J. Geomag. Geoelectr., 47, 1127-1132, 1995

The Interplanetary and Solar Causes of Major Geomagnetic Storms

Volker BOTHMER* and Rainer SCHWENN

M=-Planck-Institut fur Aeronomie, 37189 Katlenburg-Lindau, Germany

(Received November 28, 1994; Revised May 18, 1995; Accepted September 29, 1995)

We have investigated the solar wind input conditions for geomagnetic storms with Kp greater or equal 8- during the years 1966-1990 in order to clarify the interplanetary and solar causes of major geomagnetic storms. Forty-one out of the forty-three analyzed storms were found to be caused by transient shock-associated solar wind ejecta (driver gases and magnetic clouds), one storm was caused by a slow moving magnetic cloud followed by a corotating interaction region (CIR) and one by a CIR. Independent of the phase of solar activity coronal mass ejections (CMEs) CMEs are found to be the sources of major geomagnetic storms. A similar result was obtained using the D,-index as an indicator of major geomagnetic storms. The maximum Kp-values of the individual events were directly related to the peak southward components of the interplanetary magnetic field (IMF). Draping of the IMF near the front edges of driver gases (magnetic clouds) or/and the magnetic field configurations of magnetic clouds were the sources of the extreme negative B, values. The intensity of the southward components was often substantially amplified at the leading and rear edges of magnetic clouds due to their interaction with the ambient solar wind. This was found to be of particular importance for cases where magnetic clouds were followed by CIRs or interplanetary shocks, especially during very disturbed interplanetary conditions, i.e. during sequences of CMEs.

1. Introduction

In a recent analysis of ISEE 3 data for the years 1978-1982, Gosling et al. (1990) found that during high solar activity strong geomagnetic storms with Kp >-8- were caused by fast coronal mass ejections (CMEs) at the Sun. These fast CMEs had produced solar wind shock-disturbances at 1 AU associated with strong southward magnetic fields. In nearly all cases the Earth was encountered by both the shock-wave and the outward propelled coronal material driving the shock-wave. Tsurutani et al. (1992) have investigated the interplanetary sources of the five largest geomagnetic storms between 1971 and 1986 identified from the variation of the geomagnetic D,t-index. The intense negative B,events that have triggered the five stones were either caused by draping of the interplanetary magnetic field (IMF) in the sheath region ahead of driver gases or/and through the internal magnetic field configuration of the driver gas itself which is commonly called a magnetic cloud if it exhibits a coherent internal rotation of the magnetic field vector (e.g. Klein and Burlaga, 1982). Both studies were concerned with relatively small subsets of intense geomagnetic storms. While energetic CMEs occur mainly during periods ofhigh solar activity, high speed streams from coronal holes and associated corotating interaction regions (CIRs) are dominant features during minor solar activity. Coronal holes are another well known source of geomagnetic stones (e.g. Sheeley et al., 1976; Burlaga, 1975). Near-Earth solar wind data, gathered for more than thirty years now, allow a thorough search for the interplanetary and solar sources of intense geomagnetic storms during different phases ofthe solar activity cycle to help further clarify the overall importance of high speed streams and CMEs for geomagnetic disturbances.

*Now at Space Science Department of ESA, ESTEC, 2200 AG Noordwijk, The Netherlands.

1127 1129 V. BOTHMER and R. SCHWENN

2. Selection of Major Geomagnetic Storms and Investigation of Solar Wind Data

We have based our analysis primarily on the geomagnetic Kp-index and have selected storms with intensities ofKp >_8-in analogy to the previous study by Gosling et al. (1990). Kp-, D,,-values and hourly averaged near-Earth solar wind data are available from the OMNI data base at the National Space Science Data Center (NSSDC), Greenbelt Maryland. A detailed description of this data set has been given, e.g. by King (1991). Additionally Helios 1/2 data were used in a few cases when the spacecraft were close to the Earth during a geomagnetic storm. The plasma and magnetic field parameters were carefully examined to identify the characteristics of each storm, with results from previous analyses being included (e.g. Borrini et al., 1982). Our investigations yielded a total number of 43 storms (approximately 50% of all storms with Kp ? 8- during 1966-1990) for which a sufficiently well data coverage allowed us to identify properly their solar wind sources.

3. Interplanetary and Solar Sources of Geomagnetic Storms with Kp ? 8-

Interplanetary magnetic clouds were identified as important sources of geomagnetic storms with

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I' l rn 1 1 4 1 1 1 i Hour 12 18 0 6 12 18 0 6 12 18 0 6 12 10 0 6 12 I SHOCK 1 Day 352 984 358

Fig. 1. One hour averages of magnetic field (Be, B, 0, m), plasma (V, N, T) and geomagnetic (Kp, D,, (nT)) data for the strong geomagnetic storm in Dec. 1980 (KpMa. = 8-). Highest geomagnetic activity was related to the intense negative Bi values at the leading edge of the SN-cloud (Bothmer, 1993; see also Zhang and Burlaga, 1988).

