey, 1973; Gosling et al., 1974) in images from the $7^{\text{th}}$
Orbiting Solar Observatory (OSO-7) coronagraph (Koomen et al., 1975) and the
coronagraph onboard the Skylab space station that formed part of the Apollo
Telescope Mount (ATM; Tousey, 1977) suite of solar instruments. Around the
same time, the low-coronal counterparts of white-light CMEs were being
observed through multi-wavelength solar disc imagery, e.g. in extreme ultra-
violet (EUV), soft X-rays, and H$\alpha$, using data from OSO-7, Skylab, and
ground-based observatories (e.g., Demastus et al., 1973; Munro et al., 1979).
The typical low-coronal signatures of CMEs were reviewed by Hudson and Cliver
(2001). These include the appearance of coronal dimmings (e.g., Thompson et
al., 2000; Kahler and Hudson, 2001), flare ribbons (e.g., Rust and Bar, 1973;
Martin, 1979), post-eruption arcades (e.g., Rust and Webb, 1977; Tripathi et
al., 2004), and EUV waves (e.g., Thompson et al., 1998; Zhukov and Auchère,
2004), as well as the disappearance of filament material (e.g., Rust et al.,
1975; Sheeley et al., 1975) and X-ray sigmoids (e.g., Rust and Kumar, 1996;
Green et al., 2007). A major step forward in CME research was achieved in the
early 1980’s, when Howard et al. (1982) reported observations of the first
Earth-directed CME (in that case, a halo CME111Note that a halo CME is not
necessarily Earth-directed, and a CME does not need to be a halo in order to
impact Earth.) in white-light images from the Solwind coronagraph (Michels et
al., 1980) and linked this ‘coronal transient’222CMEs have been called by
various names since their discovery, ranging from ‘coronal transient
phenomena’ (MacQueen et al., 1974) to ‘mass ejections from the Sun’ (Gosling
et al., 1974). The term ‘coronal mass ejection’ was first introduced by
Gosling (1975), but came into common usage only in the early 1980’s. with a
disappearing filament on the solar disc and with a shock wave detected near
Earth about 3 days later. The following decade saw the launch of two missions
that have made a major impact on the field of solar physics, namely Yohkoh
(also known as Solar-A; Ogawara et al., 1991) and the Solar and Heliospheric
Observatory (SOHO; Domingo et al., 1995). Yohkoh carried several solar
imagers, including the Soft X-ray Telescope (SXT; Tsuneta et al., 1991),
whilst the SOHO payload includes remote-sensing and in-situ instruments,
several of which are still operational, including the venerable Large Angle
Spectroscopic Coronagraph (LASCO; Brueckner et al., 1995). By then, the whole
Sun-to-Earth picture of CMEs was seemingly quite clear: The presence of low-
coronal signatures preceding a halo CME observed by LASCO would signify that
the CME is Earth-directed, whereas the lack of visible activity on the solar
disc would indicate that the CME was associated with a far-sided eruption.

However, it was not long before this picture was shown to not always hold
true. Studies of CMEs detected in situ near Earth noted that a large number of
such events lacked clear solar associations (e.g., Cane and Richardson, 2003;
Richardson and Cane, 2010). Specifically, Schwenn et al. (2005) reported that
20% of interplanetary CMEs could not be linked to a front-sided (partial or
full) halo CME source, and Zhang et al. (2007) reported that 11% of CME-driven
storms with minimum $Dst\leq-100$ nT could not be linked to eruptive
signatures in the low corona. A turning point came with the launch of the
Solar Terrestrial Relations Observatory (STEREO; Kaiser et al., 2008) mission
in 2006. The STEREO mission consisted of twin spacecraft, one advancing ahead
of Earth in its orbit (STEREO-A) and one trailing behind (STEREO-B), thus
enabling observations of the Sun from multiple viewpoints. Using data from the
Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI; Howard et
al., 2008) suite onboard both STEREO spacecraft when they were separated by
$53^{\circ}$, Robbrecht et al. (2009) reported that a CME erupted _“leaving no
trace behind on the solar disc”_. Although the authors were able to identify a
slow, streamer-blowout CME erupting off the limb in STEREO-A imagery, there
were no corresponding on-disc signatures in STEREO-B data. The Robbrecht et
al. (2009) event represents the first direct observation of what is now known
in the solar physics community as a ‘stealth CME’. In the following years,
numerous additional stealth events have been reported (Ma et al., 2010; D’Huys
et al., 2014; Kilpua et al., 2014). According to these studies, CMEs that lack
distinct low-coronal signatures tend to occur close to solar minimum, are
generally slow and narrow, and often form at higher altitudes in the solar
atmosphere. One major question that started to be raised is whether the
eruption mechanism for stealth CMEs is fundamentally different from that of
“ordinary” CMEs, or if stealth CMEs simply represent the lowest end of the
full energy spectrum of solar eruptions (Howard and Harrison, 2013; Lynch et
al., 2016). Additionally, it was soon clear that stealth CMEs can be detected
in situ, starting with the Robbrecht et al. (2009) event that was observed at
STEREO-B featuring a classic flux-rope structure (Möstl et al., 2009; Lynch et
al., 2010; Nieves-Chinchilla et al., 2011). It also became evident that the
interplanetary counterparts of stealth CMEs are able to cause significant
space weather disturbances if they encounter Earth (Nitta and Mulligan, 2017).
CME-driven storms that cannot be linked to a clear source on the Sun or to
appreciable solar activity are particularly challenging to forecast and are
known as ‘problem geomagnetic storms’ (e.g., McAllister et al., 1996).

During the last decade, routine observations of the solar disc have advanced
considerably due to the high temporal and spatial resolution of data from the
Solar Dynamics Observatory (SDO; Pesnell et al., 2012), launched in 2010.
Nevertheless, despite the improvement of EUV observations with respect to
previous instrumentation, stealth CMEs continue to be reported (e.g., Nitta
and Mulligan, 2017). It has been suggested that apparently stealth CMEs result
from observational limitations such as instrument sensitivity and bandwidth
issues, even in the SDO era (Howard and Harrison, 2013), and that advanced
image processing techniques may reveal hard-to-observe signatures in both
solar disc and coronagraph imagery (Alzate and Morgan, 2017; O’Kane et al.,
2019). Additionally, some studies have applied geometric triangulation and
reconstruction techniques to data from complementary viewpoints in order to
trace stealth CMEs back to an approximate source region on the disc (Pevtsov
et al., 2012; O’Kane et al., 2019; Talpeanu et al., 2020). These methods,
however, have not been tested on a large number of events and hence it is not
known whether they are suitable to all circumstances or whether they can be
applied only to a limited number of cases. To complicate things further, there
is currently no formal definition describing what a stealth CME is or defining
the “observational limit” below which a CME can be considered to be stealthy.

In this paper, we contemplate the following question: Since stealth CMEs can
present diverse characteristics, is it possible that there exists a class of
“extremely stealth” CMEs that cannot be revealed even with the aid of state-
of-the-art techniques? In order to address this issue, this work aims to
investigate the efficacy of various techniques applied to remote-sensing data
in revealing the signatures of CMEs that are elusive on the solar disc. We
test such techniques on four well-studied stealth CMEs for which the “true”
source is more or less known because of the availability of remote-sensing
imagery from additional viewpoints. This will ensure that any solar activity
that is observed away from the expected source region will not be mistakenly
interpreted as a signature of the stealth CME under analysis. This manuscript
is organised as follows. In Section 4, we present and describe the imaging and
geometric techniques that we employ in this study to analyse stealth CMEs. In
Section 5, we apply these techniques to four case studies and compare them
with information that can be retrieved from plain inspection of intensity
images only. Finally, in Section 6 we discuss our results and present our
conclusions.

##  4 Techniques Employed

This section summarises the remote-sensing techniques that are used throughout
this work to analyse the four elusive CMEs under study. Imaging techniques are
described in Section 4.1, whilst geometric techniques are presented in Section
4.2.

###  4.1 Imaging Techniques

####  4.1.1 Image Differencing

The image differencing technique simply consists of subtracting from an image
a preceding one, so that changes in total intensity over time appear as
patches that are either dark (denoting an intensity decrease) or bright
(denoting an intensity increase). This method has been long used in solar
physics applications for both on-disc and coronagraph observations (e.g.,
Burlaga et al., 1982; Hudson et al., 1992). The commonly used nomenclature for
the technique is that subtracting a pre-event image yields a ‘base difference’
image, whilst subtracting successive images yields a ‘running difference’
image. Base-difference images are often used to highlight transient phenomena
that develop over larger time scales, such as coronal dimmings (e.g., Attrill
and Wills-Davey, 2010), whilst running-difference images are often used to
highlight short-term transient features such as EUV waves (e.g., Attrill et
al., 2007). Since stealth CMEs tend to be slow, i.e. they erupt and accelerate
over the course of several hours, their evolution is not expected to be
captured in running-difference images, hence we focus in this work on using
image differencing over longer time scales. In particular, Nitta and Mulligan
(2017) noted that in the case of stealth CMEs, difference images with “long
enough” temporal separations (of the order of ${\sim}10$ hours) should often
be used in order to reveal weak low-coronal signatures, including dimmings and
post-eruption arcades. In this work, in order to minimise the appearance of
artefacts at the solar limb due to rotation (which are still present even
after accounting for differential rotation), we calculate the percentage
variation between each couple of images and use a fixed temporal separation of
${\Delta}t=12$ hours.

####  4.1.2 Wavelet Packets Equalisation

The Wavelet Packets Equalisation (WPE; Stenborg and Cobelli, 2003; Stenborg et
al., 2008) technique is a multi-resolution image processing method that can be
applied to enhance features based on their multi-scale nature. In the WPE
technique, an image is first decomposed over both dimensions using spatially
localised functions known as wavelets. We have implemented the Stenborg and
Cobelli (2003) procedure using a 2D à trous wavelet transform (e.g., Shensa,
1992), where the scaling function is a $B_{3}$-spline corresponding to a
$5{\times}5$ smoothing kernel. This produces a set of wavelet planes at
different spatial scales (called wavelet scales) derived from the initial
image, together with the remaining “continuum” background image representing
the lowest frequencies and largest scales. A wavelet-processed image is
constructed by summing over all the wavelet planes and the continuum
background with user-defined weights to emphasise the desired scale sizes. In
general, the weighting strategy must be fine-tuned to both the particular
image (e.g., different EUV wavelengths or different coronagraphs) and to the
spatial scales of interest. In the case of stealth CMEs, the WPE technique has
been used to enhance off-limb structures from secondary viewpoints (e.g.,
Vourlidas et al., 2011; Nieves-Chinchilla et al., 2013; Liewer et al., 2021),
and had been explored to search for elusive on-disc signatures by Robbrecht et
al. (2009).

####  4.1.3 Multi-scale Gaussian Normalisation

The Multi-scale Gaussian Normalisation (MGN; Morgan and Druckmüller, 2014)
technique, similarly to the WPE method described in Section 4.1.2, is based on
a multi-scale normalisation algorithm. In the MGN technique, a set of 2D
Gaussian kernels of different scale lengths are used to locally normalise an
image using a set of local mean and standard deviation values. A weighted
combination of the normalised components is then used to obtain a weighted
mean locally normalised image, which is finally superposed to the
corresponding global gamma-transformed image. This results in an enhancement
of the local intensity fluctuations within the images. The MGN image
processing tool has been applied to EUV images to identify the low-coronal
signatures associated with stealth events in both on-disc and off-limb
observations (Alzate and Morgan, 2017; O’Kane et al., 2019, 2021b).

###  4.2 Geometric Techniques

####  4.2.1 Latitude Projection

Since all CMEs investigated in this study have been observed off limb from at
least one additional perspective, a simple approach to adopt when
triangulation is not possible (i.e., when only one “non-stealthy” viewpoint is
available) is to project the approximate latitude from which the CME
originated onto the “stealthy” field of view. Considering a “classic” three-
part CME structure in coronagraph imagery (Illing and Hundhausen, 1985), we
trace back to the solar disc the approximate latitude of the central, bright
core that is observed off limb. We note that not all CMEs feature a three-part
structure (e.g., Vourlidas et al., 2013), but stealth CMEs often belong to the
streamer blowout category, in which flux rope signatures tend to occur at a
higher rate than in the general CME population (Vourlidas and Webb, 2018). The
projected latitude of the source region from the off-limb viewpoint naturally
focuses the search for any possible faint or ambiguous on-disc signature to a
more localised area. This enhances the potential to find any low-coronal
signatures or dynamics that, by themselves, would not have necessarily been
interpreted as being eruption-related.

