11institutetext:  NASA Postdoctoral Program Fellow, NASA Goddard Space Flight
Center, Greenbelt, MD, USA
11email<EMAIL_ADDRESS>22institutetext: Heliophysics
Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
33institutetext: Physics Department, The Catholic University of America,
Washington, DC, USA 44institutetext: Hvar Observatory, Faculty of Geodesy,
University of Zagreb, Kaciceva 26, HR-10000, Zagreb, Croatia 55institutetext:
Universidad de Alcalá, Space Research Group (SRG-UAH), Plaza de San Diego s/n,
28801 Alcalá de Henares, Madrid, Spain 66institutetext: Institute of Physics,
University of Graz, Universitätsplatz 5, 8010 Graz, Austria 77institutetext:
Department of Physics, University of Helsinki, P.O. Box 64, 00014, Helsinki,
Finland 88institutetext: Kanzelhöhe Observatory for Solar and Environmental
Research, University of Graz, Austria

# Unveiling the Journey of a Highly Inclined CME

Insights from the March 13, 2012 Event with
110$\degree{}$ Longitudinal Separation

F. Carcaboso 11 2 2 3 3 M. Dumbović 44 C. Kay 22 3 3 D. Lario 22 L. K. Jian 22
L. B. Wilson III 22 R. Gómez-Herrero 55 M. Temmer 66 S. G. Heinemann 77 T.
Nieves-Chinchilla 22 A. M. Veronig 6688

###### Abstract

Context. A fast ($\mathord{\sim}2000$ km/s) and wide (¿110°) Coronal Mass
Ejection (CME) erupted from the Sun on March 13, 2012. Its interplanetary
counterpart was detected in situ two days later by STEREO-A and near-Earth
spacecraft, such as ACE, Wind and Cluster. We suggest that at 1 au the CME
extended at least 110° in longitude, with Earth crossing its east flank and
STEREO-A crossing its west flank. Despite their separation, measurements from
both positions showed very similar in situ CME signatures. The solar source
region where the CME erupted was surrounded by three coronal holes (CHs).
Their locations with respect to the CME launch site were east (negative
polarity), southwest (positive polarity) and west (positive polarity). The
solar magnetic field polarity of the area covered by each CH matches that
observed at 1 au in situ. Suprathermal electrons at each location showed mixed
signatures with only some intervals presenting clear counterstreaming flows as
the CME transits both locations. The strahl population coming from the
shortest magnetic connection of the structure to the Sun showed more
intensity.

Aims. The aim of this work is to understand the propagation and evolution of
the CME and its interaction with the surrounding CHs, to explain the
similarities and differences between the observations at each spacecraft, and
report what it would be one of the most longitudinal expanded CME structures
measured in situ.

Methods. Known properties of the large-scale structures from a variety of
catalogues and previous studies are used to have a better overview of this
particular event. In addition, multipoint observations are used to reconstruct
the 3D geometry of the CME and determine the context of the solar and
heliospheric conditions before the CME eruption and during its propagation.
The graduated cylindrical shell model (GCS) is used to reproduce the
orientation, size and speed of the structure with a simple geometry. Also, the
Drag-Based Model (DBM) is utilised to understand better the conditions of the
interplanetary medium in terms of the drag undergone by the structure while
propagating in different directions. Finally, a comparative analysis of the
different regions of the structure through the different observatories has
been made in order to directly compare the in situ plasma and magnetic field
properties at each location.

Results. The study presents important findings regarding the in situ measured
CME on March 15, 2012, detected at a longitudinal separation of 110° in the
ecliptic plane despite its initial inclination being around 45° when erupted
(March 13). This suggests that the CME may have deformed and/or rotated,
allowing it to be observed near its legs with spacecraft at a separation angle
greater than 100°. The CME structure interacted with high-speed streams
generated by the surrounding CHs. The piled-up plasma in the sheath region
exhibited an unexpected correlation in magnetic field strength despite the
large separation in longitude. In situ observations reveal that at both
locations there was a flank encounter, where the spacecraft crossed the first
part of the CME, then encountered ambient solar wind, and finally passed near
the legs of the structure.

Conclusions. A scenario covering all evidence is proposed for both locations
with a general view of the whole structure and solar wind conditions. Also,
the study shows the necessity of having multipoint observations of large-scale
structures in the heliosphere.

###### Key Words.:

Sun: coronal mass ejections (CMEs) – Sun: heliosphere – Sun: solar-terrestrial
relations – Sun: corona – methods: data analysis

##  1 Introduction

Coronal Mass Ejections (CMEs) are large expulsions of plasma originating from
the Sun, commonly resulting from processes of magnetic instabilities and
reconnection in the solar corona. These structures are constituted by a
strongly magnetised plasma which usually remains defined until several
astronomical units (e.g. Witasse et al., 2017) as they propagate quasi-
radially outward from the Sun covering a considerable angular extent.
Occasionally, the placement of different spacecraft in the heliosphere is
appropriate to measure in situ the passage of CMEs at different heliospheric
locations. The combination of the corresponding measurements by spacecraft at
different heliospheric locations is referred to as multipoint observations.
These multipoint observations allow a better understanding of the global and
local topology of the large-scale structures, as well as their evolution and
possible interaction with the surrounding medium, as well as with other solar
wind (SW) structures (e.g. Möstl et al., 2009; Winslow et al., 2016).

The interplanetary counterparts of CMEs are usually divided into
distinguishable parts sequentially observed in the following order (e.g.
Rouillard, 2011):

  * •

Shock. Recognised by abrupt plasma parameters increase (Krall, 1997).
Depending primarily on the CME speed relative to the upstream SW. In the case
of subsonic or sub-Alfvenic speeds the shock cannot form.

  * •

Sheath. Not all CMEs develop sheaths (see e.g. Liu et al., 2006; Salman et
al., 2020). The ambient SW piles up around the CME, compressing it, resulting
in an irregular and turbulent plasma and magnetic field. Typically, sheath
crossings last $\mathord{\sim}10$ hours at 1 au (Temmer & Bothmer, 2022).

  * •

Magnetic obstacle or ejecta. A region with an intense magnetic field and
commonly few plasma fluctuations. It can be a pre-existing flux rope (FR) or
be formed during the eruption of the CME (Zurbuchen & Richardson, 2006;
Richardson & Cane, 2010; Nieves-Chinchilla et al., 2019). It may also present
single or multiple FRs or none. If there is/are FR/s and show/s additional
properties like low plasma $\beta$ or bidirectional suprathermal electrons
(BDEs), it is often denominated as a magnetic cloud (MC, Burlaga et al., 1981;
Gosling et al., 1987).

  * •

Post-CME. After the magnetic obstacle transit, a loosely defined region
exists, with mixed characteristics from ambient SW and CME (Vršnak et al.,
2009). Signatures like BDEs can still be found (Carcaboso et al., 2020).
Duration at 1 au can last longer than the actual CME transit (Rodriguez et
al., 2016; Temmer et al., 2017; Carcaboso et al., 2020).

Previous studies such as Vourlidas et al. (2017) provide some statistics about
the properties of the CMEs and estimate that the average angular width of
these structures increases with the solar cycle, reaching
$\mathord{\sim}80\degree{}$ as maximum. There are also case-study articles
that broadened our knowledge about the complexity of the CMEs, often with the
combination of remote sensing observations and in situ measurements.
Sometimes, these large-scale structures are captured in situ by multiple
spacecraft separated in longitude, which helps to understand the global
topology and their inherent dynamics, as well as the interaction with the
surrounding SW or other structures. For example, the following studies sorted
in longitudinal angular separation analysed different events: Lugaz et al.
(2022), 55°; Liu et al. (2008), 74°; Rodríguez-García et al. (2021), 80°; Chen
et al. (2022), 90°, and Richardson et al. (2002) 160°. On those lines, Winslow
et al. (2022) describe the importance of the multipoint analysis to understand
these large-scale structures in the SW, while Scolini et al. (2023) theorises
about the amount of spatially-separated spacecraft needed for their complete
understanding and characterisation.

Often, the presence of coronal holes (CHs) surrounding the region where CMEs
originate, and the SW streams emanating from them (usually, high-speed SW
streams) may play a significant role in their propagation. For example, they
could act as ”magnetic walls” that hamper their propagation (Gopalswamy et
al., 2009). The interaction between CMEs and high-speed SW streams can lead to
significant changes in CME properties during their journey through the
heliosphere, e.g. forcing the internal structure of the CMEs to deform,
deflect, kink or rotate (Riley & Crooker, 2004; He et al., 2018; Chen et al.,
2019; Geyer, Paul et al., 2023). Also, high-speed streams could potentially
accelerate CMEs, shortening their propagation time from the Sun to Earth
(Cargill et al., 1996; Vršnak, 2001; Vršnak & Gopalswamy, 2002).

