Solar Wind Disturbances and Their Sources in the EUV
Solar Corona
A. N. Zhukov † , I. S. Veselovsky† , F. Clette , J.-F. Hochedez , A. V. Dmitriev† ,
E. P. Romashets , V. Bothmer‡ and P. Cargill§
Royal Observatory of Belgium, Avenue Circulaire 3, B–1180 Brussels, Belgium
Institute of Nuclear
Physics, Moscow State University, Moscow 119992, Russia
IZMIRAN, Troitsk, Moscow Region 142190, Russia
‡
Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, D–37191 Katlenburg-Lindau, Germany
§
Imperial College of Science, Technology and Medicine, London, SW7, 2BZ, UK
†
Abstract. We investigate possible links between the activity manifestations in the solar corona and conditions in the solar
wind. For the reduction of this immense task we have selected 206 events in the solar wind in 1997 – 2000 corresponding
to geomagnetic events with Ap > 20 (compiled into a database at http://alpha.sinp.msu.ru/apev). Up to now,
24 events during the epoch of low solar activity (January 1997 – January 1998) are investigated. The solar wind conditions
monitored by ACE and WIND spacecraft were traced back to the solar corona observed by SOHO/EIT. The search for coronal
signatures which are probably associated with the disturbed solar wind conditions was performed. The coronal sources of
these 24 events are identified, namely: eruptions in active regions, filament eruptions and coronal holes. It is shown that
halo and partial halo CMEs observed within the SOHO/LASCO sensitivity limits are not necessary indicators of Earthdirected eruptions, and coronal EUV dimmings can be used as a complementary indicator. We also found that a structure now
conventionally called a “sigmoid” cannot be represented as a single S-shaped loop (flux tube), but exhibits an assembly of
many smaller structures. It could be formed and destroyed via eruptions.
INTRODUCTION
upper limit of the one standard deviation interval in 1997
– 2000). The statistical analysis of these events was done
by Bothmer et al. [1]. The events were compiled into
the database located at http://alpha.sinp.msu.
ru/apev. To date 24 events occurred in January 1997
– January 1998 were investigated, thus the epoch of low
solar activity is considered in this paper.
The procedure of analysis is as follows. First, using
ACE and WIND spacecraft data, we identify the solar
wind disturbance which produced increase of A p index
(shock, discontinuity of another type, region of strong
negative Bz , increase in velocity, pressure etc). Average
velocity of the disturbance is found and the approximate
start time from the Sun is estimated assuming constant
solar wind velocity en route from the Sun to the Earth.
To establish
more precise timing, halo (angular width
360 ) and partial halo (angular width more than 120 )
CMEs detected by SOHO/LASCO close to the estimated
start time were identified using the CSPSW/NRL CME
catalog (http://cdaw.gsfc.nasa.gov/CME_
list/index.html). In most cases a possible source
halo/partial halo CME is found to happen in one –
two days interval around the estimated start time. The
coronal activity observed by EIT close to the initiation
Physical conditions on the Sun responsible for the production of appreciable geomagnetic perturbations are
still under investigation. The biggest geomagnetic storms
develop in association with the appearance of strong heliospheric perturbations seen in plasma and magnetic
field parameters measured by satellites in the solar wind.
They are commonly related to powerful nonstationary
processes in the solar corona and deeper layers of the solar atmosphere visualized by different observations. Impulsive and long-duration solar flares, disappearing filaments, coronal mass ejections (CMEs), transient brightenings (dimmings) and coronal holes represent the most
popular signatures to date used for diagnostic purposes.
The aim of this paper is to report several examples illustrating the preliminary results on the search of the geomagnetic disturbances sources in the SOHO/EIT data.
ANALYSIS AND RESULTS
We have selected 206 events in 1997 – 2000 corresponding to the days with daily A p index more than 20 (an
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
711
FIGURE 1. NOAA AR 8040 observed by SOHO/EIT in the Fe XII bandpass (195 Å) on May 21, 1997 at 06:13:09 UT (left
panel) and 06:43:10 UT (right panel). Note the dimming in the SW part of the AR in the right panel.
is shown in Figure 1). Hence, halo/partial halo CMEs
(as observed by LASCO) are not necessary indicators of
Earth-directed eruptions.
