PRIMARY STUDYING OF RELATIONSHIP AMONG CORONAL MASS EJECTIONS, Bs EVENTS AND GEOMAGNETIC STORMS
Liu Siqing1 Gong Jiancun1 J.K. Chao2 Xue Bingshen1
1 Center for Space Science and Applied Research, CAS,
P.O. Box 8701, Beijing 100080, China Fax: 86-10-62542551, email: [email protected]
2 Institute of Space Sciences, National Central University, Chong-li, Taiwan, China
Abstract. Two interesting solar events that took place in July 14, 2000 and Sept. 12, 2000 were studied in this paper. Two brilliant full-halo coronal mass ejections (CME) were recorded by the Coronagraphs on SOHO during these two events. The first CME accompany with a X5.7 x-ray flare which is one of the most powerful solar flares in the current solar cycle. A filament disappearance and a powerful x-ray solar flare were observed in the second event. However, the effects of these two solar events are quite different from each other. An extreme geomagnetic storm was triggered by the first CME, but the second one just led geomagnetic disturbances. It is well known that long duration of large southward interplanetary magnetic field (Bs event) is the primary cause of geomagnetic storms. Comparing these two events, we find that the IMF structures of the associated CME events are different. It is shown that the central axial field direction in magnetic cloud (MC) is related to magnetic field structure of the source region on the disc. Therefore we can predict the possibility of Bs events through the magnetic structure on the photosphere, because of the variation of IMF Bz can be deduced by MC Model.
1. Introduction
With the arrival of the maximum of 23rd solar cycle, more and more extreme solar events are observed. Since intense geomagnetic storms triggered by these events can cause disruption of satellite operation, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses, they become the key factor of space environment forecast (space weather). It is well known that solar eruption events, such as flares, coronal mass ejection (CME), filament disappearance, and eruptive prominences, are able to cause geomagnetic disturbance and geomagnetic storms. But it doesn’t mean that each solar event can cause an intense geomagnetic storm. The key problem of geomagnetic storm forecast is the characters of solar eruptions which can trigger intense geomagnetic storm.
A large number of observations and studies indicate that: the primary cause of geomagnetic storms are the long-duration southward interplanetary magnetic fields (IMF) in GSM coordinate system, so-called Bs events, which play an important role in determining the amount of solar wind energy to be transferred to the magnetosphere (Arnoldy, 1971; Tsurutani and Meng, 1972; Russell and McPherron, 1981; Akasofu, 1981; Gonzalez and Tsurutani, 1987) . When the sun erupts, it can suddenly and violently release bubbles or tongues of gas, and magnetic fields called coronal mass ejections (CMEs). Bs events usually occur within the interplanetary material associated with a CME, but not every CME will cause Bs event. It may be related to the characters of CME. To find what kind of CME will cause Bs event is the key goal of predicting geomagnetic storm associated with CME.
Scientists have been trying to forecast IMF Bz component according to the configuration of the photospheric magnetic field on solar source for many years. In 1976, Pudovkin and Chertkov claimed that the strength of geomagnetic disturbances could be predicted by the north-south disturbances of the photospheric magnetic field at the site of a solar flare (Pudovkin et al, 1976). In order to study the relationship between solar magnetic field and the variations of IMF Bz, at 1AU, Tang et al (Tang et al. 1985, 1989) examined the relationship between the polarity of the transient variation of IMF Bz and the associated flare field. Recently, Zhao et al (Zhao and Hoeksema, 1998) found that magnetic cloud central axial field directions were correlated with the central axial field directions of the associated filament on the Sun.
In this paper, two interesting events which took place in 2000 are analyzed. The relationships among the solar source magnetic field structure on photosphere, the transient variation of IMF Bz and the strength of the related geomagnetic disturbance are studied. It is confirmed that the interior magnetic structure of MC is close related to the photospheric magnetic field of the solar eruption source on the Sun. This result may be helpful in forecasting IMF Bs event and geomagnetic disturbance using magnetic field data on the solar surface.
2. Solar events of July 14, 2000 and Sept. 12 2000
At 1003UT on July 14, 2000 (Event A) , one of the most powerful solar flares of current solar cycle was recorded by NOAA satellites and the orbiting Solar and Heliospheric Observatory (SOHO). Its x-ray flux in 1.0-8.0 Angstrom band measurement onboard GOES-8 satellite reached 5.34×10-4watt/m2. Energetic proton arrived at the Earth about 15 minutes after the eruption. About 20 minutes after the breakup of the solar flare, coronagraphs (LASCO C2 and LASCO C3) on board SOHO recorded a bright “full-halo” CME. Solar material appeared to be heading toward our planet with a speed of 1300 to 1800 km/s. The interplanetary shock wave related to the CME hit the Earth at 1418UT, July 15. Meanwhile an extreme geomagnetic storm took place. Geomagnetic field index Kp equals 9 for 9 hours.