A very fast and wide CME erupted from the Sun on March 13, 2012. We discuss
the possibility that its interplanetary counterpart was detected in situ two
days later by spacecraft located close to Earth (particularly, by the European
Space Agency’s (ESA’s) Cluster (Escoubet et al., 2001), and the National
Aeronautics and Space Administration’s (NASA’s) Wind (Ogilvie & Desch, 1997;
Wilson III et al., 2021) and the Advanced Composition Explorer (ACE, Stone et
al., 1998)) as well as by the Ahead spacecraft of the NASA’s Solar Terrestrial
Relations Observatory (STEREO-A, Kaiser et al., 2008). We present a scenario
where Earth intercepted the east flank of the CME, whereas STEREO-A, separated
by 110° in longitude with respect to Earth, crossed the west flank. Despite
the large longitudinal separation between STEREO-A and Earth, measurements
from both positions showed similar in situ CME signatures, especially during
the passage of the leading and rear regions of the CME transit at both
locations. When the CME was released from the Sun, three CHs were observed
surrounding the active region (AR) where the CME originated, resulting in a
particular propagation behaviour of this structure towards the different
observatories.

This study aims to explain the propagation and topology of this extremely
broad CME released from the Sun on March 13, 2012, and its interaction with
the surrounding CHs, using both remote-sensing observations and in situ
measurements. Section 2 shows the evidence from the images provided by the
different observatories, and Section 3 shows the local SW properties during
the arrival and crossing of the CME. Section 4 describes the application of
different methodologies supporting the hypothesis that the observed structure
resulted from the same solar eruption and were in fact the same CME despite
the large longitudinal separation of the locations where the in situ
measurements were taken. Section 5 outlines a possible scenario consistent
with the observations. Finally, Section 6 summarises the work and Section 7
discusses the results implied by the scenario that reconstructs the
observations.

##  2 Remote-sensing Observations

The locations in the heliosphere of both STEREO spacecraft (STEREO-A and
STEREO-B) and near-Earth spacecraft in March 2012 enabled remote-sensing
observations that covered the entire Sun in longitude, and thus provided a
perfect opportunity to analyse CME structures propagating away from the Sun.
The orbital configuration of the spacecraft can be seen in Figure 1, whereas
the coordinates of the spacecraft used in this study, are listed in Table 1.
The instruments used for the remote-sensing observations are listed in
Appendix A.

Figure 1: Orbital configuration in HEE coordinates of the analysed spacecraft during the event. Black arrow shows the longitude of the AR at the moment of the eruption. Earth’s magnetospheric bow shock (orange) and magnetopause (purple) are calculated as in Ipavich et al. (1981) with a speed of 500 km/s (black line indicates the bow shock without the influence of the SW). Crosses in Earth’s perspective indicate the Lagrange points. Earth’s radius is scaled by a factor of 4. Sun not to scale. Note the x, y, and z grid sizes are different in some of the panels.  s/c |  $\phi$ [deg] |  $\theta$ [deg] | r [au]
---|---|---|---
STEREO-B | -118.27 $\pm$ 0.12 | 0.29 $\pm$ 0.00 | 1.02
Wind | -0.24 $\pm$ 0.00 | 0.00 $\pm$ 0.01 | 0.99
Cluster | 0.01 $\pm$ 0.02 | -0.02 $\pm$ 0.02 | 1.00
ACE | 0.10 $\pm$ 0.00 | -0.05 $\pm$ 0.00 | 0.99
STEREO-A | 110.12 $\pm$ 0.21 | 0.10 $\pm$ 0.00 | 0.96
Table 1: Mean orbital position of the spacecraft in Heliocentric Earth-
Ecliptic (HEE) coordinate system during the dates 2012-03-14 and 2012-03-20
sorted by longitude, where $\phi$ is longitude, $\theta$ is latitude, and r is
heliocentric radial distance. Uncertainties correspond to the maximum
variation during that period (r did not vary significantly).

In the following subsections, the solar-corona conditions and part of the
propagation of the CME captured by the remote-sensing instruments are
described.

###  2.1 Solar Flare and Coronal Mass Ejection

The CME under analysis occurred during the rising phase to the solar maximum
of solar cycle 24, with multiple events occurring hours before and after (e.g.
Tsurutani et al., 2014; Su et al., 2015). Noteworthy is the CMEs on March 7,
which erupted from the same AR as the CME under analysis and caused a
widespread solar energetic particle (SEP) event that has been analysed in
detail from different approaches (e.g. Lario et al., 2013; Patsourakos et al.,
2016; Feng et al., 2020; Hudson et al., 2023; Soni et al., 2023).

Preceding the main CME analysed in this paper, there is a small event seen in
STEREO-A/EUVI that happened on March 13 at approximately 15:34 UT from AR11430
located at N22W81 as seen from Earth (catalogued by the National Oceanic and
Atmospheric Administration, NOAA, Space Weather Prediction Center, SWPC). This
small previous eruption might have triggered the flare and CME eruption under
study in this paper due to global instabilities related to large eruptions
(e.g., waves, magnetic pressure change, etc.). The full coverage in longitude
of the Sun provided by the different spacecraft thanks to their orbital
position allows us to confirm that there were no other potential CMEs in the
direction of Earth or STEREO-A in a period of 24 hours prior to and after the
occurrence of the main CME studied here.

The main CME under study is related to an M7.9 X-ray solar flare from AR11429
at N18W64 (NOAA/SWPC) with the onset of the soft X-ray emission on March 13 at
17:12 UT. The longitude of this AR is indicated by the black arrow in the top
left panel of Figure 1. The CME was first observed by SOHO/LASCO/C2 at 17:36
UT, and by both STEREO-A and STEREO-B COR1 at $\mathord{\sim}\text{17:30}$ UT
(see Section 4.1 for more details). The AR configuration was classified as a
complex $\beta\gamma\delta$ (see Jaeggli & Norton, 2016, and references
therein for a detailed explanation). Prior to this event, the AR was very
dynamic, with multiple CME eruptions (see e.g. Liu et al., 2013, 2014; Dhakal
et al., 2020)111Images of the event provided by SDO/AIA can be found under the
following link:
https://sdowww.lmsal.com/sdomedia/ssw/media/ssw/ssw_client/data/ssw_service_120313_105302_91045/www.

Palmerio et al. (2018) determined that the CME on March 13 had a left-handed
helicity as derived from different multiwavelength proxies (i.e. magnetic
tongues X-ray/EUV sigmoids, the skew of coronal arcades, flare ribbons, and
filament details. See also Palmerio et al. (2017) and references therein).
Harker & Pevtsov (2013) describe in detail the magnetic conditions during the
eruption, and the appearance of a magnetic transient during the flare.

The CME was observed from different white light coronagraphs, appearing as a
backside halo CME from STEREO-B’s perspective, i.e. moving in the opposite
direction of the spacecraft with a main northward propagation. STEREO-A and
SOHO observed the CME propagating in the northeast and northwest directions,
respectively. STEREO-A/HI and STEREO-B/HI data provided valuable observations
of the eastern and western flanks of the wide CME during its heliospheric
propagation. The first observation of the structure in STEREO-A/HI and
STEREO-B/HI occurred at 18:49 UT and 19:29 UT on March 13, respectively, and
was catalogued as HCME_B__20120313_01 and HCME_A__20120313_01 by the
Heliospheric Cataloguing, Analysis and Techniques Service
(HELCATS)222https://www.helcats-fp7.eu/catalogues/data/HCME_WP2_V06.txt. As
seen by STEREO-B/HI, the northern position angle of the CME span was ¿325°,
while the southern was 210°. By March 14 at 07:29 UT, the CME’s propagating
structure showed some distortion on its leading edge, particularly in the
northern section, as indicated in the right panel of Figure 2.

Figure 2: Running difference image from STEREO-B/HI observed on March 14 at
07:29 UT. White line at the right panel indicates approximately the leading
edge of the CME. Captured planet (brightest point) corresponds to Mercury.
Adapted image from HELCATS.

As a first estimate, extracted from the propagation directions in the
coronagraphs and STEREO/HI observations, the CME propagated in the region
between Earth and STEREO-A. As reported by HELCATS, the shock speed derived
from the Jmap (Davies et al., 2009) is around 1050 $\pm$ 450 km/s. HELCATS
also estimates an arrival time at Earth on March 15 at 12:28 UT and with a
speed of 850 km/s, which differs by 50 km/s for the speed and
$\mathord{\sim}35$ minutes in advance of the actual arrival time (see ACE,
Wind and Cluster observations in Section 3 and Table 2 below for more detailed
information).

