ON THE SOLAR ORIGINS OF INTENSE GEOMAGNETIC STORMS OBSERVED DURING 6–11 MARCH 1993 NANDITA SRIVASTAVA 1,* , WALTER D. GONZALEZ 1 , ALICIA L. C. GONZALEZ 1 and SATOSHI MASUDA 2 1 National Institute for Space Research (INPE), São José dos Campos (SP), 12201–970, Brazil 2 STELAB, Nagoya University, Honohara, 3–13, Toyokawa 442, Japan (Received 24 April 1998; accepted 17 August 1998) Abstract. Intense geomagnetic storms with D ST index ≤-100 nT were recorded on 9 March and 11 March 1993 associated with solar activity on 6 March and 9–10 March, respectively. In this paper, we discuss the characteristic features of the solar origins of the two events that gave rise to coronal and interplanetary disturbances and as a consequence produced strong geomagnetic activity at the Earth. The source of the activity in one case is attributed to a major 3M7.0 flare that occurred on 6 March 1993 and in the other case, to two large filament disruptions on the disk during 9– 10 March, 1993. Both these sources were found to be located near changing or varying low-latitude coronal holes. They were also located close to the heliospheric currents sheets. Distinct X-ray activity was observed for both the events as observed by the Yohkoh SXT telescope. The detailed evolution and a comparison of these events on the basis of Yohkoh soft X-ray observations are presented here. 1. Introduction The solar origin of coronal mass ejections (CMEs) and the related interplane- tary shocks which lead to geomagnetic activity, are not yet clearly understood. Several authors have reported on the solar origins of CMEs and the associated in- terplanetary and geomagnetic activity, for example, Burlaga, Behannon, and Klein (1987), Tang et al. (1989), Gosling et al. (1991), Tsurutani et al. (1988). Recent observations made by Soft X-ray Telescope (SXT) aboard Yohkoh have shown that CME events occurring in the corona and reaching the interplanetary medium often belong to a class of soft X-ray events, termed long-duration events or LDE’s by Webb and Hundhausen (1987), wherein the X-ray emission from the hot plasma decays slowly with time, from tens of minutes to hours after the peak intensity is attained. They also showed that LDEs are statistically more strongly associated with prominence eruptions (68%) than the flares of importance (>1) in Hα (42%) and metric type II and/or IV bursts (42%) which were in agreement with earlier studies by Sheeley et al. (1983), Sheeley et al. (1984) and Kahler et al. (1984). In order to understand solar-terrestrial relationship, a number of studies related to eruptive filaments or prominences that led to intense geomagnetic activity have been made, e.g., Joselyn and McIntosh (1981) and Watanabe and Schwenn (1989). * Now at Max-Planck-Institut für Aeronomie, 37191, Katlenburg-Lindau, Germany. Solar Physics 183: 419–434, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands.