1998年5月空间天气大事件的地磁场响应

地磁场与1998年5月空间大事件相对应的是5月1日至16日发生的大磁暴（k=8)。磁暴主相开始的几个小时伴随有丰富的Pc型地磁脉动，包括P c2,Pc3,Pc4等。在增暴的恢复相，甚至还有Pc5巨型脉动，在行星标磁场Bz由北向转向南向时，磁暴主相开始；南向分量达到最大值后大约2小时，地磁H分量达到最小值，恢复相开始，并且，这次磁暴与太阳风电场也存在一定的对应关系。

THE GEOMAGNETIC RESPONSE OF MAY 1998 EVENT

During the May 1998 great event time, there was a strong storm. This storm commenced on about 2200 UT of Mad 1. The initial phase was from 2200 of May 1 to 0300 of May 2, with a sudden commencement. The main phase was from 0300 of May 2 to 0600 of May 4. The recovery phase lasted quite a few days. Observations show that there are Pc2-Pc5 geomagnetic pulsations after sudden commencement of initial phase, and these pulsations, perhaps are due to the large scale lessening or expanding of the magnetosphere. The Beijing Obs. 1 Hz Data of horizontal component H during this storm on 1-5 May 1998 are analysed. It is found that there are a lot of Pc2-Pc3 pulsations on the first 4 hours in the main phase. The power spectrum firstly, then use the band pass filter are calculated. There are some Pc5 in the recovery phase, although in China latitudes are low: L =1-2. There are some Pc5-Pc6 in the storm day time in the India. In Beijing, the Pc5 magnitude is about 5nT. It is at 18 hourLT. The band of frequencies of Pc2 pulsation is 0.1-0.15 Hz, the band of Pc3 is 0.02-0.1 Hz, and the band of pc5 is 2.0-4.0 mHz. The storm recovery phase begins when the IMF turns less southward, with delays of about 2 hours. As Fig.2 shows, the Bz. component of IMF is turning to southward when the main phase of the storm begins, and the southward IMFs (Bz < 0) decrease to the minimum on 0400 of May 4, two hours later the storm recovery phase begins. The solar wind electrical field vBz is also calculated, where v is the SW speed, the Bz is the north-south component of IMF. Some results: vBz change suddenly when the initial phase is beginning; the main phase of the storm begins at 0300 of May 2, vBz is decreasing; the recovery phase begins when vBz turns to the minimum, with delays of about 2 hours are got.