###  2.2 Coronal Holes Overview

Three CHs, apart from the northern polar CH, were located surrounding the
parent AR. The properties of the CHs were analysed using the program
Collection of Analysis Tools for Coronal Holes (CATCH, Heinemann et al.,
2019b). In Figure 3, their locations with respect to the parent AR (indicated
by a pink circle) can be seen. The East one (CH1, red) had inward (negative)
magnetic polarity, while the southwest (CH2, green) and west (CH3, blue) CHs
had outward (positive) magnetic polarity. At the moment of the eruption, each
CH had the following properties:

  * •

CH1 was the largest among the three, covering a total area of
$\mathord{\sim}12\times 10^{10}\leavevmode\nobreak\ {km}^{2}$, with its centre
of mass at approximately -3° in latitude and -3° in longitude (Stonyhurst
coordinates. Please note that the reference system is different from the one
shown in Figure 3). This CH has been studied in more detail, providing
information on its evolution and properties by Heinemann et al. (2018a, b).

  * •

CH2 covered an area of $\mathord{\sim}4.5\times 10^{10}\leavevmode\nobreak\
km^{2}$, and it is the furthest one with respect to the AR, with its centre of
mass located at -36° (latitude), 99° (longitude).

  * •

CH3 is the smallest CH, covering an area of $\mathord{\sim}3\times
10^{10}\leavevmode\nobreak\ km^{2}$ at the moment of the eruption. Its centre
of mass was located at 29° (latitude), 132° (longitude).

Figure 3: Projection of a composed image from SDO/AIA 193 Å, STEREO-A and
STEREO-B EUVI 195 Å at the moment of the solar flare preceding the CME
release. The axes correspond to latitude and longitude as seen from STEREO-A
in the ecliptic plane. Pink ellipse shows the AR, while the red, blue, and
green boxes indicate the surrounding CHs. Stars represent the projection of
each location (green, Earth; red, STEREO-A; blue, STEREO-B). See text for more
details.

EUV images also show the presence of a diffused region surrounding CH2 and
CH3. Due to this reason and the fact that both of them share the same
polarity, it could be argued that these two CHs are darker regions within the
same large and less-defined CH.

The presence of a low coronal shock wave produced by the CME (see Section 2.1)
that clearly interacted with the CHs can be easily observed in the SDO/AIA
multi-wavelength composed
image333https://suntoday.lmsal.com/sdomedia/SunInTime/2012/03/13/AIAtriratio-211-193-171-2012-03-13T1200.mov.mp4.
CH1 acted as a barrier hampering the propagation of the shock to further
eastern distances, and the boundaries of them are reflected in the video.
Figure 4 shows a snapshot of that moment at 17:53 UT on March 13, when the EUV
wave reflection with CH1 is clearly observed (the red arrows mark the
boundaries of CH1, while the pink arrow marks the AR site where the flare
occurred). Due to the clear features of this EUV wave and its size, although
beyond the scope of this study, a more detailed analysis of the remote-sensing
observations of this event in the lower corona is encouraged.

Figure 4: Composed running difference image from SDO/AIA 211 Å, 193 Å, and 171
Å observed on March 13 at 17:52:37 UT. Red arrows indicate the appearance of
the boundary of CH1 revealed by the interaction with the CME shock wave. The
pink arrow points to the AR where the CME originated.

##  3 In situ Observations

Two days after the eruption of the CME, a shock associated with a CME was
detected in situ by STEREO-A and by near-Earth spacecraft with a relatively
short time difference. At that time, STEREO-A was located at 0.96 au from the
Sun and separated 110° west from Earth (see Figure 1). In the following
subsections, the interplanetary context and the plasma properties during the
transit of the relative crossings measured at the different locations are
described. The in situ instruments utilised for this purpose are listed in
Appendix A. All the represented data have been acquired from the NASA’s
Coordinated Data Analysis Web (CDAWeb444https://cdaweb.gsfc.nasa.gov), part of
the Space Physics Data Facility (SPDF).

###  3.1 Near-Earth Observations

Earth’s surrounding ambient SW properties were measured by Wind, ACE and the
four spacecraft of the Cluster mission. All these spacecraft detected similar
time profiles of the magnetic field and plasma parameters (with the exception
of Cluster’s magnetospheric crossing). Figure 5 shows the in situ plasma and
magnetic field parameters observed by ACE555During this period, SWEPAM only
provided SW speed. SW density and temperature are obtained from SWICS, whereas
the total pressure and the plasma $\beta$ parameter are computed using SWICS
and ACE/MAG data. (left) and Wind666SW plasma parameters are obtained from 3DP
(orange) and SWE (blue) data resampled to 1-minute cadence. 3DP density was
multiplied by a factor of 5 to be comparable with the other sensors. Due to
the low values of density as measured from 3DP, temperature measurements are
inaccurate. For this reason, temperature, density and their derivatives from
3DP have not been used for the analysis. (right) between March 14 (day of year
74) and March 20 (day of year 80), 2012. The panels show from top to bottom:
SW bulk speed, proton density, kinetic proton temperature (blue/orange)
together with an empirically predicted temperature (yellow) based on the
proton speed as explained in Lopez (1987) and Elliott et al. (2012), magnetic
field strength, its vector components in the spacecraft-centred radial-
tangential-normal (RTN) coordinates, the azimuthal angle in the RTN coordinate
system (deg) complemented with the two possible nominal Parker spiral angles
calculated from the proton speed and accompanied by its polarity (red,
negative; green, positive; yellow, ambiguous)777Nominal Parker spiral angle in
the ecliptic plane is calculated as $\phi=arctan(\Omega\cdot r/V_{sw})$, being
$\Omega=2.87\times 10^{-6}s^{-1}$. The polarities are determined with a span
of $\pm$60° with respect to the theoretical vectors., interplanetary magnetic
field latitudinal angle in the RTN coordinate system, total pressure888Total
pressure is calculated as the addition of the plasma thermal pressure
($P_{g}$) plus the magnetic field pressure ($P_{B}$). $P_{g}$ is computed as
$P_{g}=N_{p}KT_{p}+N_{e}KT_{e}+N_{He}KT_{He}$, where $p\equiv$ protons,
$e\equiv$ electrons, $He\equiv$ helium; $N$ corresponds to density, $K$ to the
Boltzmann constant, and $T$ to the temperature. $T_{e}$ and an alpha/proton
ratio are considered constant with a value of 140,000 K and 0.04 respectively
(Bürgi, 1992; Newbury, 1996; Mullan & Smith, 2006). $P_{B}$ is computed as
${P}_{B}=B^{2}\,/\,2\mu_{0}$, being $B$ the magnetic field strength, and
$\mu_{0}$ the vacuum magnetic permeability., and plasma $\beta$.

The shaded areas indicate different regions based on identifiable bulk speed
features, which are associated with different conditions of the SW during the
passage of the CME and which may differ from the four distinguishable parts
explained in Section 1 (see Section 4.3 and Table 2 for more details). The SW
conditions preceding the arrival of the CME show no remarkable features, with
a SW speed of $\mathord{\sim}500$ km/s, low density, and constant negative
(inward) polarity, almost following the nominal Parker spiral. The vertical
black dashed line indicates the passage of the shock associated with the CME.
The shock reaches ACE at 12:31 UT on March 15 (day of year 75), similar to
Wind’s time (12:33 UT). The shock parameters are estimated solving numerically
the Rankine-Hugoniot relations (e.g. Viñas & Scudder, 1986; Koval & Szabo,
2008) using the Space Plasma Missions IDL Software
Library999https://doi.org/10.5281/zenodo.6141586 (Wilson III, 2022). For Wind,
the analysis found a quasi-perpendicular shock (the angle between the upstream
magnetic field and the normal to the shock, $\theta_{B}{}_{N}$, was 76.2°
$\pm$ 0.7°) with a normal vector ¡-0.771, -0.538, -0.340¿ $\pm$ ¡0.007, 0.005,
0.008¿ in GSE coordinates (uncertainties are due to statistical errors only;
they do not include systematic errors or proper measurement errors), and a
fast magnetosonic Mach number of $\mathord{\sim}3.1$.

After the MC, a region with more or less stable SW (with a speed of around 600
km/s) can be found. This region covering the periods #4, #5, and #6 in Figure
5 shows higher temperature and lower magnetic field strength (and
consequently, higher plasma $\beta$) than the MC. The period is divided into
three different parts based on the trend of the SW speed. However, the
suprathermal electron pitch-angle distributions (PADs. See Figure 11 in
Section 4.3.1) show a faint patchy presence of BDEs. This region is more
likely to be part of the ambient SW, although there are features that may be
related to the MC. According to Tsurutani et al. (2014), these regions
correspond to a high-speed stream following the CME which would have ended in
region #3.

Two additional regions (#7, #8) correspond to another expanding regime of the
SW, with low magnetic field, low proton density, and a variable magnetic field
with a predominant negative polarity. The expansion in these two regions is
more pronounced than the one in the previous region #3.

