The Inhomogeneity of Composition along the Magnetic Cloud Axis
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The Inhomogeneity of Composition along the
Magnetic Cloud Axis
Hongqiang Song 1,2∗ , Qiang Hu 3 , Xin Cheng 4 , Jie Zhang 5 , Leping Li 2 , Ake
Zhao 6 , Bing Wang 1 ,Ruisheng Zheng 1 , and Yao Chen 1
1 Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial
Environment, and Institute of Space Sciences, Shandong University, Weihai, China
arXiv:2106.15834v1 [astro-ph.SR] 30 Jun 2021
2 CAS Key Laboratory of Solar Activity, National Astronomical Observatories,
Chinese Academy of Sciences, Beijing, China
3 Department of Space Science and CSPAR, University of Alabama in Huntsville,
Huntsville, Alabama, USA
4 School of Astronomy and Space Science, Nanjing University, Nanjing, China
5 Department of Physics and Astronomy, George Mason University, Fairfax,
Virginia, USA
6 College of Physics and Electric Information, Luoyang Normal University, Luoyang,
China
Correspondence*:
Hongqiang Song
hqsong@sdu.edu.cn
ABSTRACT
Coronal mass ejections (CMEs) are one of the most energetic explosions in the solar system. It
is generally accepted that CMEs result from eruptions of magnetic flux ropes, which are dubbed
as magnetic clouds in interplanetary space. The composition (including the ionic charge states
and elemental abundances) is determined prior to and/or during CME eruptions in the solar
atmosphere, and does not alter during magnetic cloud propagation to 1 AU and beyond. It
has been known that the composition is not uniform within a cross section perpendicular to
magnetic cloud axis, and the distribution of ionic charge states within a cross section provides
us an important clue to investigate the formation and eruption processes of flux ropes due to
the freeze-in effect. The flux rope is a three-dimensional magnetic structure intrinsically, and it
remains unclear whether the composition is uniform along the flux rope axis as most magnetic
clouds are only detected by one spacecraft. In this paper we report a magnetic cloud that was
observed by ACE near 1 AU on 1998 March 4–6 and Ulysses near 5.4 AU on March 24–28
sequentially. At these times, both spacecraft were located around the ecliptic plane, and the
latitudinal and longitudinal separations between them were ∼2.2◦ and ∼5.5◦ , respectively. It
provides us an excellent opportunity to explore the axial inhomogeneity of flux rope composition,
as both spacecraft almost intersected the cloud center at different sites along its axis. Our
study shows that the average values of ionic charge states exhibit significant difference along
the axis for carbon, and the differences are relatively slight but still obvious for charge states
of oxygen and iron, as well as the elemental abundances of iron and helium. Besides the
1Song et al. The Inhomogeneity of Composition means, the composition profiles within the cloud measured by both spacecraft also exhibit some discrepancies. We conclude that the inhomogeneity of composition exists along the cloud axis. Keywords: coronal mass ejection, magnetic flux rope, interplanetary coronal mass ejection, magnetic cloud, ionic charge state, elemental abundance This is a provisional file, not the final typeset article 2
Song et al. The Inhomogeneity of Composition
1 INTRODUCTION within ICMEs are frozen-in near the Sun [33] and
Coronal mass ejections (CMEs) are an energetic the relative abundances of elements with different
explosive phenomenon in the solar atmosphere first ionization potentials (FIPs) are different
[1, 2, 3, 4], and they are called interplanetary obviously in the corona and photosphere [34, 35].
coronal mass ejections (ICMEs) after leaving the As the composition does not alter during CME
corona. When ICMEs interact with the Earth’s propagation to 1 AU and beyond [36], the in
magnetosphere, they can cause geomagnetic situ data are also employed to analyze the MFR
storms [5, 6, 7] and influence the normal work of formation [37, 28, 38] and plasma origin [39, 40]
high-tech equipments, such as the satellites, power of CMEs. So far the most complete composition
grids and GPS navigation systems [8, 9]. Therefore, data of ICMEs are provided by the solar wind
it is of great significance to grasp the trigger ion composition spectrometer (SWICS) aboard
mechanisms and eruption processes of CMEs. the advanced composition explorer (ACE) and
Ulysses, which can provide the charge states and
The researchers of solar physics community elemental abundances of ∼10 elements [41].
