An interplanetary origin of great geomagnetic
storms: Multiple magnetic clouds
Yuming Wang, Pinzhong Ye, Shui Wang
School of Earth and Space Sci., Univ. of Sci. and Tech. of China, Hefei, Anhui 230026, China
E-mail: wym@mail.ustc.edu.cn
Abstract
An event of multiple magnetic cloud (Multi-MC) in interplanetary space on
March 31, 2001, which caused the largest geomagnetic storm with Dst = − 387
nT during the 23th solar maximum (2000−2001), is studied. By analyzing the
data from the ACE spacecraft, we described the characteristics of magnetic
fields and plasma within this Multi-MC at 1 AU. We also identify its solar
sources, coronal mass ejections (CMEs), by utilizing the observations from the
Solar and Heliospheric Observatory (SOHO) and the GOES satellite. In this
instance, the Multi-MC became more geoeffective with the extraordinary
enhanced southward magnetic fields, and largely extended the duration of the
Dst storm. The observations and the theoretical analysis reveal that the
strength of the magnetic fields including the southward component increased
several times due to the compression between the sub-clouds in the Multi-MC,
and therefore intensified the corresponding geoeffectiveness. Thus, we
suggest that the compression between the sub-clouds of Multi-MC should be a
mechanism in causing great geomagnetic storms. In addition, we found that
the CMEs, which formed the Multi-MC, seemed not to originate from the same
solar region.
1 Introduction
Geomagnetic storm is primarily defined by the enhanced ring current which produces
a magnetic field disturbance on the Earth. It was found that solar wind speed (V),
south-directed interplanetary magnetic field (Bs), and duration of the Bs (∆T) have greatest
effects on causing geomagnetic storms[1] – [6]. Magnetic reconnection is proposed as the
prime energy transfer mechanism for the creation of geomagnetic storms during Bs
events[7] – [9]. Magnetic clouds (MCs), which are the interplanetary structures associated
with enhanced magnetic fields strength, long and smooth rotation of the magnetic fields
vector and low proton temperature[10], have been thought one of the major sources of
strong southward interplanetary magnetic fields, for example the Bastille event occurred
on July 14, 2001[11]. Due to MCs' comparatively regular fields, flux rope model has been
developed to describe the properties of MCs[12] – [15].
As another cause of strong Bs in interplanetary space, the combination of draping and
compression of the ambient southward interplanetary magnetic field produced by shock or
high-density plasmoid should be noticed[16] – [18]. By study five great geomagnetic storms,
Tsurutani et al.[19] concluded that precursor southward fields ahead of the high speed
streams allow the shock compression mechanism to be particularly geoeffective. The
upstream southward field is compressed at the shock leading to intense Bs which causes
the onset of the great geomagnetic storms. The stronger the shock is, the greater is the
compression ratio of the downstream to upstream field value. In their five cases, three
storms were caused by shock compression of preexisting southward fields and the other
two were produced by fields within clouds. Recently, Wang et al.[20] found that shock is
able to propagate in low β cloud, compress the southward magnetic field of the cloud, and
form large Bs event, which will causes large geomagnetic storm.
In this paper, we suggest another probable mechanism in causing great geomagnetic
storms (Dst ≤ -200 nT) by studying an event of multiple magnetic cloud (Multi-MC).
Multi-MC is formed due to the overtaking of a serial of clouds[21]. It is similar to the
complex ejecta[22], which usually originated from several CMEs. But such complex ejecta
is of non-geoeffectiveness with irregular magnetic fields. The Multi-MC can be divided into
several parts. In each part, the interplanetary parameters are primarily satisfied with the
criteria of MC. Therefore, there is relatively regular field in a Multi-MC, and geomagnetic
storm can be expected commonly.
2 The Multi-MC on March 31, 2001 associated with a great
geomagnetic storm
2.1 ACE observations
A complex interplanetary structure passing Earth was observed by the ACE
spacecraft on 31 March 2001 (Fig. 1). The structure began at 0020 UT on Mar. 31 with two
leading shocks in the front part. Shock one (S1) arrived at 0020 UT and the other (S2)
arrived at 0150 UT. The corresponding sheath fields were separated into two regions: SH1
and SH2 (as seen in the first panel of Fig. 1). The fields of SH1 were compressed largely
due to the interaction between the two shocks. At S1, magnetic field strength increased
from ~ 10 nT to ~ 60 nT. The strength of this sheath fields reached to a maximum value B
= 73 nT. In the rear of SH1, magnitude of field dropped. At S2, the field strength increased
again due to the shock compression. In the rear of SH2, the field directed southward,
which lasted more than 1 hour, and its peak value was -36 nT.