Finally, in contrast to regions #1 through #8, region A corresponds to a
period for which no correspondence in STEREO-A observations was found (see
Section 3.2). Nevertheless, this region seems to be still part of the
expanding structure at Earth and to be interacting with a glancing cross of a
SW stream interaction region (SIR) at the end, reduced by the interaction of
fast SW generated from likely CH1 and SW left behind the CME, as discussed
below. The different parts of this transit are analysed in detail and compared
to other spacecraft observations in Section 4.3.

It is also noteworthy the presence of a previous interplanetary CME crossing
Earth (not shown) from March 12 at 08:28 UT to March 14 at 02:52 UT, according
to the Wind’s in situ CME catalogue (Nieves-Chinchilla et al., 2018). This CME
may have influenced the upstream conditions of the SW that the CME under study
encountered as discussed in Section 4.

Figure 5: ACE and Wind observations during the period from 2012-03-14 to
2012-03-20. From top to bottom: SW proton speed, proton density, proton
temperature, magnetic field magnitude, magnetic field RTN components, magnetic
field azimuthal angle in the RTN coordinate system complemented with the two
possible nominal Parker spiral angles (red, negative; green, positive)
calculated from the proton speed and accompanied by its polarity (red,
negative; green, positive; yellow, ambiguous), magnetic field latitudinal
angle in the RTN coordinate system, total pressure, and plasma $\beta$.

The four Cluster spacecraft were out of the magnetosphere during the arrival
of the CME (not shown). All four spacecraft detected the shock on March 15 at
$\mathord{\sim}13$:08 UT (in an interval of 4.4 seconds between the first and
last observer, 13:07:58.550 UT and 13:08:02.950 UT). Similarly to Wind and
ACE, prior to the shock arrival, the SW had negative polarity with a constant
magnetic field strength of approximately 6.5 nT. Again, the sheath of the CME
could be subdivided into two main parts: a very variable and weak magnetic
field (#1), and a stronger but rapidly decreasing field with with a smoother
pointing direction (#2). The following region (#3) displays fluctuating
magnetic field, but with a clear tendency to increase in strength, until the
arrival to the Earth’s bow shock at 21:53 UT when the four spacecraft
reentered the magnetosphere. The different regions are analysed further in
Section 4.

The passage of the CME through the Earth’s environment caused a geomagnetic
storm with Kp ¿ 5 between 12:00 UT and 21:00 UT on March 15 and reached a
minimum equatorial Dst value of -88 nT at 21:00 UT (not shown), while crossing
the strongest values of the negative $B_{N}$ component observed during the
passage of region #2. Then, the Kp index kept a value between 3 and 4 until
the first half of March 18, while the Dst was approximately on average -47 nT,
and a maximum value of -28 nT for the same period. A Kp = 4 and Dst of
$\mathord{\sim}\text{-40}$ nT were reached between 03:00 and 07:00 UT on March
19, coinciding with the passage of the SIR partially covered in region A (see
Figure 5).

###  3.2 STEREO-A

The crossing of a CME through STEREO-A on the same day (March 15) is also
recorded in different in situ CME catalogues (see for example the
STEREO/IMPACT catalogue of CMEs101010https://stereo-
ssc.nascom.nasa.gov/data/ins_data/impact/level3/ICMEs.pdf, listed as the
number 8 of year 2012). The criteria for selecting in situ CMEs on that
particular catalogue can be found in a series of publications by Jian et al.
(2006b, 2013, 2018). Figure 6 displays, using the same format as Figure 5, the
STEREO-A in situ measurements for that period.

The black-dashed vertical line at 22:33 UT on March 15 (day of year 75)
indicates the passage of an interplanetary shock. Following the same procedure
used for Wind (see Section 3.1), the orientation of the shock normal was
obtained: ¡-0.079, -0.997, -0.002¿ $\pm$ ¡0.026, 0.002, 0.017¿ (in GSE
coordinates to be directly comparable to the shock normal estimate calculated
using Wind data). $\theta_{B}{}_{N}$ was very similar to the one measured at
Wind (66.5° $\pm$ 3.7°, i.e. a quasi-perpendicular shock), and the
magnetosonic Mach number was $\mathord{\sim}2.1$. We note that in contrast to
near-Earth observations, the shock normal at STEREO-A was mostly in the
ecliptic plane with a predominant western direction, whereas at Wind was
mostly southwestern directed.

The shock is preceded by a relatively high-speed stream of $\mathord{\sim}470$
km/s with positive polarity (green horizontal line in the eighth panel of
Figure 6), and previous to that, a heliospheric current sheet crossing at
21:30 UT on March 14 (day of the year 74). The increase of SW speed observed
early on March 15 preceded by a period of elevated SW density and magnetic
field magnitude late on March 14 can be interpreted as the arrival of a SIR,
which presumably originated from CH2 or CH3, as their longitude and polarity
agree with the observed features. According to the Predictive Science Inc.
(PSI) magnetic back-mapping
results111111https://www.predsci.com/hmi/spacecraft_mapping.php (not shown),
STEREO-A is more likely connected to the northern hemisphere on March 15,
where CH3 is located. The period with elevated magnetic field and density is
also reported in the STEREO/IMPACT catalogue of
SIRs121212https://stereodata.nascom.nasa.gov/pub/ins_data/impact/level3/STEREO_Level3_SIR.pdf
See Jian et al. (2006a, 2019) for the selection criteria. (shaded in blue).
The time of the stream interface passage according to the catalogue is marked
by the vertical dashed grey line in Figure 6 within that area, and corresponds
to the highest pressure during the passage.

Similarly to the observations at Earth, the sheath (region #1) has a variable
but intense magnetic field strength, and high proton density and temperature,
and it is followed by another region with an even more intense and smoother
magnetic field magnitude, and less dense too (region #2). Both of them show
elevated temperatures and have a high plasma $\beta$. Right after them, an
expanding region (#3) with low temperatures and constant magnetic field
strength (around 7 nT) can be found. Later on, a period with more or less
constant SW speed followed the previous regions, with even lower proton
density on average (corresponding to regions #4, #5, #6). These regions are
even colder and less dense than the previous ones, their magnetic field is
decreasing in strength and is pointing with an inclination of
$\mathord{\sim}45\degree{}$ with respect to the ecliptic with a main positive
(outward) polarity. Consequently, the plasma $\beta$ in region #6 is lower
than the surroundings.

The last part (regions #7, #8) is another expanding regime of the SW, with a
very weak magnetic field and similar plasma properties as the previous
regions, which draws a higher plasma $\beta$. The magnetic field is weak but
smooth, going from a south-pointing direction to the nominal behaviour of the
Parker spiral within the ecliptic plane.

A few hours after this last region (on March 18 at $\mathord{\sim}$19:30 UT),
another interplanetary shock produced by a different CME arrives at the
spacecraft (not indicated in Figure 6).

Figure 6: In situ observations from STEREO-A (2012-03-14 to 2012-03-20). Blue-
shaded area corresponds to the catalogued SIR (see text for more details).
Figure follows the same panel format as in Figure 5.

A more detailed study of the remote-sensing observations and in situ
measurements is performed in the following sections aiming to explain the most
probable scenario providing evidence that the structure intercepted at both
locations with $\mathord{\sim}110\degree{}$ of longitudinal separations
corresponds to the same CME that erupted from the Sun on March 13.

###  3.3 Other locations

We have considered other missions that were also located in the trajectory of
the CME or at least in its vicinity and may have been potentially impacted.
Nevertheless, they are not shown in the present article due to the complexity
of the analysis of their data in addition to evolutionary processes which are
out of the scope of the paper.

As a note, one of them was the NASA’s Mercury Surface, Space Environment,
Geochemistry and Ranging (MESSENGER) mission (Solomon et al., 2001) which was
orbiting about Mercury. MESSENGER magnetometer detected the passage of an
interplanetary shock at 07:08 UT on March 14 with clear signatures of the
passage of a CME at the moment that MESSENGER was in the SW (i.e. outside of
Mercury’s magnetosphere). At that moment, the spacecraft was located at -31.2°
in longitude, 0.4° in latitude and 0.34 au (HEE). Owing to the absence of
previous solar events, the observing time ($\mathord{\sim}8.5$ hours after the
eruption) and the MESSENGER location, we suggest that the observed structure
corresponds to the same CME under study. Nevertheless, the in situ CME has not
been catalogued in previous studies (e.g. Winslow et al., 2015). Note that the
longitudinal separation between the site of the parent eruption and the
MESSENGER spacecraft was $\mathord{\sim}85\degree{}$, which is not
inconsistent with the typical width of CME-driven shocks in the interplanetary
space (e.g. de Lucas et al., 2011).

