Global Structure of Interplanetary Coronal Mass Ejections
Retrieved from the Model Fitting Analysis of Radio
Scintillation Observations
Munetoshi Tokumaru , Masayoshi Kojima , Ken’ichi Fujiki and Masahiro
Yamashita
Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 442-8507, Japan
Abstract. Interplanetary scintillation (IPS) measurements made with the Solar-Terrestrial Environment Laboratory, Nagoya
University, multi-station system at 327 MHz are used to study the global structure of the interplanetary counterpart of coronal
mass ejections (so-called interplanetary CMEs or ICMEs). We have analyzed our IPS data of four ICME events which occurred
successively in July 2000; those are 2000 July 10, 11, 12 and 14 events (the last one corresponds to "Bastille day event").
We have employed the model fitting method to obtain unbiased three-dimensional properties of ICME from IPS observations.
The parameters determined here include the location of the ICME center, the e-folding radial thickness and angular span,
anisotropy of the angular extent, the local enhancement factor, the (average) expansion speed, and its angular dependence.
The results suggest that the global shape of July 12 and 14 events were loop-like; the latitude angular span of these events was
much smaller than the longitude one. The July 10 and 11 events are found to be explained excellently by a shell-shape ICME
model. The mass contained by the ICME has been estimated using information on the global structure obtained here.
INTRODUCTION
formed the model fitting analysis to extract unbiased
three-dimensional information from our IPS data. In this
paper, we have analyzed another three ICME events,
which occurred successively just prior to the Bastille Day
event, by employing the model fitting method developed
in the previous study.
The global properties of the interplanetary counterpart of
coronal mass ejections (interplanetary CMEs or ICMEs)
are little understood owing to a lack of imaging observations for the solar wind plasma, although they provide
crucial information for studying the dynamics of CMEs
in the solar wind. Interplanetary scintillation (IPS) measurements with a high sensitivity system have an unrivalled potential to clarify the global properties of ICMEs,
since they allow us to produce an plane-of-sky projection
map of the solar wind plasma (e.g. [1], [2], [3]). The spatial resolution of the IPS solar wind map depends on the
number of line-of-sight (los) employed for IPS observations in a day. Although it has proven from earlier studies
that IPS solar wind maps act as an effective tool to detect
and track ICMEs traveling from the sun to beyond the
earth orbit, the three-dimensional structure of ICMEs is
still unclear, since an apparent feature of ICMEs in the
IPS solar wind map is influenced significantly by the los
integration effect and the plane-of-sky projection effect.
So that, we need to remove these effects in order to clarify three-dimensional properties of ICMEs from IPS observations.
Recently, we have investigated the global structure of
ICME associated with the 2000 July 14 flare event (socalled the Bastille day event), using our IPS observations [4] (hereafter Paper I). In Paper I, we have per-
IPS OBSERVATIONS OF ICME
IPS observations at 327 MHz have been carried out daily
between April and December of every year with the fourstation system of the Solar-Terrestrial Environment Laboratory (STEL), Nagoya University [5]. Our IPS observations covers the radial distance range between 0.2 AU
and 1 AU from the sun, and number of line-of-sight (los)
used in a day for our IPS observations is about 40. From
our IPS data, we derive solar wind velocity and g-value,
which represents the relative level of solar wind (density)
turbulence, ∆Ne . In this study, the g-value data have been
analyzed to study global properties of ICMEs.
The g-value is normalized to the level of the ambient
solar wind. When a highly turbulent plasma associated
with an ICME passes across the los for a given radio
source, the g-value increases abruptly. By plotting the gvalue data of an ICME event on the plane of sky, we can
make the spatial distribution map of turbulent (high ∆N e )
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
729
FIGURE 1. (right) All-sky g-map obtained from our IPS observations for July 11 22h UT – July 12 7h UT, 2000, and (left)
SOHO/LASCO C3 image for the CME on July 10, 2000.
TABLE 1. Solar-terrestrial phenomena associated with 2000 July ICME events analyzed.
Information on flare and ssc, which acts as a proxy of the IP shock occurrence at the earth,
is from Solar Geophysical Data, and information on CME is from SOHO/LASCO observations.