####  4.2.2 Tie-point Technique

The Tie-point (TP) triangulation technique was formulated by Inhester (2006)
and first employed by Thompson (2009) to study a Sun-grazing comet. The
principle on which the TP technique is based is that two separate observers
and the point of interest in space (to be triangulated) form a plane called
‘epipolar plane’, which is reduced to a line (‘epipolar line’) in image
projections. A point identified in an image from the first observer must lie
on the same epipolar line in the corresponding image from the second observer.
Larger-scale features can be tracked by finding correspondences between
different pixels along epipolar line pairs in images from both spacecraft. The
3D reconstruction or triangulation is then achieved by finding the
intersection of the two lines of sight (for each pixel of interest) along the
corresponding epipolar plane, which is unambiguously defined. In solar physics
applications, the TP method has been used to evaluate the 3D morphology of
erupting filaments (e.g., Bemporad et al., 2011; Thompson et al., 2012;
Panasenco et al., 2013; Palmerio et al., 2021) and the evolution of CME fronts
or cores in coronagraph data (e.g., Mierla et al., 2008, 2009; Srivastava et
al., 2009; Liewer et al., 2011).

####  4.2.3 Graduated Cylindrical Shell

The Graduated Cylindrical Shell (GCS; Thernisien et al., 2006, 2009;
Thernisien, 2011) model is a reconstruction technique usually applied to
white-light coronagraph images. In the GCS model, a wireframe describing the
geometry of flux ropes is used to fit CMEs from one or more simultaneous
viewpoints. Within a single reconstruction, six free parameters (latitude,
longitude, axis tilt, apex height, half-angle, and aspect ratio) can be
adjusted until they best match the CME morphology observed in one or more
images. The geometry of the model itself is often referred to as a “hollow
croissant” and consists of a half-torus frontal part with two conical legs
connected to the Sun. The resulting shape, reminiscent of a croissant, is
“hollow” in the sense that the electron density is placed uniquely on the
shell of the model. Thus, fits performed with the GCS model can provide
information on the morphology of CMEs, but not on their magnetic field
structure. The GCS technique is widely used in solar physics and space weather
applications to determine geometric and kinematic parameters of CMEs and their
shocks through the corona (e.g., Mierla et al., 2010; Shi et al., 2015;
Schmidt et al., 2016), also in the case of stealth CMEs (e.g., Lynch et al.,
2010; He et al., 2018). More recently, O’Kane et al. (2019) and Freiherr von
Forstner et al. (2021) used the inferred propagation latitude and longitude
from GCS reconstructions to obtain an approximate location for the source
region of the stealth CMEs they analysed. The full list of GCS-reconstructed
parameters for all CMEs studied in this work can be found in Supplementary
Table 1.

##  5 Analysis of Sample Stealth Events

We present in this section the remote-sensing analysis of four CMEs with
elusive on-disc signatures: 2008 June 1, 2011 March 3, 2012 February 4, and
2016 October 8. These events were selected based on two main factors. First of
all, they were all observed as classic three-part CMEs including a flux rope
by at least one spacecraft, i.e., they do not belong to the so-called ‘jet’
and/or ‘blob’ categories. Furthermore, each event was observed from at least
one additional viewpoint, enabling estimation of its approximate source region
on the solar disc. Such observations are provided for each case study as
supplementary videos in which EUV data have been enhanced with a radial
filter, in order to bring out off-limb emission. The reader is invited to
initially rapidly move the video player slider back and forth, so that the
motion catches the eye. As stealth CMEs are usually slower than average, it
may be difficult to identify erupting structures that evolve over extremely
long time scales when played at the speeds shown in the videos (i.e., 2–4
hours per second). Additionally, kinematic (height–time) plots for each event
based on observations from these additional viewpoints are provided in Figure
S1.

###  5.1 Event 1: 2008 June 1

The first CME that we focus on in this study (Event 1) erupted on 2008 June 1
and was a stealth event as seen from STEREO-B. This CME was first reported by
Robbrecht et al. (2009) and its eruption was later modelled by Lynch et al.
(2016). As mentioned in the Introduction, this event marked the first direct
observation of a CME that left _“no trace behind”_ in EUV imagery from one
viewpoint, hence we treat it here as a “stealth CME prototype”. At the time of
this event, STEREO-A was located $28^{\circ}$ west of Earth and STEREO-B was
positioned $25^{\circ}$ east of Earth. The pre-eruptive configuration and
eruption process were well observed by STEREO-A, as shown in Extreme
UltraViolet Imager (EUVI) and COR1 coronagraph data shown in Supplementary
Video 1. From the STEREO-A perspective, a flux rope structure (observed as a
characteristic cavity in off-limb imagery; Gibson et al., 2006) can be seen to
lie at relatively high altitudes below a coronal streamer above the
southeastern limb. The eruption itself took place over large time scales
(${\sim}$1.5 days), during which the flux rope slowly lifted off (starting
around 15:00 UT on May 31), causing the streamer to swell and resulting in a
classic streamer-blowout CME that reached the COR2-A coronagraph field of view
around 22:00 UT on June 1. We remark that corresponding images from the
Extreme-ultraviolet Imaging Telescope (EIT; Delaboudinière et al., 1995)
onboard SOHO are not available because of a data gap.

Figure 1: Imaging techniques applied to the 2008 June 1 CME (Event 1).
STEREO/SECCHI/EUVI-B 195 Å images are shown at four different times and
processed with four different methods. (a–d) Intensity images. (e–h)
Difference images with fixed $\Delta t=12$ hours. (i–l) WPE-processed images.
(m–p) MGN-processed images. Two faint dimmings are indicated with arrows (see
text for details).

EUVI images from STEREO-B for the eruption period processed with different
techniques are shown in Figure 1 and Supplementary Video 2. It is clear that
the succession of images in Figure 1 does not show any strong indication that
an eruption has occurred. Nevertheless, it is possible to note two extremely
faint dimmings (indicated with arrows) developing in the southern hemisphere
starting around 15:00 UT on June 1 (see also Supplementary Video 2). These
dimmings are not straightforward to identify even in difference data, possibly
because they are rather weak and hence appear “camouflaged” by other intensity
fluctuations on the solar disc. We also note that these signatures appear
equally visible in images produced using the other three methods (i.e.,
intensity, WPE, and MGN). The dimmings seem to be spatially consistent with
the approximate CME source region deduced from STEREO-A imagery (see
Supplementary Video 1), but because they are so faint it is not possible to
draw strong conclusions as to their association with the 2008 June 1 CME.