Ii The Interplanetary and Solar Causes of Major Geomagnetic Storms 1129

Kp 2:8-. Figure 1 shows a typical example of a geomagnetic storm (December 1980, KpMSX= 8-) that was caused by a shock-associated magnetic cloud. Note the smooth south (S) to north (N) turning of the magnetic field's latitudinal angle Band the response in Kp and D,,. Peak geomagnetic activity was directly related to the intense interplanetary southward magnetic fields in the front part of the magnetic cloud. Data for a storm in April 1973 (KpMa==8-) are presented in Fig. 2. In contrast to Fig. 1 this storm was triggered by a slow moving NS-cloud that was not driving a shock-wave. The cloud was compressed at its rear edge. Note the increase of the magnetic field strength towards the rear edge of the cloud. This compression was caused by a CIR of a recurrent high speed stream from a coronal hole. The high speed stream can be identified from its higher than average plasma speed and temperature and its lower density. It is obvious from this example that interaction of solar wind streams can substantially amplify geomagnetically efficient interplanetary conditions. Directional changes of the IMF (IMF-draping, see e.g. McComas et al., 1989) in the sheath region between shock-waves and driver gases including magnetic clouds were identified as a second main triggering mechanism for intense geomagnetic storms. Figure 3 reveals that highest geomagnetic activity during the storm in April 1979 (KpMax= 8) was caused by draping of the IMF ahead of a fast moving magnetic cloud driving a shock. A substantial number of strong storms was also found to be triggered by both effects, IMF-draping and the internal magnetic

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Fig. 2. Data for the geomagnetic storm in April 1973 (KpM„ = 8-) caused by a slow (without shock) moving NS-cloud. Peak geomagnetic activity was related to the strong southward magnetic fields at the cloud's rear part due to compression of a high speed stream overtaking the cloud (the magnetic cloud was identified by Klein and Burlaga, 1982).

N1I9 1130 V. BOTHMER and R. SCHWENN

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Fig. 3. A geomagnetic storm with KpMa,= 8 caused by draping of the IMF ahead of a shock-associated magnetic cloud (the cloud was identified by Burlaga et al., 1987).

fields of magnetic clouds. Figure 4 shows the interplanetary conditions during the very intense storm in July 1974 (KpMax=9-)that occurred close to a solar activity minimum. The source ofthis storm was a series of shock- associated driver gases, i.e. fast CMEs. Pre-existing southward magnetic fields associated with a preceding CME were enhanced through compression by a faster moving CME overtaking the slower one.

4. Results and Conclusions

From an investigation of the solar wind conditions during 43 geomagnetic storms with Kp ? 8- between 1966-1990 it is found that 41 storms were caused by shock-associated driver gases including magnetic clouds. One storm was caused by a slow (without shock) moving magnetic cloud followed by a CIR. Only a single storm was caused by a CIR, i.e. a high speed stream from a coronal hole. The fact that intense interplanetary southward fields were observed during all storms documents that a southward IMF is necessarily required for an enhanced energy transfer into the Earth's magnetosphere, presumably via magnetic reconnection. Draping of the IMF near the leading edges of driver gases (magnetic clouds) or/and the magnetic configurations of magnetic clouds have been identified as the interplanetary sources of the extreme negative B, values during the storms. The intensity of the southward fields has often been substantially

i1 The Interplanetary and Solar Causes of Major Geomagnetic Storms 1131

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Fig. 4. A geomagnetic storm with KpMax= 9- triggered through the passage of 3 CMEs, Note the sequence of three shocks associated with driver gas signatures (see Borrini et al., 1982).

amplified at the leading/rear edges of magnetic clouds due to their interaction with ambient solar wind streams (e.g. clouds followedby CIRs or interplanetary shocks), especially when multiple energetic CMEs cause strong disturbances in the heliosphere. Intense geomagnetic storms were found independent of the phase of the solar cycle to be caused by fast CMEs at the Sun. This can be explained by the observation that CMEs can produce interplanetary disturbances associated with strong magnetic field strengths and long-lasting southward fields at 1 AU, whereas high speed streams from coronal holes and associated CIRs are usually associated with minor magnetic field strengths and thus produce weaker geomagnetic activity.

REFERENCES

Borrini, G., J. T. Gosling, S. J. Bame, and W. C. Feldman, An analysis of shock wave disturbances observed at 1 AU from 1971 through 1978, J. Geophys. Res., 87, 4365-4373, 1982. Bothmer, V., Die Struktur magnetiscber Wolken im Sonnenwind, Ph.D. Thesis, Universitat Gbttingen, 1993. Burlaga, L. F., Interaction with the Earth, Space Sci. Rev., 17, 327-352, 1975. Burlaga, L. F., K. W. Behannon, andL. W. Klein, Compound streams, magnetic clouds, andmajorgeomagnetic storms,J. Geophys. Res., 92, 5725-5734, 1987. Gosling, J. T., S. J. Bame, D. J. McComas, and 1. L. Phillips, Coronet mass ejections and large geomagnetic storms, Geophys. Res. Lett., 17, 901-904, 1990.

iF 1132 V. BOTHMER and R. SCHWENN

King, J. H., Long-term solar wind variations and associated data sources, J. Geomag. Geoelectr., 43, 865-880, 1991. Klein, L. W. and L. F. Burlaga, Interplanetary magnetic clouds at 1 AU, J. Geophys. Res., 87, 613-624, 1982. McComas, D. J., J. T. Gosling, S. J. Bame, E. J. Smith, and H. V. Cane, A test of magnetic field draping induced Bz perturbations ahead of fast coronal mass ejects, J. Geophys. Res., 94,1465-1471, 1989. Sheeley, N. R., Jr., J. W. Harvey, and W. C. Feldman, Coronal holes, solarwind streams, and recurrent geomagnetic disturbances: 1973-1976, Solar Phys., 49, 271-278, 1976. Tsurutani, B. T, W. D. Gonzalez, F. Tang, and Y. T. Lee, Great magnetic storms, Geophys. Res. Lett., 19, 73-76, 1992. Zhang, G. and L. F. Burlaga, Magnetic clouds, geomagnetic disturbances, and cosmic ray decreases, J. Geophys. Res., 93, 2511- 2518,1988.