##  4 Data Analysis

This section focuses on the morphology and dynamics of the CME from its solar
origin to the in situ interplanetary measurements, assuming that the in situ
measured structure at Earth and STEREO-A, with a relative separation larger
than 100°, corresponds to the same CME. We also show that the structure
deflected from an initial considerable out-of-the-ecliptic inclination, to be
observed in the ecliptic plane with that separation.

###  4.1 Reconstruction of the CME near the Sun

For the reconstruction of the size, orientation and kinematics of the
structure, the graduated cylindrical shell (GCS) geometrical model has been
used (Thernisien, 2011) utilising the Python-based PyThea tool (Kouloumvakos
et al., 2022). This model assumes a structure similar to a croissant in shape
composed of two cones and part of a torus, where different parameters can be
adjusted to reconstruct the coronagraph observations, and thus reproduce the
leading edge of the structure and obtain, among others, its 3D shape and
orientation. A snapshot of the fits applied simultaneously to STEREO-A,
STEREO-B and SOHO coronagraph observations is shown in the top row of Figure
7. The bottom panel of Figure 7 shows the derived height of the leading edge
of the GCS model (i.e. apex) as a function of time, providing a speed of 1982
$\pm$ 75 km/s when fitted to a first-order function, similar to the one listed
in the SOHO/LASCO catalogue (1884–2054
km/s)131313https://cdaw.gsfc.nasa.gov/CME_list/UNIVERSAL/2012_03/univ2012_03.html
. The two values 1884 km/s and 2054 km/s correspond to a linear and second-
order fit to the leading edge of the CME (position angle 286°) versus time as
seen in plane-of-sky LASCO/C2 and LASCO/C3 observations.. The GCS results show
an average orientation of $\mathord{\sim}18\degree{}$ in latitude and
$\mathord{\sim}55\degree{}$ in longitude in Stonyhurst coordinates, a tilt
angle of approximately -45° measured counterclockwise positive from the Earth-
Sun direction. A similar orientation was obtained in the study Palmerio et al.
(2018). Our GCS reconstruction is consistent in longitude and latitude to the
AR location on the Sun (N18W64). Also, despite the observed interactions in
the lower corona with the surrounding CHs (see Section 2.1) and the high-speed
streams coming from them, the direction and orientation of the CME did not
considerably change in the coronagraphs’ field of view.

Figure 7: Top: Running difference images provided by STEREO-B/COR2 (left),
SOHO/LASCO/C2 and STEREO-A/COR2 (right) on 2012-03-13 at $\mathord{\sim}18:24$
UT. GCS reconstruction is shown in green. See text for more details. Bottom:
Height of the apex as a function of time along with linear fit to the data.

###  4.2 CME Propagation

Taking into account the arrival time of the shock at both locations (see
Section 3) and that the eruption appeared on the coronagraphs at
$\mathord{\sim}\text{17:40}$ UT on March 13 as indicated by the onset of the
soft X-rays, the transit time of the CME to reach Earth is
$\mathord{\sim}42.96$ hours, while for STEREO-A it takes $\mathord{\sim}52.88$
hours (both times have been computed considering that the eruption started at
the onset time of the soft X-ray emission of the associated flare). Also,
according to the GCS reconstruction (see Section 4.1), the CME centre (i.e.
apex) was propagating $\mathord{\sim}55\degree{}$ apart from both Earth and
STEREO-A. In order to have an estimation of the drag undergone by the CME
while propagating in the interplanetary medium, the 1D Drag-Based Model
(DBM)141414Available at https://oh.geof.unizg.hr has been used (Vršnak et al.,
2013; Dumbović et al., 2021), which provides a drag parameter ($\gamma$) based
on the propagation time. By using the in situ measured SW speed, and assuming
that the speed of the structure was the same in both directions and similar to
the apex ($\mathord{\sim}2000$ km/s) , $\gamma$ obtained for Earth is
$0.3753\times 10^{7}km^{-1}$, while for STEREO-A is $0.5464\times
10^{7}km^{-1}$, suggesting that the structure underwent more drag
($\mathord{\sim}46\%$ more) during its propagation towards STEREO-A than along
the Sun-Earth line.

There are two distinct processes that may explain the difference in drag
although the CME propagated with similar longitudinal separation with respect
to both spacecraft. These are the swept ambient SW in the Sun-Earth line due
to a previous CME (see Section 3) and the presence of the SIRs produced by CH2
and CH3. The less-dense conditions left behind the previous CME allow a faster
propagation of the CME under study towards Earth (Liu et al., 2014; Temmer &
Nitta, 2015), while the presence of SIRs hampers the propagation in STEREO-A
direction (Gopalswamy et al., 2009). This can also be appreciated in the
higher proton temperature at STEREO-A than at Earth during the first part of
the structure, and the difference in density in the upstream SW
($\mathord{\sim}5\leavevmode\nobreak\ cm^{-3}$ for Earth,
$\mathord{\sim}8\leavevmode\nobreak\ cm^{-3}$ for STEREO-A), but especially
during the transit of the sheath ($\mathord{\sim}9.5\leavevmode\nobreak\
cm^{-3}$ for Earth, $\mathord{\sim}27\leavevmode\nobreak\ cm^{-3}$ for
STEREO-A. See Figure 9), which is more representative of the conditions that
the CME experienced during its evolution through the interplanetary medium.

###  4.3 Longitudinal SW Comparison

There is clear evidence suggesting that the plasma measurements at both
locations (i.e. at STEREO-A and near-Earth) correspond to different parts of
the same structure. In order to prove this, in this Section we perform a more
detailed comparison of the plasma properties measured at the subdivided
regions of the passage of the CME at both locations.

A timeline of the identified crossed regions is shown in Figure 8. Table 2
describes the different regions as well as lists the time interval when they
were observed, the duration of their passage, and the comparison of the time
span with respect to Wind observations. It should be noted that, as described
in Section 3, those regions do not correspond one-to-one to the traditional
identification of the different parts of an in situ measured CME (see Section
1), but they are purely based on the SW speed tendency.

As mentioned above, the bulk SW speed profile shows extremely similar
behaviours, although there is an almost-systematic offset of
$\mathord{\sim}200$ km/s less in the case of STEREO-A, most likely produced by
the interaction of the CME with the preceding SIR (see e.g. Heinemann et al.,
2019a, for a similar case). The different regions have been defined based on
the discontinuities and changes of tendency of this plasma property. A brief
summary of the particular features observed in each region is also described
at the bottom of Table 2.

Figure 8: Timeline of the different identified crossed regions (see Section 3
for the definition of the regions) for each spacecraft. Each colour represents
each of them (see text for more details). Note that Cluster did not observe
the complete sequence of regions due to its entry into the magnetosphere (grey
areas)

s/c | Region |  | Start (March 2012)
---
(day HH:MM UT)
| End (March 2012)
---
(day HH:MM UT)
| Duration
---
(minutes)
|  $\Delta t$ wrt
---
Wind (minutes)
Wind | #1 | 15 12:34 | 15 15:48 | 194 | -
#2 | 15 15:48 | 15 20:46 | 298 | -
#3 | 15 20:46 | 16 09:14 | 748 | -
#4 | 16 09:14 | 16 14:21 | 308 | -
#5 | 16 14:21 | 16 23:52 | 570 | -
#6 | 16 23:52 | 17 21:57 | 1325 | -
#7 | 17 21:57 | 18 03:13 | 316 | -
#8 | 18 03:13 | 18 11:08 | 475 | -
A | 18 11:08 | 19 03:30 | 982 | -
ACE | #1 | 15 12:32 | 15 15:40 | 188 | -6
#2 | 15 15:40 | 15 21:03 | 323 | +25
#3 | 15 21:03 | 16 08:12 | 669 | -79
#4 | 16 08:12 | 16 14:27 | 375 | +67
#5 | 16 14:27 | 16 22:32 | 485 | -85
#6 | 16 22:32 | 17 22:00 | 1408 | +83
#7 | 17 22:00 | 18 03:11 | 311 | -5
#8 | 18 03:11 | 18 10:19 | 428 | -47
A | 18 10:19 | 19 03:09 | 1010 | +28
Cluster 1 | Magnetosphere | ¡15 00:00 | 15 04:12 | - | -
#1 | 15 13:08 | 15 16:27 | 199 | +5
#2 | 15 16:27 | 15 21:53 | 326 | +28
Interaction | 15 21:53 | 16 05:09 | 436 | -
Magnetosphere | 16 05:09 | 17 00:15 | 1146 | -
Interaction | 17 00:15 | 17 20:04 | 1189 | -
#6 | 17 20:04 | 17 21:50 | 106* | -*
#7 | 17 21:50 | 18 03:32 | 342 | 26
Interaction | 18 03:32 | 18 08:24 | 291 | -
| Magnetosphere | 18 08:24 | 19 06:54 | 1351 | -
STA | #1 | 15 22:29 | 16 00:44 | 135 | -59
#2 | 16 00:44 | 16 09:04 | 314 | -16
#3 | 16 09:04 | 16 16:48 | 463 | -350
#4 | 16 16:48 | 16 21:11 | 263 | +20
#5 | 16 21:11 | 16 23:56 | 165 | -405
#6 | 16 23:56 | 17 14:14 | 858 | -467
#7 | 17 14:14 | 17 20:54 | 400 | +84
#8 | 17 20:54 | 18 06:22 | 568 | -414