Figure 2: Geometric techniques applied to the 2008 June 1 CME (Event 1). (a–c)
Latitude (thick red line) of the CME core (indicated with an arrow) projected
onto the solar disc as seen by STEREO-A, SOHO, and STEREO-B. (d–f) GCS
reconstruction applied to the white-light structure seen in STEREO-A, SOHO,
and STEREO-B imagery. (g–i) The GCS-reconstructed CME displayed together with
the ‘source latitude’ in a 3D representation and shown from the three
viewpoints of STEREO-A, Earth, and STEREO-B.

Results from the application of geometric techniques to Event 1 are shown in
Figure 2. In the top row, the approximate latitude of the footpoints of the
flux rope structure seen in off-limb imagery from STEREO-A (marked with an
arrow in Figure 2(a), see also Supplementary Video 1) is projected onto the
solar disc from all three available viewpoints. The flux rope lifted off from
${\sim}$S27∘, roughly consistently with the location of the faint dimmings
shown in Figure 1, which extend between ${\sim}$S20∘ and ${\sim}$S50∘. We also
perform a GCS reconstruction of the large-scale CME in the corona, shown in
the middle row of Figure 2. We remark that, given that the eruption is a halo
from STEREO-B and rather close to the central meridian as seen from Earth, the
first available time for a meaningful fitting is ${\sim}$3.5 hours after the
first appearance of the CME in the COR2-A field of view. According to GCS
results, the CME apex is located at S04∘E29∘ as seen from Earth, which
converts into a longitude of E04∘ as seen from STEREO-B. Whilst the value for
latitude is significantly different, the longitude is quite consistent with
that of the faint dimmings indicated in the last column of Figure 1, which
cover a longitudinal span of E25∘–W25∘ in the STEREO-B reference frame (note
that the images in the middle row of Figure 2 and those in the last column of
Figure 1 are taken at the same time). The ‘source latitude’ and the
reconstructed GCS wireframe are shown together in a 3D representation in the
bottom row of Figure 2. It is clear that the CME deflected significantly
towards the solar equatorial plane during its early evolution, which is also
visible from Supplementary Video 1 and is consistent with the tendency of CMEs
to align themselves with the heliospheric current sheet during solar minimum
(e.g., Yurchyshyn et al., 2009; Isavnin et al., 2014). Hence, we conclude that
searching for the source of the eruption based on coronagraph images and GCS
reconstructions alone would have likely resulted in a somewhat misleading
region.

###  5.2 Event 2: 2011 March 3

The second CME that we analyse in this work (Event 2) erupted on 2011 March 3
and was a stealth event as seen from Earth. This case study was previously
analysed by Pevtsov et al. (2012), Nitta and Mulligan (2017), O’Kane et al.
(2019), and O’Kane et al. (2021b), who all placed the CME source region in the
vicinity of active region AR 11165, located close to the central meridian from
Earth’s perspective. Hence, we investigate this event as a possible case of a
more localised, active region stealth CME. At the time of this eruption,
STEREO-A was located $87^{\circ}$ west of Earth and STEREO-B was positioned
$95^{\circ}$ west of Earth, meaning that the two spacecraft had a nearly-
quadrature view of the event from opposite sides. Such observations are shown
in Supplementary Video 3, which presents simultaneous EUVI and COR1 data from
the twin STEREOs. In the video, a large bubble-shaped set of loops can be
initially seen to lie off the limb above AR 11165, before slowly inflating
(starting around 18:00 UT on March 2) and erupting as a flux rope CME that
reached the COR2 field of view around 04:00 UT on March 3. From the STEREO-A
perspective, high-altitude post-eruption arcades could be observed after the
CME lifted off as AR 11165 rotated into view, forming just above the longer-
lived active region loops. It is clear from these observations alone that this
CME indeed originated from AR 11165, albeit from higher altitudes than usual
and thus away from stronger active region fields. This may be a contributing
factor to the stealthiness of the event in on-disc imagery, as suggested by
O’Kane et al. (2019).

Figure 3: Imaging techniques applied to the 2011 March 3 CME (Event 2).
SDO/AIA 211 Å images are shown at four different times and processed with four
different methods. (a–d) Intensity images. (e–h) Difference images with fixed
$\Delta t=12$ hours. (i–l) WPE-processed images. (m–p) MGN-processed images.
AR 11165 is circled in the first column. Dimming and brightening regions are
indicated with arrows (see text for details).

Images from the Atmospheric Imaging Assembly (AIA; Lemen et al., 2012)
instrument onboard SDO corresponding to the eruption period are shown in
Figure 3 and Supplementary Video 4. Whilst the intensity, MGN, and WPE images
do not show strong eruptive signatures, the difference images in the second
row display clearer signs of activity on the solar disc. First of all, an
extended pair of dimmings and flare loops at the southern periphery of a large
coronal hole in the western hemisphere can be observed in Figure 3(e)
(indicated with arrows). These are found to be associated with a filament
eruption that occurred on March 1, which possibly destabilised the nearby
active region fields and thus triggered the later stealth CME. The successive
panels are characterised by a series of dimmings (starting around 22:00 UT on
March 2 and marked in panels (f) and (g)) and a strong brightening (starting
around 01:00 UT on March 3 and marked in panel (h)) around AR 11165 (indicated
with a circle in the first column of Figure 3). It can be seen that initially,
a pair of dimmings develops north–south of the active region and later a
third, more diffuse dimming appears to the east of the fading southern one.
Corresponding images obtained with other techniques, on the other hand, show
more elusive signatures. A faint pair of dimmings north and south of AR 11165
can be seen in Supplementary Video 4 (the northern dimming is marked in panels
(c), (k), and (o) of Figure 3), together with the appearance of a set of loops
that seem to correspond to the post-eruption arcade seen by STEREO-A (marked
in panels (d), (l), and (p) of Figure 3). We emphasise that these signatures
are rather weak, and thus without prior knowledge of an eruption having
occurred they would have been easily overlooked. Furthermore, we note that
although the arcade is well observed in all non-differenced data (i.e.,
intensity, WPE, and MGN), its fine structure is more easily revealed in images
processed with advanced techniques.