Figure 1
Observations by the ACE spacecraft from 1200 UT March 30 to 1200 UT
April 1, 2001. From top to bottom are plotted: magnetic field strength B, the elevation θ
and azimuthal φ angles of the field direction, z component field B z, solar wind speed V,
proton density N, the proton temperature Tp, the ratio of thermal press to magnetic
press β, the density ratio of He++ to proton Nα/Np, and geomagnetic index Dst.
From 0505 UT to 2138 UT, there is the complex ejecta itself. The corresponding ratio
of the density of He++ to the density of proton was relatively high (~ 0.1), which indicated
the presence of ejecta from the corona[23 – 25]. As denoted by dot-dashed lines in figure 1,
we separated the complex ejecta into three parts. The first part (I) began at 0505 UT and
ended at 1015 UT. It was satisfied with all of the characteristics of typical MC basically. In
part I, the magnetic field was enhanced and the peak value of B was about 49 nT. It
largely exceeded the ambient field. The orientation of magnetic field rotated from south to
north smoothly and largely. Relative to the borders, the proton temperature (Tp) was lower
in Part I. Especially the ratio of thermal pressure to magnetic pressure (β) was less than
0.1 approximately, which was consistent with the previous observations of clouds (β ≈
0.06±0.04)[22], [26]. Therefore, we considered that part I was a magnetic cloud (MC1).
Similarly, the third part (III) was also considered an MC (MC2), which began at 1234 UT
and ended at 2138 UT. In this part, the magnetic field strength reached to 41 nT, and the
duration of Bs ≥ 10 nT was more than 6 hours. Tp was not as low as in the typical cloud,
but β was also lower than 0.1 roughly. Maybe the compression between the clouds made
the enhancement of the proton temperature. The second part (II) corresponded to the
interacting region between these two clouds. In this interacting region, the field strength
reached to minimum, and β increased again with peak value > 1.0 (as marked by circle in
the eighth panel of Fig. 1). In MC1, the solar wind speed did not decrease continuously. In
the rear part of MC1, the solar wind speed increased again that indicates the compression
of MC2 with MC1. Hence, we believe that part I, II and III constructed a Multi-MC.
Moreover, we thought that the shock S2 was associated with the second cloud. It
penetrated through the first cloud and was merging with the first shock S1. The merger of
shocks has been discussed in previous work[27].
There was another ejecta following this Multi-MC. Its shock (S3) arrived at 2138 UT,
just behind MC2. The magnetic field strength in the ejecta was smaller and the interaction
of the ejecta with the preceding Multi-MC was weak comparably, so we excluded it from
studying the complex structure.
2.2 The corresponding CMEs
The analysis of spacecraft data reveals that MCs are a well-defined subset of coronal
mass ejections (CMEs)[28]. Almost half of all ejecta are MCs[29], [30]. According to the data
from Large Angle Spectroscopic Coronagraph (LASCO) and Extreme Ultraviolet Imaging
Telescope (EIT) onboard of Solar and Heliospheric Observatory (SOHO), we could find
out the corresponding solar sources, CMEs, of the Multi-MC. Here, we also refer to the
‘CME catalog' (see http://cdaw.gsfc.nasa.gov/CME_list/) to choose the candidates.
Generally, halo or semi-halo CMEs are considered moving toward or away from the
Earth[31] – [33]. If their associated solar activities may be observed on the frontside of the
solar surface, such CMEs are Earth-directed certainly. Within five days before the arrival
of the Multi-MC at 1 AU, i.e., from 26 to 30 Mar. 2001, only three halo CMEs were
observed (Figure 2).
Figure 2
Running-difference images of these three halo CMEs observed by
LASCO/C2 (top) and EIT195Å (bottom).
Some observations were listed in Table 1. The first halo CME was visible in
LASCO/C2 at 0127 UT on 28 Mar. 2001. The projected initial speed was about 427 km/s
averagely. EIT 195Ǻ showed several activities on solar surface around the occurrence of
the CME. Thus there was ambiguity in identifying the CME's source region. According to
the observations from GOES about X-ray flares, we tentatively considered that the CME
was associated with a C5.6 X-ray flare which erupted from AR9401 (N20E22) at 0129 UT.