have reached a consensus that CMEs result from
When an ICME has its nose pass through a
eruptions of magnetic flux ropes (MFRs), which
spacecraft, the MFR will be detected as a magnetic
refer to a volumetric current channel with the cloud (MC) [42, 43, 44]. This is schematically
helical magnetic field lines wrapping around
shown in Figure 1(a) (also see [45, 46] for a
the central axial field [e.g., 10, 11]. In white- similar cartoon), where the purple arrow depicts a
light coronagraph images, CMEs often exhibit a
spacecraft trajectory crossing one ICME through
three-part structure, i.e., a bright front, a dark its nose portion as marked with the blue rectangle.
cavity and a bright core [12]. The cavity and
Figure 1(b) displays the MFR within the rectangle,
core have been considered as the MFR cross
and the green dots represent the center of each
section and erupted filament, respectively, for
cross section. The black, blue and red arrows
several decades. However, recent studies clearly
depict three different trajectories.
demonstrate that both the filaments and hot-
channel MFRs can appear as the bright core [13, Several statistical studies have been conducted
14, 15, 16]. The hot channels are first revealed on ICME composition. Huang et al. [47] analyzed
through extreme-ultraviolet passbands sensitive to the composition inside 124 MCs and reported that
high temperatures (e.g., 131 and 94 Å) [17], and fast MCs have higher charge states and relative
they can also be observed in hard X-ray [18] and elemental abundances (except the C/O) than slow
microwave [19] images. Researchers also suggest ones. Owens [48] analyzed the charge states of
that the dark cavity corresponds to a low-density carbon, oxygen, and iron within 215 ICMEs,
region with sheared magnetic field in the early including 97 MCs and 118 non-cloud events,
eruption stage [16]. and found that MCs exhibit higher ionic charge
states than non-cloud events. Zurbuchen et al.
Both theoretical and observational studies reveal [49] performed a comprehensive analysis of the
that MFRs can form prior to [20, 21, 17, elemental abundances of 310 ICMEs from 1998
22, 23] and during [24, 25, 26, 27, 28] solar March to 2011 August. They reported that the
eruptions, while they might exist before eruptions abundances of low-FIP elements within ICMEs
in more events [29]. The numerical simulations exhibit a systematic increase compared to the solar
demonstrate that the repetitive magnetic reconnections wind, and the ICMEs with elevated iron charge
could play an important role during the MFR states possess higher FIP fractionation than the
evolution [30]. The remote-sensing observations other ICMEs. Very recently, Song et al. [50]
have been widely used to investigate the MFR reported that all the ICME compositions possess
formation process [26, 31, 32]. The charge states the solar cycle dependence.
Frontiers 3Song et al. The Inhomogeneity of Composition
In the meantime, some attentions are paid This implies that the two spacecraft crosses the
on the composition distribution inside each MC. MC along two trajectories resembling the black
Song et al. [37] found that the average values and red arrows in Figure 1(b), respectively, and
of iron charge states () can present four provides us an excellent opportunity to explore
regular profiles along the spacecraft trajectories whether the composition is uniform along the axis.
throughout MCs, i.e., (i) a bimodal profile with We introduce the data in Section 2, and give the
both peaks higher than 12+, (ii) a unimodal profile observations in Section 3. Section 4 presents the
with peak higher than 12+, (iii) and (iv) the conclusion and discussion.
profile remains beyond and below 12+
throughout the spacecraft trajectory inside an MC, 2 DATA
respectively. Their studies demonstrated that the The data used in this paper are provided by several
charge states can be nonuniform within the cross payloads on board the ACE and Ulysses spacecraft.
section of a specified MC, and suggested that the ACE is in a halo orbit around the first Lagrangian
above profiles are tightly correlated with both the point between the Earth and the Sun since it was
impact factor of spacecraft trajectories and the launched in 1997. Ulysses was launched in 1990
formation process of MFRs. For example, the and entered an elliptical and heliocentric orbit
bimodal profile implies that the MFR exists prior with an aphelion at ∼5.4 AU from the Sun and a
to eruption, see Figure 8 in [37] for more details. In perihelion distance of ∼1.34 AU. Magnetic field
addition, the elemental abundances are not uniform data are provided by the ACE/MAG [54] and
within one cross section either [39]. Therefore, a Ulysses/Magnetic field [55] instruments. The bulk
spacecraft can detect different composition profiles solar wind properties and the helium abundances
when it cross one MC along the blue and black are from the Solar Wind Electron Proton Alpha
arrows as shown in Figure 1(b), which are located Monitor (SWEPAM) [56] on board ACE and the
in the same cross section perpendicular to the Solar Wind Observations Over the Poles of the Sun
axis but with different impact factors. However, (SWOOPS) [57] on board Ulysses. The SWICS
whether the inhomogeneity of composition exists instruments on board both spacecraft [58, 59] offer
along the MC axis remains unclear because most the composition of heavy ions.