Solar Event |
July 14, 2000(Event A) |
Sept. 12, 2000(Event B) |
Flare class |
X5.7/SF |
M1.0/2N |
CME |
Full-Halo CME |
Filament Disappearance, Full-halo CME |
Position |
N16E02 |
S12W18 |
Proton Event |
Yes |
Yes |
Radio Storm |
II IV |
II |
Shock Wave |
Yes |
Yes |
Bs Event |
Bz<-10 for 7 hours |
None |
Geomagnetic Storm |
Extreme Geomagnetic Storm(Kp=9) |
None |
Table1:Comparison of these two solar events and their terrestrial effects
The other event was produced on Sept. 12 (Event B). A filament collapsed, spawning a powerful x-ray solar flare and a brilliant full-halo CME. The shock wave hit the Earth at about 0400UT on Sept. 15, however it didn’t cause a geomagnetic storm.
(a) (b)
Fig. 1. A filament disappearance recorded at the Big Bear Solar Observatory through a red Hydrogen-alpha filter
Comparing these two solar events (table 1), some similarities can be found. (1) Both of them have some important solar eruptive phenomena, such as flare, CME, Type II radio storm; (2) The eruption positions are near the center of the disc, and the CMEs aimed at the Earth; (3) Interplanetary shock waves were observed; (4) Proton events were observed in both events. The differences between these two events are: (1) The intensity of these two events are different: the eruption in Event A is stronger than Event B; (2) In Event A, the related IMF has a southward component (Bz< -10nT) for at least 7 hours, whereas in Event B, the IMF is northward; (3) Extreme geomagnetic storm was triggered in Event A. The Kp index equals 9 for about 9 hours; Dst index reached –295. On the contrary, no geomagnetic storm took place during Event B.
3. the Relationship between the Structure on the Photosphere and IMF Structure
From the above analysis, it is found that the most important reason of the different geomagnetic effects of these two events should be the intensity of the eruption and configuration of IMF within the solar material. The intense southward IMF Bz is associated with two types of origins (Gonzalez and Tsurutani, 1987). One of them has an intrinsic solar origin. The other type has an interplanetary origin.
(a) (b)
Fig. 2. The magnetogram observed as the solar erupted
In
the event of first type,
the southward IMF Bs event is
just a part of the internal magnetic field in the ejected plasmas
from the sun. At this
case, the magnetic
structure in the flare regions must have some characteristic
features. Fig. 2 is the
solar magnetogram of these two events. In these pictures, blue and
green indicate the
magnetic field is outwards,
red and yellow indicate
the magnetic
field is inwards.
The brightness represents the intensity of the magnetic field. A
big
difference can be found
in these two pictures:
The eruption in Event A comes from an unusual north-south oriented
active region. The magnetic neutral line in
the source area is
parallel with
the equatorial plane (Fig.
2a). The eruption in
Event B comes from a usual east-west oriented active region with
the magnetic neutral
line perpendicular to
the equatorial plane (Fig.2b).
Comparing Fig.1a and
Fig. 2b, it is also shown that
the filament lay on the magnetic neutral
line.
The Burlaga’s magnetic cloud model (Fig. 3) can be used to study the relationship between the solar magnetic field and the IMF Bz component. We suppose that the solar magnetic field from the solar surface is carried by the solar material and transports into the interplanetary region with the magnetic structure described by the Burlaga’s model.
W
Fig.
3. Magnetic cloud (Burlaga Model)
ith this assumption,
the axis of magnetic cloud must be parallel with
the magnetic neutral
line of the active region. In Event
A, the axis is almost
parallel with the ecliptic
plane and lays
on the Y axis of GSE coordinate (Fig.
4a). Fig. 4b is a sketch for the
MC in Event A
projected in the XZ plane. The spacecraft trajectory is displayed as
the dashdotted line. When
the spacecraft goes across the MC from L to N, Bz component changes
from the minimum (southward, point L), to zero (point M), and then to
the maximum (northward, point N). At the same time, Bx component
keeps positive, with its
intensity changing from the minimum (L) to the maximum (M), and then
again to the minimum (N). In the MC model described by Burlaga, the
field strength decreases as we go away from the cloud’s axis.
The axial component decreases while the tangential component
increases. Therefore, the intensity of By component changes in the
same way as that of Bx, that is changing from the minimum (L) to the
maximum (M), and then to the minimum (N). The orientation remains
positive.