Region |  Description
---|---
#1 |  It starts with the shock and forms part of the sheath region.
#2 |  It is defined between a sudden enhancement of the bulk speed and the sudden depletion of this physical property. It is partially covering the sheath, and from the near-Earth observations, part of the magnetic obstacle of the structure too.
#3 |  Expanding region. It has a clear FR structure at Earth, but not at STEREO-A. For Earth, it can be considered as a magnetic cloud (Nieves-Chinchilla et al., 2019), while for STEREO-A is considered part of the sheath (Jian et al., 2018).
#4 |  Increasing speed from $\mathord{\sim}625$ to $\mathord{\sim}700$ km/s at Earth, and from $\mathord{\sim}425$ to $\mathord{\sim}500$ km/s at STEREO-A.
#5 |  The speed profile presents a ”u” shape or parabola behaviour.
#6 |  Speed remains more or less constant with a very fluctuating behaviour.
#7 |  It shows a gradual decrease of the speed with smooth behaviour for the three s/c, indicating an expansion. The magnetic field strength matches for all the observers.
#8 |  Expanding region.
A |  Expanding region ending with a bulk speed plateau that ends up in a SIR. (Observed only near Earth)
Interaction |  It is a mixture of the magnetopause and the transit of the in situ CME.

Table 2: Start, end, and duration of the different regions of the in situ
measured CME based on changes in the tendency of the SW speed for the
different spacecraft. Last column compares the duration of the different
regions at each spacecraft with respect to Wind.

*Comparison to Wind is not possible due to the previous interaction with the magnetosphere.

Considering these different regions, an analysis comparing the plasma
properties of the passage of the CME through each spacecraft has been
performed using normalisation of the time series to initial and end times of
observed structures for each region. Figure 9 shows from top to bottom: the
bulk SW speed, the components of the velocity in RTN coordinates ($V_{R}$,
$V_{T}$, $V_{N}$), the magnetic field magnitude, the components of the
magnetic field vector in RTN ($B_{R}$, $B_{T}$, $B_{N}$), and the proton SW
density. The red traces indicate STEREO-A observations; dark green, Wind
observations; olive green, ACE; and purple, Cluster-4. The SW speed and
magnetic field strength exhibit an extraordinary similarity at the different
locations and throughout all the regions despite the lower values (by about
$\mathord{\sim}200$ km/s) measured by STEREO-A, and some small differences in
the field and velocity components. We outline the following details when
comparing the measurements of the spacecraft:

  * •

The bulk SW speed obviously displays similar behaviour during all regions, as
this parameter was used to select the boundaries. However, a systematic offset
of $\mathord{\sim}200$ km/s between STEREO-A and Earth can be observed.

  * •

Most of the speed corresponds to the radial component, as it is the
predominant direction of the SW. In this case, the velocity in the T-N plane
would be representative of the lateral expansion of the structure (Owens &
Cargill, 2004). Regions #2, #3, #7 and #8 show similar behaviour of the
velocity components.

  * •

The magnetic field magnitude is very similar during the entire transit,
especially during regions #1, #2, #7 and #8.

  * •

The radial and tangential components of the magnetic field vector display both
similar values and tendency during region #2, and stable values with similar
behaviour during regions #7 and #8.

  * •

Regions #4, #5, #6 present fluctuating velocity components and less correlated
magnetic field strength and components. These differences, together with the
similarities of the surrounding areas, suggest that these regions (#4, #5, #6)
are under different plasma conditions. Also, it supports the idea that at both
locations (i.e. STEREO-A and Earth), the main structure is being crossed twice
(in, out, in. See Section 5 for more details).

  * •

In all regions, the proton density shows a clear difference between STEREO-A
and Wind, being almost systematically $\mathord{\sim}3$ times greater at
STEREO-A, most likely due to the interaction with the SIR, which also slows
down the CME propagation in the Sun–STEREO-A direction.

As shown in Figure 9, there are certain parts during the transit of the CME
through the spacecraft with such similarity that evinces the presence of the
same structure crossing STEREO-A and near-Earth observatories despite their
notable separation.

In order to quantify the similarity between the SW speed, velocity components,
magnetic field magnitude, magnetic field components, and SW density at
STEREO-A and near-Earth in each region of the CME passage, Figure 10 shows the
Pearson correlation coefficient between them computed for each region. The
calculation has been performed correlating the observations of each in situ
parameter from STEREO-A and near-Earth spacecraft. Then, the largest
correlation out of the three combinations (STEREO-A and Wind, STEREO-A and
ACE, or STEREO-A and Cluster) has been selected. Despite the long separation
between STEREO-A and near-Earth spacecraft, local particularities, and the
evolutionary processes undergone by the structure along the different
directions (apart from the instrumental differences and cadences), the speed
and magnetic field strength are correlated with a Pearson correlation
coefficient greater than 0.5 for most of the crossed regions. In particular,
the magnetic field displays a better correlation during the front (regions #1
and #2) and rear portions of the CME (regions #7 and #8), while the SW speed
does for regions #1, #3, #4, #7 and #8. Although the main property used for
selecting the boundaries of each region was the bulk speed, the correlation is
higher for the magnetic field. The above results suggest that the structure is
magnetically coherent despite the local plasma properties.

Apart from the difference in time between ACE and Wind observations of the
different features of the bulk speed profile (that can be more than one hour
apart. See Table 2), there is also a difference in the velocity components
despite their relatively small separation (see Figure 9). This may indicate
that the expansion of the structure (regions #3 and #8) is occurring
differently at both positions, probably due differences in the proximity of
both spacecraft to the central part of the expanding local plasma (i.e. the
impact parameter of both spacecraft; see e.g. Dasso et al., 2007; Démoulin et
al., 2008, for details). The quasi-stable values and the increment of the
tangential (T) and normal (N) components with respect to the radial (R) could
be also indicative of the existence of a transit through the leg of the
structure (as derived from Démoulin et al., 2008).

Figure 9: Comparison of the plasma and magnetic field properties through the
different regions. From top to bottom: plasma velocity (magnitude and RTN
components), magnetic field (magnitude and RTN components), and proton
density. X-axis corresponds to the cumulative percentage of data points of the
different regions. Each shade colour corresponds to the different regions
utilised in previous figures. All data are resampled to a 1-minute cadence.
See text for more details. Figure 10: Greatest Pearson correlation coefficient
values found between STEREO-A and near-Earth spacecraft observations for each
defined sector of the parameters shown in Figure 9.

Besides, considering the interaction of the structure with the locally
encountered upstream ambient SW, the piled-up plasma in the sheath (region #1)
presents a similar behaviour and an unexpectedly high correlation, considering
the completely different conditions of the SW that the structure suffered
while propagating to the observatories.

####  4.3.1 Suprathermal Electron Pitch-angle Distributions

Suprathermal electrons are a good tracer of the interplanetary magnetic field
topology and properties, especially when comparing their pitch-angle with
respect to the direction of the magnetic field. Figure 11 top (bottom) panel
shows the 272 eV (193.5–314.3 eV) PADs obtained from ACE/SWEPAM
(STEREO-A/SWEA) measurements from 00:00 UT on March 15 to 12:00 UT on March
19. Different colour blocks on top of each panel and dashed lines correspond
to the previously defined regions (see Section 4.3). The sporadic peaks (and
floor) are intentionally saturated, as their high (low) intensity smears the
visual contrast of the rest of the period.

Figure 11: Top: Suprathermal electron PADs (time series of the distribution
function, Fdist, with respect to the magnetic field angle) for the 272 eV
energy channel measured by ACE/SWEPAM from 2012-03-14 to 2012-03-19 at 12:00
UT. Bottom: In situ PADs of the suprathermal electrons from 193.5 to 314.3 eV
measured by STEREO-A/SWEA for the same period. Dashed lines and top colour
bars represent the different defined regions. See text for more details.