MCs are detected only by either ACE or Ulysses.
Given the MC is a three-dimensional (3D) structure 3 OBSERVATIONS
intrinsically, the axial distribution of composition The criteria used to identify MCs near 1 AU mainly
can reveal whether different portions along the include the enhanced magnetic field strength,
MFR axis experience different eruption processes smoothly changing of magnetic-field direction,
in the corona. declining profile of solar-wind velocity, low proton
temperature (or low plasma β), and elevated
In this paper, we report an intriguing event,
He2+ /H+ ratio [42, 60, 61]. ACE detected an
in which an MC was observed by both ACE
MC on 1998 March 4–6 as shown in Figure 2.
on 1998 March 4–6 and Ulysses on March 24–
The purple vertical dashed line denotes the shock
28. At these times, both spacecraft were located
driven by the ICME, and the two purple dash-
around the ecliptic plane, and the latitudinal and
dotted lines demarcate the MC boundaries.
longitudinal separations between them were ∼2.2◦
and ∼5.5◦ , respectively. The Grad-Shafranov Figure 2(a) shows the total magnetic field
(GS) reconstruction [51, 52] demonstrated that the strength and its three components in RTN
MC axis oriented in an approximate east-west coordinate, where the X-axis (R) points from Sun
direction with the axis direction at Ulysses being center to spacecraft, the Y-axis (T) is the cross
titled slightly away from that at ACE, and both product of solar rotational axis and X axis, lying
spacecraft almost intersected the MC center [53]. in the solar equatorial plane towards the west limb,
This is a provisional file, not the final typeset article 4Song et al. The Inhomogeneity of Composition
and the Z-axis (N) is the cross product of X and a latitudinal separation of ∼2.2◦ and a longitudinal
Y axes. The total magnetic field strength (black) separation of ∼5.5◦ when they detected the MC.
increased obviously compared to the background The GS reconstruction showed that the MC axis
solar wind and the Bn component (blue) changed oriented in an approximate east-west direction, and
its direction gradually within the MC, which are both spacecraft almost intersected the MC center
the typical features of MCs. Figures 2(b)–(d) [53], which support that ACE and Ulysses crossed
present the velocity, density, and temperature of the MC at different sites along its axis and provide
the ICME sequentially. The declining profile of us an excellent opportunity to explore whether the
velocity indicates that the MFR is expanding. axial composition is uniform.
Ulysses detected an MC on March 24–28 [62] We compare the composition measured by both
as shown in Figure 3, where the magnetic field, spacecraft in Figure 4, where the black and red
velocity, density, and temperature are presented lines represent the results of ACE and Ulysses,
from top to bottom panels sequentially. The respectively. Please note that we only plot the
velocity profile in Figure 3(b) shows that the MC composition within the MC, i.e, the left/right
keeps expansion during the propagation to 5.4 boundary of each panel corresponds to the MC
AU. Due to the continuous expansion, the total start/end time. The ionic charge states (C6+ /C5+ ,
magnetic field intensity within this MC decreased O7+ /O6+ , and ) and elemental abundances
obviously near 5.4 AU compared to ∼1 AU, see (Fe/O and He2+ /H+ ) are presented in Figures 4(a)–
Figures 2(a) and 3(a). A shock exists within the (e). The average values within the MC are also
MC as depicted with the red arrows in Figures shown in each panel. The blue horizontal dashed
3(a) and 3(b), and the MC rear boundary can lines represent the corresponding means in the
be identified through the He2+ /H+ ratio and the slow solar wind during solar maximum [65] for
plasma β value [53]. Note that the shock does not reference and comparison.
influence our analyses about the ionic charge states
and elemental abundances. Our study shows that the average values of
composition within an MC can possess significant
Previous studies [63, 53] have confirmed that the
differences along the axis. For example, the
MC displayed in Figure (3) corresponds to that
C6+ /C5+ ratio measured by Ulysses (3.04) is 12
in Figure (2). Skoug et al. [63] fitted both MCs
times higher than that by ACE (0.23). In the
using a force-free model of magnetic field [64], meantime, the differences could be relatively slight
and found that their central speed and cloud axis
for some compositions. For example, the O7+ /O6+
direction were very similar. The increase in MC ratio measured by Ulysses (0.41) is higher than
diameter between 1 and 5.4 AU was also consistent
ACE (0.34) by ∼21%. The means of
with an expanding MC. Besides, both MCs had left- detected by both spacecraft are nearly identical
handed field structure and contained the similar
(∼10). As to the elemental abundance, the Fe/O
magnetic fluxes, which were further confirmed
ratio by ACE (0.17) is ∼42% higher than that by
by Du et al. [53] with the GS reconstruction
Ulysses (0.12), and the He2+ /H+ ratio of ACE
technique. In addition, Du et al. [53] input the
(0.093) is higher than Ulysses (0.053) by ∼75%.