(a) (b)
Fig. 4. The sketch of the changes of the magnetic field components as spacecraft passes across the MC in Event A
Fig. 5. The Magnetic field data ACE satellite observed during July 14-16, 2000
Fig. 5 shows the intensity and three components of the IMF as ACE satellite passes through the magnetic cloud in Event A. Three vertical lines represent the three points (LMN) described in Fig. 4b.It can be seen that the variations of the three components of the IMF follow the model described above. For example, the Bz component changes from –52nT (L) to zero (M), and then to 32nT(N).
The axis of the second MC is almost perpendicular to the ecliptic plane and lays on the Z axis of GSE coordinate. Fig. 6a shows the second MC structure in Burlaga Model. The intensity and three components of the IMF in event B observed by ACE satellite are displayed in Fig. 7. It is very obvious that the magnitude and the components are small. This kind of observations can be obtained if we assume the satellite passed through the MC from the limb region. Fig. 6b is a sketchy plot for the second MC projected in the XZ plane. When the satellite passed through the MC from L to N point, Bz would be small, Bx would be positive, and By would keep negative. The observations show agreement with the analysis using Burlaga’s Model in Fig. 6b.
(a) (b)
Fig. 6. The sketch of the changes of the magnetic field components as spacecraft passes across the MC in Event B
Fig. 7. The Magnetic field data observed by ACE satellite during Sept. 14-16, 2000
4. Discussion and Conclusions
From the analyses of the relationships among the magnetic structure on the solar source, IMF and the geomagnetic field disturbance, several important conclusions can be obtained.
It is confirmed that there is an important relationship between the characteristics of the internal magnetic field of MC and the magnetic structure on the solar surface.
The position of the magnetic neutral line is related to the position of the filament.
It is possible to presume the structure of MC through the magnetic field observed on the solar surface, and predict the possibility of Bs events and geomagnetic storms.
We think this result is useful in deducing whether a CME can cause geomagnetic storm. Further study is needed. First, more events should be included to verify this result. Second, it’s not enough to analyze the structure only from the solar magnetic field data, because the orientation of the axial component can hardly be determined by the photospheric magnetic field. Research has shown that the outer solar atmosphere observation (such as, Extreme ultraviolet Imaging Telescope on board SOHO) and calculation result may provide more help in determining the orientation. Lastly, which part of MC that sweeps the Earth is also an important factor that will affect the IMF orientation near 1AU.
Acknowledge: The magnetogram data is from the synoptic program at the150-Foot Solar Tower of the Mt. Wilson Observatory. The Mt. Wilson 150-Foot Solar Tower is operated by UCLA, with funding from NASA, ONR and NSF, under agreement with the Mt. Wilson Institute. The authors thank the ACE science center for providing the IMF data from ACE satellite. The Hydrogen-alpha images of the Sun captured at the Big Bear Solar Observatory. The authors would like to thank Prof. S. Y. Fu for useful comments.
References:
Akasofu, S. –I., “Energy Coupling between the Solar Wind and the Magnetosphere”, Space Sci. Rev. 28, 111, 1981.
Arnoldy, R. L., Signature in the interplanetary medium for substorms, J. Geophys. Res., 76, 5189, 1971.
Bothmer, V., and R. Schwenn, Signatures of fast CMEs in interplanetary space, Adv. Space Res., 17, 319-322, 1966.
Gonzales, W. D. and B. T. Tsurutani, Criteria of interplanetary parameters causing intense geomagnetic storms (Dst<-100nT). Planet. Space Sci., 35, 1101-1109, 1987.
Burlag, L. F., E. Sittler, F. Mariani, and R. Schwenn, Magnetic loop behind an interplanetary shock: Voyager, Helios and IMP-8 observations, J. Geophys. Res., 86, 6673, 1981.
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Russell, C. T. and R. J. McPherron, Semiannual variation of geomagnetic activity. J. Geophys. Res., 78, 92-95, 1973.
Tang, F., S. –I. Akasofu, E. Smith, and B. Tsurutani, Magnetic fields on the sun and the north-south component of transient variations of the interplanetary magnetic field at 1 AU, J Geophys. Res., 90, 2703-2712, 1985.
Tang, F., B. T. Tsurutani, W. D. Gonzalex, S. –I. Akasofu and E. J. Smith, Solar sources of interplanetary southward Bz events responsible for major magnetic storms, J. Geophys. Res., 94, 3535-3541, 1989
Tsurutani, B. T. and C. –I. Meng, Interplanetary magnetic-field variations and substorm activity, J. Geophys. Res, 77, 2964-2970, 1972.
Zhao, X. P., J. T. Hoeksema, Central axial field direction in magnetic clouds and its relation to southward and dependence on disappearing solar filaments, J. Geophys. Res., 103, 2077-2083, 1998
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