Figure 4: Geometric techniques applied to the 2011 March 3 CME (Event 2).
(a–c) Application of the TP technique to the pre-eruptive structure. The
technique is applied to STEREO-A and STEREO-B images and the latitudinal
extent of the triangulated loop is projected onto the SDO/AIA field of view
(and displayed over the central meridian). (d–f) GCS reconstruction applied to
the white-light structure seen in STEREO-A, SOHO, and STEREO-B imagery. (g–i)
Results of the TP and GCS reconstruction techniques displayed together in a 3D
representation and shown from three viewpoints: STEREO-A, Earth, and STEREO-B.

Results from the application of geometric techniques to Event 2 are shown in
Figure 4. Since the pre-eruptive structure was well observed by both STEREO
spacecraft, we analyse it using the TP technique. The triangulated balloon-
shaped feature is overlaid onto EUVI images in panels (a) and (c). However,
despite the favourable viewing perspective of nearly-quadrature with Earth
from both observers, it is not possible to obtain a meaningful triangulation
of the loop onto the solar disc imaged by SDO. This is because the uncertainty
associated with the TP technique depends on the angle between the observing
spacecraft, and specifically is proportional to the inverse of the sine of the
separation angle (Inhester, 2006). In the case of this event, the separation
of ${\sim}180^{\circ}$ between the two STEREOs results in a large uncertainty
in the east–west direction. Hence, the only information that can be retrieved
from the TP technique applied to Event 2 is the latitudinal position of each
point part of the triangulated loop, resulting in a vertical bar in Figure
4(b) that is arbitrarily placed at central meridian relative to Earth since
its exact longitude cannot be retrieved. The extent of the loop in the
north–south direction is consistent with the location of AR 11165, as
expected. Results of the GCS reconstruction are shown in the middle panels of
Figure 4. Since the CME was well observed as a limb event from both STEREO
spacecraft, we perform our reconstruction while the transient is still visible
in SECCHI/COR1 and before it reaches the LASCO/C2 field of view, in order to
capture the eruption as close as possible to its initial state. This results
in a propagation direction of S27∘W07∘ as seen from Earth, rather close to
location of AR 11165 at the reconstruction time (S17∘W12∘), but still
suggesting a slight southward deflection. Results from the two geometric
techniques are shown together in a 3D representation in the bottom panels of
Figure 4. Despite the issues with the application of the TP technique for this
particular event discussed above, the two structures obtained match fairly
well. Even if an exact longitude of the source region cannot be retrieved from
TP, it may be argued that the viewing geometry of the two STEREO spacecraft
implies that the triangulated structure was located close to central meridian.
An alternative would be to perform a GCS reconstruction based on the EUV
images, as made e.g. by O’Kane et al. (2021a).

###  5.3 Event 3: 2012 February 4

The third CME that we analyse in this work (Event 3) erupted on 2012 February
4 and was a stealth event as seen from Earth. This CME was first reported by
D’Huys et al. (2014) and was further analysed by Alzate and Morgan (2017).
This event was also well observed off limb by both STEREO spacecraft, with
STEREO-A being located $108^{\circ}$ west of Earth and STEREO-B being placed
$115^{\circ}$ east of Earth. Observations from both EUVI and COR1 telescopes
are provided in Supplementary Video 5 and show the CME of interest being
ejected off the northeastern limb from STEREO-A’s viewpoint and off the
northwestern limb from STEREO-B’s perspective, indicating that the eruption
originated from the Earth-facing Sun. The pre-eruptive structure could be
observed for several hours above the limb from both spacecraft before its slow
lift-off (starting around 04:00 UT on February 4), indicating that the CME
flux rope erupted from unusually high altitudes. The CME reached the COR2
field of view around 10:00 UT on February 4.

Figure 5: Imaging techniques applied to the 2012 February 4 CME (Event 3).
SDO/AIA 211 Å images are shown at four different times and processed with four
different methods. (a–d) Intensity images. (e–h) Difference images with fixed
$\Delta t=12$ hours. (i–l) WPE-processed images. (m–p) MGN-processed images.
Dimming and brightening regions are indicated with arrows (see text for
details).

SDO/AIA images corresponding to the eruption period are shown in Figure 5 and
Supplementary Video 6. As is often the case for stealth CMEs, these data do
not show “explosive” eruption signatures that are more typical of active-
region CMEs. Nevertheless, it is possible to distinguish rather clear
indications that an eruption occurred, as marked with arrows in the difference
image in Figure 5(h). Specifically, we observe an elongated pair of
brightenings attributable to flare ribbons, the first being rather prominent
and close to the northwestern limb and the second being more diffuse and
culminating in the vicinity of the solar north pole, whit faint loops
reminiscent of a post-eruption arcade in between. These features start to
develop around 12:00 UT on February 4. In non-differenced images, the
westernmost ribbon appears significantly less prominent and the more diffuse
one is not visible at all. The structure that we recognised as a post-eruption
arcade based on difference images is somewhat visible, but less clearly
attributable to a post-eruption arcade. Although all these coronal features
can be observed in intensity images as well, their structure appears sharper
in WPE- and MGN-processed data. Furthermore, all images reveal an especially
faint dimming region close to the solar north pole (with onset around 06:00 UT
on February 4 and indicated with arrows in Figure 5(d), (l), and (p)). This
darkening feature appears equally visible in intensity, MGN, and WPE images.
Supplementary Video 6 demonstrates that while it is not possible to establish
with certainty whether a large-scale eruption occurred from intensity images
alone, the development of the various brightening features in difference
images unambiguously links the CME seen off limb from the twin STEREOs to a
rather defined source region on the Earth-facing disc.

