Properties of the Geomagnetic Storm Main Phase and the Corresponding Solar Wind Parameters on 21–22 October 1999

Abstract

We use the SYM-H index to indicate the ring current index. We find that there were two periods during which the SYM-H index decreased quickly during the main phase of the geomagnetic storm on 21–22 October 1999. The first period from 11:44 p.m. UT on 21 October 1999 to 1:35 a.m. UT on 22 October 1999 is defined as step 1. Another period from 3:36 a.m. UT to 5:49 a.m. UT on 22 October 1999 is defined as step 3. The durations of step 1 and step 3 are defined as Δt1 and Δt3, respectively. The variation of the pressure-corrected SYM-H index during step 1 and step 3 are defined as ΔSYMHob1∗ and ΔSYMHob3∗, respectively. The interplanetary (IP) sources responsible for ΔSYMHob1∗ and ΔSYMHob3∗ are determined as the solar wind during period 1 and period 3, respectively. We find that the largest southward component of the interplanetary magnetic field (Bsmax) during period 3 was larger than that during period 1, and the largest solar wind dawn-to-dusk electric field (Eymax) during period 3 was also larger than that during period 1. We also find that the time integral of Ey during period 3 was much larger than that during period 1. However, we find that |ΔSYMHob1∗| was larger than |ΔSYMHob3∗|, and |ΔSYMHob1∗/Δt1| was larger than |ΔSYMHob3∗/Δt3|, indicating that the geomagnetic activity intensity during a period does not depend on Bsmax or Eymax, nor does it depend on the time integral of Ey. What is the reason for this? We find that the solar wind dynamic pressure during period 1 was larger than that during period 3, indicating that the geomagnetic storm intensity during a period not only depends on the solar wind speed and Bs, but it also depends on the solar wind dynamic pressure. The magnetosphere took 4 min to respond to the IP shock. When the z-component of the interplanetary magnetic field (IMF) turned from northward to southward, the response time of the SYM-H index to the southward component of the IMF was 21 min.