Event Date
Solar Flare
Peak Time (UT)
Class/Importance
Heliographic Location
CME
Occurrence
IP shock/ssc
Occurrence Time (UT)
Vshock (km/s)
July 10, 2000
July 11, 2000
July 12, 2000
July 14, 2000
21:42
M5.7/2B
N18E49
13:10
X1/2N
N18E27
10:37
X1.9/2B
N17E21
10:24
X5.7/3B
N22W07
Y
Y (Full-halo)
Y
Y (Fast-halo)
July 14
15:32
790
July 15
14:37
1470
July 13
09:42
690
MODEL FITTING ANALYSIS OF
G-VALUE DATA
plasma. Such a plane-of-sky projection map of the solar
wind is called an all-sky g-map. In the right panel of Figure 1, an all-sky g-map obtained from our IPS observations between July 11 22h UT and July 12 7h UT, 2000,
is indicated. The center of this map corresponds to the
location of the sun, and dotted concentric circles are isoR contours for 0.3, 0.6, and 0.9 AU. Here, R RSE sin ε ,
and RSE , ε are the Sun-Earth distance (i.e. 1 AU) and
the solar offset angle, respectively. Circles in the figure
show the location of los, and the size of circles denotes
the strength of g-value. Enhancements of g-value found
in the north-east quadrant of the map are considered to be
an interplanetary consequence by the CME which was
observed on July 10, 2000, with SOHO/LASCO measurements [7] (the left panel of Figure 1). Here, we note
that the location of the g-enhanced region in the map
shows an excellent agreement with the radial extension
of brightness enhancements in the LASCO image. While
the crescent-shape appearance of g-enhanced region may
suggest that the ICME had a shell-shape structure with a
thin radial thickness, we must draw attention to the fact
that this feature includes effects of the los integration effect and the plane-of-sky projection.
The g-value, g, is related to the integration of solar wind
density fluctuations ∆N e via the following formula,
g2 ∝
∆Ne 2 wzdz
(1)
for weak scattering of a radio wave (i.e. for R 02 AU
in the case of 327 MHz wave, where R is the radial distance). Here, z is the distance along the line-of-sight, and
wz is the IPS weightening function [6]. Equation (1) allows us to calculate the g-value, provided that the threedimensional ∆Ne distribution in the solar wind is given
by a suitable model. Thus, we can determine the best-fit
model of ∆Ne distribution by comparing the model calculations to observed g-value data. This analysis yields
unbiased information of three-dimensional ∆N e distribution in the solar wind [8]. In the present analysis, we have
made the model fitting analysis for three ICME events
which occurred just prior to the Bastille day event; those
are July 10 (Figure 1), July 11, and July 12. The solarterrestrial phenomena associated with these ICME events
730
FIGURE 2. All-sky g-maps obtained from our IPS observations on (left) July 13, (middle) July 14, and (right) July 15, 2000.
These events correspond to the July 11, 12 and 14 flare/CME events.
In the case of such broadside observations, anisotropy of
the angular extent is hardly determined from the analysis of g-value data without a priori information, even if it
exists (At least, we may conclude from our IPS data that
the longitude extent of ICME is not so large as to arise g
enhancements in the western hemisphere). Therefore, we
assume in the analysis for these events that the ICME had
a shell shape structure with an isotropic angular extent;
i.e. AR 1 . This is equivalent to an assumption that the
latitude extent of observed g-enhanced region is roughly
equal to the longitude extent, and a similar assumption
is often employed in the analysis of plane-of-projection
data such as coronagraph data.
(including the Bastille day event) are summarized in Table 1. All-sky g-maps obtained from our IPS observations for July 11, 12 and 14 events are indicated in Figure 2.
For the fitting analysis, we have employed a simple
∆Ne model, which assumes a Gaussian-form enhancement by an ICME and R 2 radial fall of the background
turbulence level. In this model, an anisotropic angular extent of ICME is taken into account, and an iso-∆N e contour at a given radial distance is assumed to be in an elliptic form which is defined by an e-folding major angular
span, θ0 , the ratio of the minor angular span to the major
one, AR, and the twist angle of the major angular span
direction to the heliographic equator, β . The expansion
speed VS of ICME is assumed to depend on the separation angle θ to the ICME center axis, and a function of
VS VS0 cosα θ 2 (where VS0 , α are constant) is used
as the angular dependence of VS . The effect due to deceleration or acceleration of ICME is neglected in this
model. Besides θ0 , AR, β , VS0 , and α , free parameters
of this model are the Carrington coordinate of the ICME
center axis (λ0 , φ0 ), the e-folding radial thickness D, and
the local enhancement factor at the center C1 (The ∆Ne
model used here is the same as one used in Paper I, and
more detailed description on it is presented therein).