Figure 6 shows the results of the geometric techniques applied to Event 3.
Being well-observed off limb from two different perspectives, the 2012
February 4 CME is well suited to be analysed using the TP technique.
Furthermore, the ${\sim}135^{\circ}$ separation between the two STEREO
spacecraft does not lead to the uncertainty issues that were encountered for
Event 2. In the top row of Figure 6, a pre-eruptive loop (on 2012 February 3
at 18:00 UT) is traced with the TP technique in 195 Å images from both STEREO
spacecraft and then projected onto Earth’s view, resulting in a structure
rooted around N55∘W30∘. Note that the triangulated loop has been projected
into a 174 Å image from the Sun Watcher using Active Pixel System Detector and
Image Processing (SWAP; Halain et al., 2013; Seaton et al., 2013) telescope
onboard the Project for On Board Autonomy 2 (PROBA2; Santandrea et al., 2013)
satellite, which has a larger field of view than SDO/AIA. The highest point in
the reconstructed loop lies at $1.81\,R_{\odot}$ from the solar centre,
confirming that a high-altitude flux rope was involved in the eruption. The
middle row of Figure 6 shows the GCS technique applied to simultaneous images
of the white-light CME from the SECCHI/COR2 coronagraphs onboard STEREO and
the LASCO/C2 coronagraph onboard SOHO. The CME apex has the direction
N50∘W03∘, consistent with a high-latitude eruption. Results from the two
reconstructions are combined in the bottom row of Figure 6. Whilst the
latitudes retrieved from the two methods are in agreement, larger differences
are found in the longitudes, which is not surprising considering, for example,
that the GCS precision is typically around $\pm 4^{\circ}$ for latitude and
$\pm 17^{\circ}$ for longitude (Thernisien et al., 2009). Nevertheless, both
techniques yield a source region that is (at least to some extent) consistent
with the bright features indicated in Figure 5(h).

Figure 6: Geometric techniques applied to the 2012 February 4 CME (Event 3).
(a–c) Application of the TP technique to the pre-eruptive loop structure. The
technique is applied to STEREO-A and STEREO-B images and then projected into
the PROBA2 field of view. (d–f) GCS reconstruction applied to the white-light
structure seen in STEREO-A, SOHO, and STEREO-B imagery. (g–i) Results of the
TP and GCS reconstruction techniques displayed together in a 3D representation
and shown from the viewpoints of STEREO-A, Earth, and STEREO-B.

###  5.4 Event 4: 2016 October 9

The fourth CME that we analyse in this work (Event 4) erupted on 2016 October
8 and was a stealth event as seen from Earth. This CME was first reported by
Nitta and Mulligan (2017) and was further analysed by He et al. (2018). Having
taken place in 2016, only the viewpoint from STEREO-A located $148^{\circ}$
east of Earth is available since contact with STEREO-B was lost in October
2014. Observations from the EUVI and COR1 telescopes onboard STEREO-A are
presented in Supplementary Video 7, where the eruption can be seen to
originate close to the solar equator off the western limb, corresponding to an
ejection off the Earth-facing disc. In addition, the CME cavity extended
significantly beyond the central core early in the eruption, suggestive of a
much larger-scale event. Loops corresponding to the outer CME rim can be
observed to lift off ${\sim}25^{\circ}$ south of the equator around 16:00 UT
on October 8, whilst structures to the north of the central core cannot be
discerned with clarity in the EUVI field of view, most likely because of the
presence of a bright helmet streamer. The resulting CME reached the COR2 field
of view around 22:00 UT on October 8.

Figure 7: Imaging techniques applied to the 2016 October 8 CME (Event 4).
SDO/AIA 211 Å images are shown at four different times and processed with four
different methods. (a–d) Intensity images. (e–h) Difference images with fixed
$\Delta t=12$ hours. (i–l) WPE-processed images. (m–p) MGN-processed images.
Dimming and brightening regions are indicated with arrows (see text for
details).

Images from SDO/AIA corresponding to the eruption period are shown in Figure 7
and Supplementary Video 8. It is possible to discern signatures of two
different eruptions in the presented data. The first can be noted clearly in
the figure in difference images (indicated with an arrow in panel (e)) and in
all panels in Supplementary Video 8, and corresponds to a small filament
eruption from the northeastern quadrant. Despite the rather evident low-
coronal signatures, this eruption does not correspond to the CME seen off limb
in STEREO-A imagery (see Supplementary Video 7) because of its timing
($\sim$14:00 UT on October 8, several hours too early compared to its first
appearance in COR1), its source region (${\sim}40^{\circ}$ north of the
equator), and its localised nature (in contrast to the large-scale CME
observed by STEREO-A). Nevertheless, it is possible that this minor eruption
destabilised the overlying field(s) and thus facilitated the onset of the
following, larger CME. Over the following hours, starting around 18:00 UT on
October 8, a pair of coronal dimmings developed, marked by arrows in Figure 7
(d) and evident in Supplementary Video 8. In the difference images in panels
(f) and (g), it is evident that the eastern dimming appears deeper than the
western one. In data processed with WPE or MGN in panels (i) to (l) and (m) to
(p), respectively, the progressive darkening of these two areas over the
presented interval shows that the dimmings developed over a remarkably long
time span (i.e., even longer than the $\Delta t=12$ hours used here for
difference images). We do not note differences in the appearance of the
dimmings in the intensity, WPE, and MGN images. Their extent in latitude (from
${\sim}$N02∘ to ${\sim}$S22∘) is in agreement with the location of the off-
limb signatures observed from the STEREO-A viewpoint and shown in
Supplementary Video 8.

Figure 8: Geometric techniques applied to the 2016 October 8 CME (Event 4).
(a–b) Latitude (thick red line) of the CME core (indicated with an arrow)
projected onto the solar disc as seen by STEREO-A and SDO. (c–d) GCS
reconstruction applied to the white-light structure seen in STEREO-A and SOHO.
(e–f) The GCS-reconstructed CME displayed together with the ‘source latitude’
from (a) and (b) in a 3D representation and shown from the two viewpoints of
STEREO-A and Earth.

Results from applying geometric techniques to Event 4 are shown in Figure 8.
In the top row, the red line indicates the approximate latitude of the CME
core as derived from STEREO-A observations (marked with an arrow in Figure
8(a), see also Supplementary Video 7) projected onto the solar disc from both
available viewpoints. The latitude of ${\sim}$S02∘ is rather consistent with
the dimming locations, but less compatible with the GCS reconstruction shown
in the middle row of Figure 8, which gives a CME apex propagation direction of
N12∘E05∘, indicating a significant deflection and/or non-radial propagation of
the structure towards the north after eruption (this aspect can also be noted
in Supplementary Video 8). The dimmings covered a longitudinal span of
E44∘–W22∘ at the time of the GCS reconstruction, highlighting the large-scale
nature of the event, and were centred around E12∘, i.e., well within the GCS
uncertainties mentioned in Section 5.3. As was the case for Event 2 and Event
3, this eruption can be convincingly linked to low-coronal signatures (albeit
weak), and as was the case for Event 1, relying uniquely on coronagraph
imagery and GCS reconstructions would have resulted in a somewhat misleading
estimated source region. The fact that this CME was a full halo as seen by
SOHO and that only two viewpoints were available certainly contributed to the
late reconstruction time, when the apex was already at ${\sim}10$ $R_{\odot}$.
Hence, the geometric parameters were only determined after the CME had already
experienced significant alterations to its trajectory.