plasma and magnetic field data observed by ACE
to their magnetohydrodynamic model to simulate Besides the average values, the composition
the MC propagation and evolution to the Ulysses profiles measured by both spacecraft also exhibit
location. They compared the model predictions discrepancy. Figure 4(a) shows that the C6+ /C5+
and the Ulysses observations, and identified further of Ulysses elevated at the MC center, while
that Ulysses and ACE observed the same MC. As the ACE profile did not exhibit the central peak.
mentioned, the ACE (at ∼1 AU) and Ulysses (at The O7+ /O6+ of Ulysses presented a multi-peak
∼5.4 AU) were located near the ecliptic plane with profile, while ACE did not detect obvious peaks
Frontiers 5Song et al. The Inhomogeneity of Composition
as shown in Figure 4(b). The He2+ /H+ of ACE that Ulysses passed through the ICME along a path
elevated in the second half as displayed in Figure a little far from the MC center than ACE. Figure
4(e), different from the profile of Ulysses that 4(a) showed that Ulysses detected high C6+ /C5+ at
did not have large variation along the whole its central portion, which should also be observed
path. These can rule out the possibility that the by ACE if the composition is uniform along the
inhomogeneity of composition is induced by the MC axis. However, the C6+ /C5+ profile of ACE
erosion [66] completely during propagation from did not present the elevated center. Therefore,
1 AU to 5.4 AU. Moreover, the erosion effect if assuming there were some uncertainties about
should be small for this event as both MCs have the spacecraft path in the GS reconstruction, it
the similar magnetic fluxes as mentioned. The will not change our conclusion about the axial
profiles of and Fe/O measured by both inhomogeneity of MC composition.
spacecraft also exhibit some different fluctuation The charge states of carbon, oxygen and iron
characteristics as displayed in Figures 4(c) and are frozen-in sequentially in the corona, i.e., the
4(d). The above results prove that the composition frozen-in altitudes of carbon and iron are the
is inhomogeneous along the MC axis. lowest and highest, respectively, in these three
elements. For example, the carbon are frozen-
4 CONCLUSION AND DISCUSSION in below 1.5 solar radii [67, 68], while the iron
An MC was detected by ACE at ∼1 AU and around 3–4 solar radii [69, 70]. Therefore, the
Ulysses at ∼5.4 AU sequentially during March obvious differences of C6+ /C5+ along the MC
1998, when both spacecraft were located around axis imply that the different portions of MFR
the ecliptic plane. The latitudinal and longitudinal along the axis experience eruption processes with
separations between them were ∼2.2◦ and ∼5.5◦ , different physical parameters (e.g., temperature,
respectively. The GS reconstruction [53] showed density, and velocity) in the low corona. The
that the axis oriented in an approximate east-west similar values of indicate that the physical
direction, and both spacecraft almost intersected parameters along the axis approached in the high
the MC center, which provided an excellent corona. These should be taken into account in 3D
opportunity to explore whether the composition simulations of CMEs. The axial inhomogeneity of
is uniform along the axis. We compared the elemental abundances implies that the abundances
ionic charge states of carbon, oxygen, and iron are not uniform throughout the MC source region
(C6+ /C5+ , O7+ /O6+ , and ), as well as the on the Sun.
elemental abundances of iron and helium (Fe/O
Our study demonstrated that the axial composition
and He2+ /H+ ) along the two trajectories. The
is nonuniform inside an MC, while we can not
results showed that the average values of C6+ /C5+
conclude that this large inhomogeneity exists
exhibit significant difference along the axis, while
within each MC. More events are necessary
the differences are relatively slight but still obvious
to investigate the inhomogeneity of composition
for O7+ /O6+ , , Fe/O, and He2+ /H+ .
along the MC axis, which needs a CME
Besides the means, the composition profiles within
being detected by several spacecraft sequentially
the MC measured by both spacecraft also exhibit
or simultaneously at different locations. This
obvious discrepancies. We conclude that the
becomes more realizable as Solar Orbiter was
inhomogeneity of composition exists along the MC
launched in 2020 [71]. Besides, Chinese solar
axis.