Focusing on the top panel, ACE observed a simple strahl coming from the anti-
parallel direction before the arrival of the CME. The width of the strahl
becomes broader while approaching the CME shock. Within region #1, the PAD is
nearly isotropic, which is a typical signature of the sheaths (e.g. Carcaboso
et al., 2020). Region #2 displays periods of BDE especially at the beginning
and the strahl electron width coming from 180° becomes larger until the end of
the period. This behaviour suggests there is a still linked-to-the-Sun tube
(i.e., a FR) of particles which is undergoing local disturbances that broaden
the strahl predominantly coming from 180° (see e.g. Horaites et al., 2018, for
potential sources of scattering). The reason underneath this observation may
result from a local feature within region #2 because the smoothness of the
magnetic field, in this overall expanding region, will cause the electrons to
focus more easily than in the ambient SW due to adiabatic expansion (Anderson
et al., 2012).

The top panel of Figure 11 shows that region #3 was characterised by clear
BDEs, with the electrons coming from 0° presenting larger width but less
intensity than those coming from 180°. The intensities of the electrons coming
from 0° decrease in region #4, and the width of those coming from 180°
increases. Region #5 has a similar behaviour to region #2 in the predominant
strahl. Region #6 shows sporadic periods of BDE and isotropy, and predominant
simple strahl, which suggests that there is a mix of open, closed, and
completely detached magnetic field lines. Overall, region #7 shows a simple
strahl narrower than in previous regions. Region #8 shows a weaker intensity
of those electrons coming from 180°. Also, it displays some intermittent
periods of BDE and isotropy. Finally, region A shows simple strahl with some
isotropic periods. At the end, the counterstreaming electrons are becoming
clearer due to the proximity of the upcoming SIR.

The whole transit of ACE through the structure presents periods of more or
less clear BDEs (sometimes longer, sometimes patchy behaviour), and
systematically, a strahl population with pitch angles
$\mathord{\sim}0\degree{}$ less intense than the population with pitch angles
$\mathord{\sim}180\degree{}$. This could be related to having a flank crossing
close to one leg of the structure, which apparently owns a preferential
negative (inward) direction (see examples in Kahler et al., 1996). In
addition, it agrees with the remotely-observed handedness (left) previously
mentioned in Section 2.1 and reported by Palmerio et al. (2018), having the
more intense strahl population in the opposite direction than the dominant
magnetic field. As discussed below, this is consistent with the spacecraft
crossing the east leg of the structure. On the other hand, the absence of BDEs
may be an indicator that only part of the CME remained magnetically closed,
whereas the other parts were eroded (possibly due to interaction with the fast
SW) or originally were open magnetic field lines (e.g. Winslow et al., 2016;
Carcaboso et al., 2020).

The bottom panel of Figure 11 shows STEREO-A observations. The blue-shaded
period marked in Figure 6 is also in the bottom panel of Figure 11, and
corresponds to the passage of a SIR that started to cross STEREO-A on March
14, as identified by Jian et al. (2019). The difference between the strahl
peak and the background is not as pronounced as it is during the passage of
the high-speed stream right before the arrival of the CME. The strahl
population of the suprathermal electrons follows the polarity of the nominal
Parker spiral (positive). Within the CME passage at STEREO-A, region #1
displays isotropic electron PADs, similarly to ACE; region #2 shows clear
BDEs, with some periods of isotropy; the boundary between region #2 and #3 is
completely isotropic, and region #3 shows a smeared strahl population,
reaching widths greater than 90° and reaching the complete isotropy by the
end, indicating the presence of magnetic in situ reconnections (e.g. Gosling
et al., 2005); region #4 shows a similar behaviour, but with clearer isotropy;
region #5 gradually shows greater BDEs, and regions #6 and #7 shows the
clearest BDE period, with higher intensities and strahl widths at 0° than
180°; finally, region #8 also shows BDE but with less gradient between peaks
and background.

The overall tendency of the electron PADs at STEREO-A during the transit of
the CME is the clear presence of BDEs with a predominant peak intensity
parallel to the magnetic field. Assuming that the magnetic field is
predominantly axial, and STEREO-A crossed closer to the west leg of the CME,
this behaviour supports the idea that the handedness derived from remote
sensing observations (left-handedness) previously reported by Palmerio et al.
(2018) also matches the one observed in situ. In this case, the peak intensity
in the BDEs could be also indicative of the proximity to the different legs of
the structure (Kahler et al., 1996). While STEREO-A observes a more intense
flux coming from the parallel direction (pitch-angle
$\mathord{\sim}0\degree{}$), near-Earth measurements show a predominant higher
peak with pitch angles at $\mathord{\sim}180\degree{}$ (anti-parallel to the
magnetic field).

Considering the longitudinal separation between STEREO-A and Earth, the
suprathermal electrons would take a longer time to travel from one location to
the other than the actual passage time of each defined CME region over each
spacecraft. As a reference, a completely field-aligned electron of 300 eV
would take approximately 13.5 hours to travel 1.62 au, which corresponds to
the absolute distance between STEREO-A and Earth (actual magnetic field path-
length would be longer in a FR structure, and the particle would have an
inherent gyroradius). Despite this fact, there are remarkable similarities in
their PADs at both locations, suggesting that the transport conditions that
those electrons may have experienced are similar regardless of having
travelled completely different paths. The different regions (purely defined by
the changes in the SW speed tendencies) are strongly linked to the behaviour
of the PADs.

##  5 Possible Scenario

Figure 12 sketches the envisaged scenario of the CME structure and the SW
conditions in the interplanetary medium during the event. This scenario is
based on the measurements and the performed analysis and represents the most
plausible reconstruction according to our assessment. However, other
alternative interpretations could exist. The top panel represents a view from
the north of the ecliptic and the bottom panel a view from the nose of the
structure directed between Earth (green sphere) and STEREO-A (red sphere).
Earth would cross the east flank of the CME $\mathord{\sim}10$ hours before
STEREO-A, which would cross the west flank. At the time Earth crosses the
shock and sheath of the structure, STEREO-A is crossing a high-speed stream
(indicated by the greenish tube in Figure 12) likely originated from CH3 (or
the combination of CH2 and CH3).

According to the GCS reconstructions, the CME originated
$\mathord{\sim}55\degree{}$ in longitude between both spacecraft whereas the
half angle of the structure was $\mathord{\sim}50\degree{}$. The structure was
also very inclined ($\mathord{\sim}45\degree{}$ with respect to the ecliptic
plane), so in order to cover a large longitudinal span over the ecliptic plane
later at 1 au, the structure had to widely expand in that direction, rotate
and/or bend to cover that space. Shock orientations (at Earth pointing
southwest, and at STEREO-A pointing west) indicate that the
deformation/rotation was very likely.

There are also indications that the portion of the CME directed toward
STEREO-A would have started to interact with the high-speed streams produced
by CH2 and CH3 at closer radial distances, such as the presence of higher
densities at STEREO-A (3 times greater than at Earth), the speed was
$\mathord{\sim}200$ km/s lower, as well as the duration of the passage
(shorter in the case of STEREO-A), suggesting that the structure was likely
more compressed on average in that portion of the CME.

Figure 12: Proposed scenario based on the observations. Spacecraft and
structures are located at the moment of the encounter with Wind. At that
moment, STEREO-A was found in the high-speed stream. The green and brown
traces represent the high-speed streams generated by CH2/CH3 and CH1,
respectively. Colour bars represent the radial cross through the conditions of
the SW following the same format as Figures 8 and 9.

The similarities of different plasma properties at the two locations (bulk SW
speed, behaviour of the velocity components, magnetic field strength, PADs…)
suggest that at least the three first regions (#1, #2, #3) and the two last
regions (#7, #8) correspond to the same structure, which matches with the CME
originated on March 13 at 17:12 UT, while regions #4 and #5 display mixed
signatures of ambient SW and magnetic obstacle, with evidence of local
reconnection as indicated by, among others, the observed suprathermal
electrons PADs. Additionally, region #6 in the case of Earth’s observations
does not show clear signatures of being part of the core of the structure,
while for STEREO-A, there is a clear inclined and smooth magnetic field, with
low temperatures, low plasma $\beta$ and BDEs, for which it could be
identified as a MC.

This scenario differs from what Tsurutani et al. (2014) found for this event
based only on near-Earth in situ observations (see Figure 5 in that article).
That study proposes that from region #4 to #8, the SW presents high-speed
stream signatures. From our observations, we did not find clear evidence for
the source of this fast SW, but the one produced by the CME itself.

##  6 Summary

A CME associated with a M7.9 flare was released on March 13, 2012, with a
speed of $\mathord{\sim}2000$ km/s, and an inclination of approximately 45°.
The CME started propagating outward from the Sun in a direction 55° east from
Earth, i.e. between Earth and STEREO-A which were separated by
$\mathord{\sim}110\degree{}$ apart from each other in longitude. The AR was
surrounded by three CHs. At least one of them interacted with the CME while it
propagated away from the Sun, hampering its propagation towards STEREO-A.