##  6 Discussion and Conclusions

In this work, we have presented and analysed four stealth CMEs that presented
diverse characteristics: a classic streamer blowout (Event 1), a CME
originating from an active region (Event 2), a flux rope lying at unusually
high altitudes prior to eruption (Event 3), and a significantly large-scale
event (Event 4). These case studies were also characterised by different
viewing geometries between the “stealthy perspective” (on the solar disc) and
the off-limb observer(s): Event 1 has STEREO-A ${\sim}50^{\circ}$ away from
STEREO-B, Event 2 had the STEREOs nearly in quadrature with Earth, Event 3 had
the STEREOs ${\sim}110^{\circ}$ away from Earth, and Event 4 had STEREO-A
separated by ${\sim}150^{\circ}$ from Earth. We have investigated these CMEs
using remote-sensing imaging and geometric techniques in order to determine
their corresponding source region on the Sun. Our analysis was based on EUV
images of the solar disc in the 195 Å (STEREO) and 211 Å (SDO) channels,
especially suited to detect dimming and brightening regions associated with
low-coronal signatures of CMEs (e.g., Nitta and Mulligan, 2017), together with
white-light images of the solar corona. Since for all events an approximate
region of origin was known due to off-limb views from additional viewpoints,
the motivation for our analysis was to test and demonstrate the effectiveness
of the different techniques over a range of events with different properties
and observation geometries. Our main findings are summarised in Table 1.

Table 1: Results from applying different imaging and geometric techniques to the events under study. For the imaging techniques, the observed eruption signatures are indicated, with A = arcade, B = brightening, and D = dimming. The upper cases denote strong signatures, whilst the lower cases denote only weak signatures. For the geometric techniques, it is indicated whether the TP method could be applied (N/A for events that only had one off-limb view available) and which features could be triangulated, as well as the radial distance from the Sun of the CME apex at which the earliest meaningful GCS reconstruction could be performed. The ‘latitude projection’ scheme could in principle be applied to all events under study, since it only requires one off-limb viewpoint of the eruption, hence it is not shown here. |  | Imaging | Geometric
---|---|---|---
|  | Intens | Diff | MGN |  WPE | TP | GCS
E1 | Streamer blowout | d | d | d | d | N/A | ${\sim}7\,R_{\odot}$
E2 | AR eruption | A,d | a,B,D | A,d | A,d | Lat only | ${\sim}3\,R_{\odot}$
E3 | High flux rope | b,d | A,B,d | b,d | b,d | Full loop | ${\sim}4\,R_{\odot}$
E4 | Large-scale CME | D | D | D | D | N/A | ${\sim}10\,R_{\odot}$

Analysis of solar disc imagery with various image processing techniques
revealed the presence of signatures for all events, as shown in Table 1,
albeit with different confidence levels. The most convincing evidence was
found for Event 2 and Event 3, whilst Event 4 was associated with weak but
reasonable signatures and Event 1 was characterised by the largest
uncertainties. At least according to the small sample investigated here, it
seems that the main factor contributing to the level of “stealthiness” is the
spatial extent of the eruption rather than the altitude from which a CME lifts
off (in particular, compare the size of the on-disc signatures for Event 1 and
Event 4 with those for Event 2 and Event 3, which are significantly more
localised). Another aspect to note is that the eruption associated with the
weakest signatures (Event 1) was also the only event that did not feature a
nearby active region or area of strong magnetic fields. Regarding the image
processing techniques used, it is evident that difference images with large
temporal separations revealed the clearest eruptive signatures, as was also
reported by Nitta and Mulligan (2017). Moreover, we note that even if dimmings
are often evident enough in intensity data, brightenings tend to appear
overwhelmingly clearer in difference images (see Event 2 and Event 3). In this
work, we have used a fixed $\Delta t=12$ hours for all events, but even longer
separations may be explored in the case of eruptions that develop extremely
slowly (as for Event 1 and Event 4), although artefacts at the solar limb
would also become more prominent and problematic. In this regard, it should be
noted that difference images are particularly prone to spurious effects due to
spatio-temporal interference; i.e., dimming and brightening features may
correspond to “true” dimmings and brightenings as well as moving structures
over long time scales. Thus, difference images may be complemented with non-
differenced data, which should be used to properly interpret the identified
large-scale changes and connect them to well-defined activity on the Sun.
Furthermore, we did not find substantial differences in the features revealed
in “normal” intensity images and those produced by more advanced processing
techniques for the cases that were only associated with (more or less defined)
dimmings, i.e. Event 1 and Event 4. Since the main purpose of these methods is
to sharpen coronal features and accentuate small-scale variations, it is not
surprising that they are as powerful as non-processed images when such
structures and alterations are missing in the first place, i.e. in the case of
the most problematic events. This overall conclusion is in agreement with
O’Kane et al. (2021a), who studied a stealth CME off limb (from a secondary
viewpoint) using MGN, but found that this technique did not also reveal the
corresponding signatures on disc, which were however completely elusive to
other data sets as well. On the other hand, the events that were associated
with more prominent structural changes in the corona, i.e. Event 2 and Event
3, showed significantly enhanced features in WPE and MGN imagery in comparison
to intensity data. This allows for deeper analysis of the onset and signatures
of these eruptions, which may in turn advance understanding of at least a
subset of elusive events. Hence, this work demonstrates that advanced image
processing techniques are also applicable to a portion of large-scale stealth
CMEs observed against the solar disc, in addition to their usefulness for
investigating small, short-lived activity and off-limb events that has been
shown in previous studies (e.g., Alzate and Morgan, 2017; O’Kane et al., 2019;
Liewer et al., 2021). In conclusion, our recommendation for identifying and
analysing the origins of stealth CMEs on the solar disc is a multi-step
approach: (1) use difference images to easily single out large-scale changes,
(2) use intensity data to properly interpret difference images and rule out
artefacts, and (3) if coronal features such as brightenings, loops, and
ribbons can be identified, use advanced image processing techniques such as
WPE and MGN to analyse their fine structure in deeper detail.