physicists are proposing several space missions
The magnetic field within the MC measured [72] to explore the Sun and solar eruption further.
by Ulysses did not exhibit the obvious changing The Lay a Finger on the Sun [73] will launch
of direction compared with the measurements of a spacecraft to explore the solar eruption near
ACE, see Figures 2(a) and 3(a). This might indicate the Sun, thus it will provide more MC cases
This is a provisional file, not the final typeset article 6Song et al. The Inhomogeneity of Composition
that are measured sequentially near the Sun and edu/ACE/ASC/level2/index.html) and the
around 1 AU combined with other spacecraft. The Ulysses Final Archive (http://ufa.esac.
Solar Ring [74] plans to deploy six spacecraft, esa.int/ufa/), respectively.
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Frontiers 11Song et al. The Inhomogeneity of Composition FIGURE CAPTIONS Figure 1. Schematic drawing of the spacecraft trajectory crossing an ICME. The black dashed and solid lines represent the shock and MFR, respectively. The red dotted lines delineate the MFR axis. The ICME nose portion is marked with the blue rectangle in Panel (a), which is enlarged for details in Panel (b). The blue, black, and red arrows describe the different trajectories of spacecraft, and the green dots denote the MC center. This is a provisional file, not the final typeset article 12
Song et al. The Inhomogeneity of Composition
15
10
(a)
Magnetic Field (nT)
5
0
-5
-10
Br Bt Bn
-15
400
(b)
380
Velocity (km s-1)
360
340
320
300
60
50
(c)
Density (cm-3)
40
30
20
10
0
8 (d)
Temperature (104 K)
6
4
2
0
08:00 16:00 00:00 08:00 16:00 00:00 08:00
Start Time (04-Mar-98 06:00:00)
Figure 2. Magnetic field and solar wind parameters measured by ACE near 1 AU. (a) Total magnetic field
strength (black) and its three components in RTN coordinate, (b)–(d) Velocity, density, and temperature
of solar wind. The purple vertical dashed line denote the shock, and the dash-dotted lines demarcate the
MC boundaries.
Frontiers 13Song et al. The Inhomogeneity of Composition
2
(a)
Magnetic Field (nT)
1
0
-1
-2
Br Bt Bn
400
390
(b)
Velocity (km s-1)
380
370
360
350
340
1.2
(c)
1.0
Density (cm-3)
0.8
0.6
0.4
0.2
0.0
5
(d)
Temperature (10 K)
4
4
3
2
1
0
24-Mar 25-Mar 26-Mar 27-Mar 28-Mar
Start Time (23-Mar-98 10:01:28)
Figure 3. Magnetic field and solar wind parameters measured by Ulysses near 5.4 AU. (a) Total
magnetic field strength (black) and its three components in RTN coordinate, (b)–(d) Velocity, density,
and temperature of solar wind. The purple vertical dashed line denote the shock, and the dash-dotted lines
demarcate the MC boundaries. The red arrows in (a) and (b) depict the shock inside the MC.
This is a provisional file, not the final typeset article 14Song et al. The Inhomogeneity of Composition
2.0 15
(a) Mean=0.23 ACE
C6+/C5+
1.5 Mean=3.04 Ulysses
C6+/C5+
10
1.0
5
0.5
0.0 0
1.0
(b) Mean=0.34
0.8
Mean=0.41
O7+/O6+
0.6
0.4
0.2
0.0
13
12 (c) Mean=9.56
11
Mean=10.13
10
9
8
7
0.5
(d) Mean=0.17
0.4 Mean=0.12
0.3
Fe/O
0.2
0.1
0.0
0.20
(e)Mean=0.093
0.15 Mean=0.053
He2+/H+
0.10
0.05
0.00
MC start MC end
Figure 4. Composition within the MC provided by SWICS aboard ACE (black) and Ulysses (red). Panels
(a)–(e) show the C6+ /C5+ , O7+ /O6+ , , Fe/O, and He2+ /H+ sequentially, and their average values
are also presented in each panel. Note that the Ulysses values in Panel (a) correspond to the right ordinate.
The blue horizonal dashed lines depict the corresponding means of slow wind during solar maximum [65].
The MC started from 14:30 UT on March 4 (3:00 UT on March 24) and ended at 5:30 UT on March 6
(4:00 UT on March 28) for ACE (Ulysses).
Frontiers 15You can also read