The GCS reconstruction and the derived kinematics seem to be consistent with
the arrival time of the CME at both locations and the in situ measurements at
$\mathord{\sim}1$ au. There is no evidence of the presence of other CMEs of
similar magnitude 24 hours prior to and after the CME, observed by LASCO and
SECCHI coronagraphs, which erupted on March 13, 2012 at $\mathord{\sim}17:30$
UT. This can be assured thanks to the orbital position of the two STEREOs and
Earth, which allowed a 360° visual coverage of the Sun.

The DBM model suggests that although it is the same structure, there is a
$\mathord{\sim}46{\%}$ more drag towards STEREO-A than towards Earth, most
likely because of two complementing effects: a previous CME towards Earth
swept the SW in that direction, and the presence of the high-speed streams
produced by CH2 and CH3.

The computation of the shock parameters presented indicates that the shock
normal vector at Wind location was pointing southwest and, at STEREO-A
location, it was contained in the ecliptic plane pointing west.

Despite the large longitudinal separation and initial inclination of the
structure, the bulk SW speed and magnetic field magnitude displayed almost
identical profiles on near-Earth observatories and STEREO-A, suggesting that
the observed CME is the same structure at both spatial locations. STEREO-A
transited the west flank, while Earth did it through the east one. The in situ
observations show opposite field polarity on STEREO-A compared to Wind/ACE
(negative for Earth’s and positive for STEREO-A’s), and the suprathermal
electron PAD during the BDE periods supports the in situ left-handedness,
matching the one reported by Palmerio et al. (2018) using remote sensing
observations. Also, the higher intensity of the strahl population at 0° on
STEREO-A, and at 180° on Earth’s observation, during the BDE periods could
indicate the proximity to the magnetic foot point of the CME.

As it can be derived from the timeline and the $\Delta{t}$ column in Table 2,
some parts of the structure seem to have transits of different duration
depending on the observer (even with the very-close-to-each-other Wind and
ACE). This has significant implications for our understanding of the CMEs, as
most of the studies are based on just single point observations.

Despite not having clear smooth magnetic field rotation in the STEREO-A region
#2, there are pronounced BDEs, similar to those observed at ACE, but with the
highest peak in the opposite direction (suggesting that both spacecraft
intercepted the flanks of the CME close to the respective legs, as mentioned
above). Assuming that the regions are part of the same structure and that no
remarkable evolutionary processes have happened (which may have resulted in a
different imprint in the PADs), the identification of what traditionally has
been called FR extends further than the actual helical magnetic field, which
would correspond to the magnetic obstacle or region #3.

A geomagnetic storm with Kp ¿ 5 was produced while Earth was crossing a strong
negative $B_{z}$ component. The geomagnetic effects (Kp $\mathord{\sim}4$)
lasted until the end of the passage of the structure by Earth. However,
previous catalogues reported a shorter in situ CME passage.

##  7 Conclusion and Discussion

The main conclusions of this work can be stated as follows:

  1. 1.

There is clear evidence of having the same CME observed by 110° separation
although its initial inclination was $\mathord{\sim}45\degree{}$. Based on
these observational facts, we propose a scenario (Section 5) in which an
originally highly inclined wide CME deformed and/or rotated in such a way that
it was observed near its legs by spacecraft in the ecliptic plane separated by
¿100° in longitude.

  2. 2.

The structure interacts with at least one of the high-speed streams produced
by the west CHs (CH2, CH3) surrounding the AR, which probably caused the
deformation and deceleration of the structure in the STEREO-A direction.

  3. 3.

The piled-up plasma observed in the sheath (region #1) has an unexpectedly
extraordinary correlation in the magnetic field strength at the two locations
separated by 110° in longitude. This plasma is supposedly a mixture of the CME
and the local ambient SW compressed downstream the shock. Despite the ambient
conditions at both locations were different, the similarities in the magnetic
field strength require a deeper analysis.

  4. 4.

The in situ observations reveal that at both locations each spacecraft crossed
the respective flanks of the CME, then ambient SW, and finally the legs’
proximity of the CME. In the case of STEREO-A, the cross through the leg is
more evident than for near-Earth spacecraft (See Figure 12, which sketches the
proposed scenario).

  5. 5.

We found no clear evidence for a potential source for a high-speed stream
crossing Earth during the period from March 16 to March 18 as suggested by
Tsurutani et al. (2014).

The similarities of the structure regardless of the large angular separation
are evidenced, in contrast to previous studies, such as the case-study event
analysed by Lugaz et al. (2022), where less clear correlations are found
despite the spacecraft locations observing the CME were closer to each other.
This shows that the factors that determine each particular observation (i.e.
how the spacecraft intercepts the CME, the morphology and evolutionary
processes of the CME, SW conditions, etc.) play the main role in the
interpretation of the observations and derived scientific results. On the
other side, having relatively close proximity does not assure similar
measurements. They may significantly differ more than one hour in the
definition of each region based on the bulk speed (e.g. see column $\Delta{t}$
for near-Earth spacecraft on Table 2). These differences play a very relevant
role in the scientific explanation, and thus in our understanding of the
large-scale structures, such as the CMEs are. Even with the extensive
catalogues available, the use of single spacecraft data during the transit of
the CMEs implies that the derived interpretations may suffer large
inconsistencies. Nevertheless, the use of the available multipoint
measurements helps, and it is even required, to support the different
encountered scenarios.

This case study opens up questions about our current view of the CME
longitudinal spread and its evolution in the interplanetary space, pointing
out the need to find more examples of widespread CMEs and analyse them. Such
analysis will help us understand the eruption, evolution and global topology
of CMEs, as well as improve our modelling efforts.

###### Acknowledgements.

The authors acknowledge the open policy of the data for the different
missions. We acknowledge use of NASA/GSFC’s Space Physics Data Facility’s
CDAWeb service. F. C. acknowledges the financial support by an appointment to
the NASA Postdoctoral Program at NASA Goddard Space Flight Center,
administered by Oak Ridge Associated Universities through a contract with NASA
and the support of the Solar Orbiter mission. FC and LKJ thank the support of
the STEREO mission. M.D. acknowledges the support by the Croatian Science
Foundation under the project IP-2020-02-9893 (ICOHOSS). RGH acknowledges the
financial support of the Spanish Ministerio de Ciencia Innovación y
Universidades Project PID2019-104863RBI00/AEI/10.13039/501100011033 and by the
European Union’s Horizon 2020 research and innovation program under grant
agreement No. 101004159 (SERPENTINE). SGH acknowledges funding by the Austrian
Science Fund (FWF): Erwin-Schrödinger fellowship J-4560. This work has been
possible thanks to the scientific discussions within the LASSOS-Goddard group.
F. C. thanks Benjamin Lynch, Angelos Vourlidas and Andreas Weiss for the
discussions, and also the referee for their feedback.

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##  Appendix A Missions and Instruments

The instruments utilised in this study are listed below. On the one hand, the
remote-sensing instrumentation listed by each mission was:

  * •

Solar Dynamics Observatory (SDO, Pesnell et al., 2011): Atmospheric Imaging
Assembly (AIA, Lemen et al., 2012).

  * •

Solar and Heliospheric Observatory (SOHO, Domingo et al., 1995): Large Angle
Spectroscopic Coronagraph (LASCO, Brueckner et al., 1995).

  * •

STEREO: Heliospheric Imager (HI, Eyles et al., 2009), Extreme UltraViolet
Imager (EUVI), and the two coronagraphs (COR1, COR2), all being part of the
suite Sun–Earth Connection Coronal and Heliospheric Investigation (SECCHI,
Thompson et al., 2003; Howard et al., 2002).

On the other hand, the in situ instrumentation was:

  * •

ACE: The Electron, Proton, and Alpha Monitor (EPAM, Gold et al., 1998), the
Solar Wind Electron Proton Alpha Monitor (SWEPAM, McComas et al., 1998), the
Solar Wind Ion Composition Spectrometer (SWICS, Gloeckler et al., 1998), and
the magnetometer of the Magnetic Fields Experiment (Smith et al., 1998).

  * •

Cluster: The magnetometer (Balogh et al., 2001) and the Cluster Ion
Spectrometry experiment (Rème et al., 2001).

  * •

STEREO-A: The suite of instruments of the In situ Measurements of Particles
And CME Transients (IMPACT, Luhmann et al., 2008), particularly the
instruments Solar Wind Electron Analyzer (SWEA, Sauvaud et al., 2008) and the
magnetometer (Acuña et al., 2008), and the Plasma and Suprathermal Ion
Composition (PLASTIC, Galvin et al., 2008).

  * •

Wind: The Solar Wind Experiment (SWE, Ogilvie et al., 1995), the Magnetic
Field Investigation (MFI, Lepping et al., 1995), and the 3-D Plasma and
Energetic Particle Investigation (3DP, Lin et al., 1995).