The second halo CME was visible in C2 at 1250 UT on the same day. The initial speed
was about 519 km/s averagely. EIT 195Ǻ showed an activity in AR9393. Meanwhile, an
M4.3 X-ray flare beginning at 1121 UT was observed in the same region (N18E02) by
GOES. The last halo CME was visible in C2 at 1026 UT on 29 Mar. 2001. The projected
initial speed was about 942 km/s averagely. EIT 195Ǻ observed an activity associated
with a X1.7 X-ray beginning at 0957 UT from AR9393 (N20W19). Hence, these three halo
CMEs were all Earth-directed and could be detected at Earth.
Table 1
No.
Date
Timea
1
Mar. 28
2
3
The corresponding halo CMEs
Speedb (km/s)
Locationc
Flare
0127
427
N20E22 ?
C5.6 ?
AR9401
Mar. 28
1250
519
N18E02
M4.3
AR9393
Mar. 29
1026
942
N20W19
X1.7
AR9393
(UT)
a
The first appearance in view of LASCO/C2.
b
CMEs’ projected speed.
c
Location of associated solar flare.
d Solar
active region.
ARd
The interval of the first two CMEs’ initiation was 11.38 hours, and that of the last two
CMEs was 21.60 hours. If their projected speeds were representative of the speeds along
the Sun-Earth direction, the second CME was moving faster than the first one. Therefore,
it could overtake the earlier one and form a Multi-MC as well as observed by ACE. On the
other hand, slow CMEs accelerate when moving outward and fast CMEs decelerate[34].
The speeds of CMEs are distributed within a narrow range at 1 AU. Thus, although the
last halo CME's initial speed was much larger than the others, it seems to just catch up
with the preceding CME because of the too long delay after the second CME's initiation
and the probable deceleration in interplanetary space. It did not interact with the early MC
sufficiently yet at 1 AU. That was why the shock S3 observed by ACE still stayed behind
the MC2. Summarily, the first two halo CMEs were responsible for the Multi-MC, and the
last halo CME was corresponding to the ejecta following the Multi-MC.
2.3 Geoeffectiveness
In MC1, the maximum value of magnetic field Bs was 48 nT and the duration, ∆T, was
nearly 3 hours with Bs ≥ 10 nT. In MC2, the maximum value of Bs was 37 nT approximately
and ∆T was more than 6 hours. Analyze the corresponding Dst index data, we found there
was a great geomagnetic storm with two major Dst peaks (as seen in the last panel of Fig.
1). The onset of the storm was at 0300 UT on Mar. 31, caused by the southward field
located at the tail of SH2 (marked by unfilled triangle in the fourth panel). Under the effect
of the Bs in the first cloud, Dst value reached the first peak (−387 nT) at 0900 UT. The
second Dst peak (−284 nT) was found at 2200 UT, which was produced by the Bs of
second cloud obviously.
Since the rate of the occurrence of CMEs is high during the solar maximum, the
Multi-MC structure in the interplanetary medium is common relatively. Only from March to
April 2001, there are three cases of Multi-MCs (March 3 – 5, March 31 and April 11 – 13,
also refer to Wang et al.[35] work) observed near the Earth. Meanwhile, there are totally 5
intense geomagnetic storms with Dst ≤ -100 nT, among which there are three great storms
with Dst ≤ -200 nT. Two of these three great storms with Dst peak values of -387 nT and
-271 nT are caused by the Multi-MCs on March 31 and April 11 – 13, respectively. Thus,
Multi-MC is one kind of the interplanetary origin of great geomagnetic storms.
3 Discussion
The great geomagnetic storms with peak Dst ≤ −200 nT are infrequent. Even during
this solar maximum (from 2000 to 2001 approximately), there were only 8 great
geomagnetic storms with Dst ≤ −200 nT. Especially, the geomagnetic storm produced by
this event was the largest one among these great storms. Thus, it is valuable to analyze
this event and find out the corresponding interplanetary cause.
In causing intense geomagnetic storms, there are three major factors: plasma speed
(V), southward component of interplanetary magnetic fields (Bs), and duration of Bs (∆T).
As for intense storms with peak Dst ≤ −100 nT, Gonzalez and Tsurutani[36] suggested
threshold values of Bs ≥ 10 nT and ∆T ≥ 3 hours. Under effect of magnetic reconnection
between interplanetary magnetic fields and the Earth's magnetosphere, a great amount of
energy can be transferred into the magnetosphere from solar wind if there is sufficient
long duration of Bs. Hence, single MC may produce major geomagnetic storm due to the
combination of its Bs field and long duration ∆T, for example the some large events
occurred from Aug. 1978 to Dec. 1979[17], [36].