Analysis of the events in this study with the aid of various geometric
techniques has revealed that triangulation and reconstruction methods can help
trace an eruption back to its source as long as they are used when the CME is
as close as possible to the Sun. Given that most CME deflections and other
non-radial propagation effects take place below a few solar radii (e.g., Kay
et al., 2015; Kay and Opher, 2015; Liewer et al., 2015), it is crucial to
determine the geometric parameters before the most dramatic evolution has
occurred. In this sense, the CME propagation direction with respect to the
observers plays a central role, as can be seen from the reconstructed CME apex
heights shown in Table 1: amongst the cases investigated here, the best
scenario was achieved for Event 2 and Event 3, in which the CMEs were
propagating in directions well away from at least two viewpoints, enabling a
meaningful GCS reconstruction to be made early on. The least favourable
configuration happened for Event 4, where the CME was a full halo from one
viewpoint and only a second observer was available, thus the apex was already
at ${\sim}10$ $R_{\odot}$ at the time of the performed GCS fitting. In this
case, the resulting CME reconstruction strongly hints at an eruption that
originated from the northern hemisphere. Hence, without complementary
observations from STEREO-A, the source of the white-light structure might have
been erroneously attributed to the previous small filament eruption preceding
the “main” event. Hence, our recommendation for tracing a CME observed in
white light back to its elusive source is to mind the viewing configuration of
the event and pay caution the farther the reconstructed CME is from the Sun.
The TP technique, on the other hand, is quite efficient in triangulating a
pre-eruptive structure and/or tracing an eruption back to a source, but is
more strongly dependent on the viewing geometry (see Table 1). Excluding cases
in which a spacecraft separation of ${\sim}180^{\circ}$ does not allow for a
unique solution (see e.g. Event 2), the method requires two well-separated
spacecraft to observe a particular feature simultaneously, so it is not
applicable to cases in which an eruption is behind the limb relative to at
least one observer. Of course, being able to analyse a stealth CME with
triangulation and reconstruction methods implies the availability of
additional viewpoints to start with. Apart from helping to discern whether a
CME is stealthy or simply far-sided, remote-sensing measurements away from the
Sun–observer line may help identify the source of an elusive event in a more
or less straightforward way, as was the case for the sample events analysed in
this study. Unfortunately after the loss of STEREO-B and with STEREO-A slowly
approaching Earth, the capability to observe stealth CMEs from a viewpoint
well-separated from the Sun–Earth line will be lost at least for a while. In
the longer term, observations away from the Sun–Earth line, made for example
by a STEREO-like, polar, or L4/L5 mission, would help to provide this
capability (e.g., Vourlidas, 2015; Lavraud et al., 2016; Gibson et al., 2018;
Bemporad, 2021).

The four events analysed here took place during different stages of the solar
cycle, with Event 1 happening at solar minimum, Event 2 and Event 3 close to
solar maximum, and Event 4 in the midst of the descending phase of the cycle.
Together with the different characteristics of each eruption summarised in
Table 1, this indicates that stealth CMEs are not restricted to a particular
set of source regions or solar activity period. Hence, although the long-
standing question on the fundamental nature of stealth CMEs has not been
officially answered yet, this study yet emphasises that these events can
present as diverse characteristics as “ordinary” eruptions. Other methods that
may help advance current understanding of stealth CMEs include the study of
the coronal environment from which these eruptions originate, as was done by
O’Kane et al. (2021b) for Event 2 in this work. The authors concluded that
flux emergence and magnetic reconnection episodes were observed in the CME
source region prior to eruption, which led to the formation of the structure
that later left the Sun as a stealth event, and that a high-altitude null
point was revealed by photospheric magnetic field extrapolations of the pre-
eruptive configuration. Moreover, images of the solar disc taken closer than 1
AU may more easily reveal the eruptive signatures of stealth CMEs. The Solar
Orbiter (Müller et al., 2020) spacecraft, launched in February 2020 to orbit
the Sun as close as ${\sim}0.3$ AU and equipped with an EUV instrument as well
as a coronagraph, will possibly be able to provide answers in this regard.
Finally, it is worth remarking on the impact of stealth CMEs in the wider
context of space weather. These events are occasionally capable of driving
large geomagnetic disturbances (e.g., Nitta and Mulligan, 2017), so it is
important to develop a framework in which they can be fully observed and
forecast. One major issue in this sense is that the lack of well-defined low-
coronal signatures does not allow for unambiguous analysis of the remote-
sensing proxies that are necessary to determine the CME pre-eruptive structure
and configuration (e.g., Palmerio et al., 2017). Nevertheless, successfully
identifying the source region of a stealth CME represents a first step towards
providing more reliable predictions.

## Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any
commercial or financial relationships that could be construed as a potential
conflict of interest.

## Author Contributions

EP and NVN selected the events examined in this study and organised the
various aspects of the analysis. EP led the investigation and wrote the draft
manuscript. All authors participated in the discussions, read and critically
reviewed the manuscript, approved the final version, and agreed to be
accountable for all aspects of the work.

## Funding

EP was supported by 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. NVN
and TM acknowledge support from NASA grant no. NNX17AB73G. NVN was partially
supported by the NASA AIA contract no. NNG04EA00C and the NASA STEREO mission
under NRL contract no. N00173-02-C-2035. MM and ANZ thank the European Space
Agency (ESA) and the Belgian Federal Science Policy Office (BELSPO) for their
support in the framework of the PRODEX Programme. JO thanks the STFC for
support via funding given in her PHD studentship. IGR was supported by the
STEREO mission and NASA program NNH17ZDA001N-LWS.

## Acknowledgements

The authors are pleased to acknowledge 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/), from which this work originated.

## Supplemental Data

The Supplementary Material for this article can be found online at:
https://www.frontiersin.org/articles/10.3389/fspas.2021.695966/

## Data Availability Statement

All the solar data available in this study can be accessed at the Virtual
Solar Observatory (http://sdac.virtualsolar.org/cgi/search). These data were
visualised, processed, and analysed using Python’s SunPy package (SunPy
Community et al., 2020), IDL SolarSoft (Freeland and Handy, 1998), and the ESA
JHelioviewer software (Müller et al., 2017).

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