As shown in the middle panel of Figure 2, g enhancements associated with the July 12 flare/CME event occur
at two areas, which are located symmetrically with respect to the Sun-Earth line in the map. This fact suggests
that the ICME had a loop- or rope-like structure which
elongated in longitude, as the case of the Bastille day
event (Paper I). On the other hand, g enhancements associated with July 10 and 11 events are found mostly in the
eastern hemisphere (see the right panel of Figure 1 and
the left panel of Figure 2). This implies that the central
part of those events propagated in the eastward direction
with a considerable offset angle to the Sun-Earth line.
RESULTS AND DISCUSSIONS
Calculated g-values gcal with the ∆Ne model have been
compared with observed g-values g obs for each ICME
event, and free parameters of the model have been adjusted to minimize the rms deviation σ between g cal and
gobs . Parameters determined by this analysis are listed
in Table 2. The correlation coefficient ρ between gcal
and gobs is also indicated in the table. The table includes
the results of the model fitting analysis for the Bastille
day event (Paper I). Provided that the global distribution of ∆Ne enhancements due to ICME is determined by
the current analysis, the mass contained by the ICME,
MICME , can be estimated by integrating the enhanced
∆Ne level over the volume occupied by the ICME [9].
Here, the solar wind density is assumed to be proportional to ∆N e , and SOHO in situ measurements are used
to determine the ambient level of the solar wind density,
which is necessary to estimate MICME . Estimated values
of MICME are shown in the bottom row of Table 2.
We summarize results of the present analysis as follows;
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TABLE 2. Results of the model fitting analysis.
Event
July 10
July 11
July 12
July 14
Location
VS0 (km/s)
α
D (AU)
θ0 (deg.)
C1
AR
β (deg.)
σ
ρ
MICME (g)
N13E19
880
2.9
0.07
51
4.2
1
S19E37
810
0.
0.08
18
6.9
1
0.205
0.798
2 6 1016
0.220
0.843
8 7 10 15
N02E41
880
1.9
0.07
202
4.8
0.04
0
0.218
0.827
6 5 10 15
N11W17
1540
3.3
0.13
130
6.4
0.15
28
0.140
0.762
5 5 10 16
agreement with a typical value of a CME mass. In comparing between ICME and CME mass estimates, it must
be noted that an ICME mass is considered to increase significantly due to the sweeping effect, which is one of important processes governing the deceleration of an ICME
in the interplanetary medium. Our results obtained here
are useful to clarify the sweeping effect and the deceleration profile of ICME, and a study from this viewpoint is
regarded as a promising future work (The deceleration of
ICMEs between the corona and the earth orbit has been
studied from our IPS observations [11]).
1). Our IPS data suggests that the ICME event associated with the 2000 July 12 flare/CME had a loop-like
(or toroidal) shape which stretched considerably in longitude. The minor angular span of this ICME event is
found to be 0.04 times as small as the major one. A
similar feature of the ICME has been reported from the
analysis of the Bastille day event (Paper I). From the
comparison with solar magnetograph measurements, it
is demonstrated that the loop orientation of July 12 and
14 (Bastille day) events is approximately normal to the
magnetic neutral line on the source surface. The looplike structure revealed for July 12 and 14 events might
be related to the magnetic structure of coronal ejecta, although the details are left to a future study.
2). For ICME events associated with July 10 and 11
flares/CMEs, it has been demonstrated that observed g
enhancements are fitted nicely by a shell shape ICME
model. Since the current analysis is based on a priori
information of AR, we cannot conclude unambiguously
that there are two types (loop or shell) of the ICME
global structure. However, g enhancements with a haloshape appearance have been observed for other halo
CME events (e.g. the 2000 June 6 event), and this fact
supports a shell shape model of ICME. Thus, we need to
investigate this point from further analysis.
3). Estimated values of the radial thickness D for
ICME events show good agreement with a typical thickness of the compression region driven by the IP shock at
1 AU [10]. The small angular span of ICME θ 0 for the
July 11 event is consistent with the missing encounter of
the IP shock to the earth. Locations of the ICME center
determined here are found to be close to those of the flare
site for all cases except for the July 11 event. Although
a slight difference of locations between the flare site and
the ICME center exists for July 10, 12 and 14 events, it
is considered to be within the error inherent to our IPS
observations. To examine whether this difference is intrinsic or not, IPS observations with a higher density are
required.
4). The estimated values of ICME mass are in good
ACKNOWLEDGMENTS
This work was supported by the IPS project of the SolarTerrestrial Environment Laboratory, Nagoya University.
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