Not like to single MC, much stronger Bs is pivotal, for the case mentioned in this paper,
to cause great geomagnetic storm. According to the statistical study, the size of MC is
0.28 AU averagely with speed of 450 km/s approximately[10], [37]. Therefore, a typical MC
usually lasts for 25 hours when passing the Earth. The first cloud of this multi-MC began at
0505 UT and ended at 1015 UT that only lasted for 5.2 hours. A large compressed degree
(25/5.2 = 4.8 times) of the cloud may be estimated. Assume a flux rope could be
described by force-free model, we could obtain that magnetic field strength is inverse
proportional to the square of its radius according to the conservation of axial magnetic
flux[12], [14]. Thus, the magnetic field strength of an MC would increase when it was
compressed. Inside the first sub-cloud of this Multi-MC, B value should increase to
4.82=23.0 times of original value theoretically with 4.8 compressed degrees. In fact, the
increment of magnetic fields strength should be not such large if the interaction, such as
penetration, reconnection, and so on, between the sub-clouds are considered. Certainly,
we believe that the extraordinary enhancement of B was existence indeed and the
corresponding Bs value also increased, which strengthened the clouds' geoeffectiveness.
The combination of field draping and shock compression are another major
interplanetary cause of great geomagnetic storms by forming strong interplanetary B s
interval[16], [19]. Compared with them, the Multi-MC also presents very large Bs interval due
to the compression. However, the difference is that the former is formed by fast overtaking
shock and the latter is formed by interaction of clouds. In this event, the second cloud
overtook the first one and compressed it, that caused the greatest Dst peak. Further more,
the existence of the second cloud largely extended the duration of Dst storm.
Based on the flux rope model[21], the geoeffectiveness of Multi-MC can be further
understood. Figure 3 and 4 show the simplest Multi-MC structure consisting of two
uniform magnetic clouds in the ecliptic plane (left) and the configuration of its magnetic
field (right). For the case of two sub-clouds with the same rotation (Fig. 3), the
z-component magnetic field fluctuates two times with the polarity of S-N-S-N or N-S-N-S
(S and N denote southward and northward respectively). The middle two extreme values
are both smaller than the other two. For the case of two sub-clouds with the opposite
rotation (Fig. 4), the configuration of magnetic field is very different from the former. Within
the Multi-MC, there is a large and wide peak or trough. The polarity is S-N-S or N-S-N.
Obviously, the N-S-N Multi-MC should be the most geoeffective in theory, whereas the
geoeffectiveness of S-N-S Multi-MC should be the smallest. What we analyzed is the
simplest cases, and in fact, the situation will be much more complicated. For example, the
axes of sub-clouds may orientate to any direction, the number of sub-clouds may be more,
and their magnetic field strength may vary. Although Multi-MC is one kind of interplanetary
origin of great geomagnetic storms, not all of Multi-MCs can produce large geomagnetic
storms.
Figure 3
Schematic picture of a Multi-MC consisting of two uniform magnetic clouds
with the same rotation in the ecliptic plane.
Figure 4
Schematic picture of a Multi-MC consisting of two uniform magnetic clouds
with the opposite rotation in the ecliptic plane.
Here, by the analysis of an example of Multi-MC, we suggest that the overtaking
between sub-clouds of Multi-MC should be a probable mechanism in causing great
geomagnetic storms. In addition, the identification of the Multi-MC’s solar sources implies
that the corresponding CMEs are not necessary to originate from the same solar active
region. Since magnetic cloud is a very large-scale structure, and its diameter can reach
0.28 AU approximately at 1 AU, the CMEs originating from different regions still may form
Multi-MC as long as - 8 -they move in the same direction roughly. How to predict the
occurrence of Multi-MC and its geoeffectiveness? Is every Multi-MC definitely more
geoeffective than isolated magnetic cloud? These are all unanswered questions and need
further study to recover.
Acknowledgment We acknowledge the use of the data from the ACE, SOHO, GOES spacecraft and
the Dst index from World Data Center. This work is supported by the National Natural Science
Foundation of China (49834030), the State Ministry of Science and Technology of China (G2000078405),
and the Chinese Academy of Sciences (KZCX2-SW-136).
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