A most interesting problem linked with eruptions is
their precursors. It was reported [3, 4, 5] that an eruption
in AR is often accompanied by the sigmoid-to-arcade restructuring of this AR seen by Yohkoh/SXT. It is argued
[5] that sigmoid represents an S-shaped loop (or a flux
tube) stretched along the photospheric magnetic neutral
line – an unstable configuration which is then erupted
[6]. AR transforms into an unsheared arcade with its axis
along the pre-eruption sigmoid.
Canfield et al. [4] found 23 sigmoidal ARs in 1997.
Later, Glover et al. [7] reported that only 7 of them were
truly sigmoidal, others being either projected sigmoidal
(when many structures collectively form an S-shaped
feature) or non-sigmoidal. Glover et al. [7] stress the
need of observational definition of the term “sigmoid”.
In the recent work Glover et al. [8] state that a truly
sigmoidal AR for most of the time exhibits two J-shaped
loops which reconnect and form a true sigmoid just
before the eruption.
Our analysis of the EIT data leads to a stronger conclusion: S-shaped loops along the neutral line are never
observed (at least in our limited data set), see e. g. NOAA
AR 8038 – truly sigmoidal according to a modified classification by Glover et al. [7] – in the right panel of Figure
3. In the SXT data “sigmoids” are either interrupted, or
partly saturated, or too diffuse to reveal their magnetic
structure. EIT images (Figure 3, right panel) show that
the overall configuration of AR indeed could be S-shaped
(it is even better seen in the 284 Å bandpass), but no single S-shaped loop (flux tube) is observed.
Another interesting feature of NOAA AR 8038 (which
erupted on May 12, 1997 to produce the event 12, see
Thompson et al. [2]) is the origin of its “sigmoidal” struc-
of this CME was studied to identify the coronal structure
and/or process which is the most probable candidate to
be the source of the CME (and of the geomagnetic disturbance). Sometimes no halo/partial halo CMEs were
observed during these days. In this case we investigate
the EIT data in the 3-day window around the estimated
start time.
The list of events with their identified sources is shown
in Table 1. Three events (numbers 2, 3, 10) are produced by flows from coronal holes (CH), either equatorial ones (event 3) or extensions of polar CHs to the
equator (events 2, 10). 7 events (9, 12, 13, 15, 16, 20, 21)
are produced by eruptions in active regions (AR). Event
11 is a combined event: plasma erupted from the AR is
pushed by a faster flow from the CH. 3 events (14, 23, 24)
are produced by the eruptions of filaments inside ARs or
connected to them. 4 events (4, 6, 17, 18) are produced
by filament eruptions outside ARs. (A filament producing event 17 is situated in the remains of decayed AR
8076, and at the time of eruption no sunspots could be
seen there.) 4 events (5, 7, 19, 22) have the same source
as events of the previous day (4, 6, 18, 21 respectively).
These events are either long-duration events or represent long (more than a day) relaxation of the A p index
to undisturbed values. Sources of 2 events (1 and 8) are
not identified as EIT was baking out.
Following examples illustrate the obtained results.
Consider the event 13. We note first that it cannot be produced by a flow from a CH because the average velocity
of the geoeffective solar wind disturbance was very low
(about 360 km s 1 ). During previous two weeks LASCO
did not observe a halo/partial halo CME. According to
e. g. Thompson et al. [2], Hudson et al. [3], eruptions
could be accompanied by coronal dimmings. Indeed,
on May 21 EIT detected two eruptions in the NOAA
AR 8040 associated with dimmings (the first eruption
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TABLE 1. List of geomagnetic events in January 1997 – January 1998 with Ap > 20 and their coronal sources observed
by SOHO/EIT
Event
number
Date
dd.mm.yy
1
2
3
4
5
6
7
8
9
10
11
10.01.97
28.01.97
08.02.97
10.02.97
11.02.97
27.02.97
28.02.97
28.03.97
11.04.97
17.04.97
01.05.97
12
13
14
15
16
17
18
19
20
21
22
23
15.05.97
27.05.97
09.06.97
03.08.97
03.09.97
01.10.97
10.10.97
11.10.97
07.11.97
22.11.97
23.11.97
30.12.97
24
07.01.98
Coronal source seen by SOHO/EIT
(EIT bakeout)
extension of the southern polar CH to the equator
CH near the disk center
southern polar crown filament eruption
same as event 4
filament eruption in the southern hemisphere
same as event 6
(EIT bakeout)
two eruptions in NOAA AR 8027
extension of the southern polar CH to the equator?
eruption in NOAA AR 8035 followed
by the flow from the southern polar CH
eruption in NOAA AR 8038
eruption in NOAA AR 8040
filament eruption from the west of NOAA AR 8048
eruption in NOAA AR 8066
eruption in decaying NOAA ARs 8076, 8078, 8079
filament eruption in remains of NOAA AR 8076
southern polar crown filament eruption
same as event 19
eruption in NOAA AR 8100
eruption in NOAA AR 8108
same as event 21?
eruption of the filament channel extending
out from NOAA AR 8124
eruption of the filament-like structure
to the east of NOAA AR 8130
Start from
the Sun
–
end of Jan. 23
Feb. 5
Feb. 7, 01:05 UT
–
Feb. 22, 01:18 UT
–
–
Apr. 6, 23:10 UT and Apr. 7, 14:00 UT
Apr. 13
Apr. 27, 03:08 UT
May 12, 04:50 UT
May 21, 06:13 UT and/or 20:42 UT
Jun. 1, 22:35 UT
Jul. 31, 01:43 UT
Aug. 29, 23:24 UT
Sep. 27, 17:58 UT
Oct. 6, 12:13 UT
–
Nov. 4, 05:58 UT
Nov. 17, 03:51 UT
–
Dec. 25, 21:49 UT
Jan. 2, 22:34 UT
FIGURE 2. Remains of NOAA ARs 8076, 8078 and 8079 observed by SOHO/EIT in the Fe XII bandpass (195 Å) on August 30,
1997 at 00:49:26 UT (left panel; note the “sigmoidal” structure) and 01:12:31 UT (right panel; note rising post-eruption loops).
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FIGURE 3. NOAA AR 8038 observed by SOHO/EIT in the Fe XII bandpass (195 Å) on May 10, 1997 at 11:06:10 UT (left
panel), 14:34:10 UT (middle panel) and 23:55:17 UT (right panel). Note the dimming associated with the eruption to the south of
the AR in the middle panel and an assembly of small-scale structures forming an overall S-shaped feature in the right panel.
ACKNOWLEDGMENTS
ture on May 10. It appears that the “sigmoid” is formed
as a result of another eruption as shown in Figure 3. This
AR exhibits an example of the process when “sigmoidal”
configuration (unstable?) is formed via an eruption and is
destroyed via an eruption as well.
Finally, consider event 16 produced by an eruption
in the decaying NOAA ARs 8076, 8078 and 8079 on
August 29–30, 1997. In the left panel of Figure 2 one
can see a “sigmoidal” structure (close to saturation). It is
interesting to note that post-eruption loops have the same
footpoints as the “sigmoid”, and there is no signature of
reconnection although the “sigmoidicity” of that part of
the AR disappears.
SOHO/LASCO and SOHO/EIT consortiums are acknowledged for the data used during the preparation
of this paper. SOHO is a joint ESA–NASA project.
The CME catalog used is generated and maintained by
the Center for Solar Physics and Space Weather, The
Catholic University of America in cooperation with the
Naval Research Laboratory and NASA. The authors are
grateful to the ACE and WIND teams for open access via
Internet to the solar wind data. This work is supported by
the Belgian OSTC program for cooperation with Central
and Eastern Europe, INTAS–ESA 99-00727 and INTAS
00-752 grants. The work in the Moscow State University
is supported by the RFBR, State Program “Astronomy”
and the Program “Universities of Russia” grants.
CONCLUSIONS
We have investigated the coronal sources of geoeffective
interplanetary disturbances during the low activity epoch
(January 1997 – January 1998). Using the EIT observations of the EUV solar corona, we have preliminarily
identified the sources of all 24 geomagnetic events with
daily A p index values more than 20. The identification of
the source is mostly unambiguous; it is doubtful only in
2 cases (events 10 and 22).
It is found that halo/partial halo CMEs observed
within LASCO sensitivity limits are not necessary indicators of Earth-directed eruptions (events 13, 14, 15). We
propose to use the coronal dimmings observed by EIT as
a complementary indicator. Indeed, transient dimmings
are associated with all of the events except those produced by flows from CHs.
A structure now conventionally called a “sigmoid”
cannot be represented as a single S-shaped loop (flux
tube) but instead exhibits an assembly of many structures
of smaller scale.
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