Local-time Distribution of Low-latitute Ground Magnetic Disturbances at Sawtooth Injections of April 18-19, 2002

A magnetic storm which occurred on April 17-19, 2002 has been studied to investigate a development of the ring current and the current structure which was set up during the magnetic storm. A Coronal Mass Ejection (CME) passed through the Earth on April 17, 2002, which caused an intense magnetic storm on the ground. The main phase of the magnetic storm started at 11 UT on April 17 and developed until 19 UT on April 18 with minimum Dst of -123 nT. On April 18, low energy electron flux observed by four LANL satellites showed quasi-periodic perturbations (2-3 h) in the energy range of 50-315keV: Events of this type has been called "saw-tooth" events. In the solar wind data from the ACE satellite, the IMF-Bz component was stable and southward (-10 nT) from 1 UT till 17 UT on April 18. On the other hand, magnetic variations on the ground showed Bay-like magnetic variations with amplitudes of 10-40 nT that were synchronized with particle injections observed by the LANL satellites. We have used magnetic data from 8 ground stations of the Circum-pan Magnetometer Network (CPMN) which are LMT (Mlat=-33.53 deg.), MUT (6.39), EWA (21.57), GAM (5.64), YAP (0.50), SMA (-19.82), LAQ (36.25) and HER (-42.12): They are widely separated in longitudinal direction in the middle and low latitudes. The Bay-like magnetic variations, which were synchronized with the particle injections, were predominant in the H-component, and showed similar waveforms at all stations. D-component variations were smaller than the H-component. Clear Pi 2 pulsations also appeared globally, corresponding to the above magnetic variations. We have so far studied the Pi 2 which occurred at 19:07 UT on April 18: In this case, each ground stations located at 04:30 MLT (LMT), 05:00 (MUT), 06:30 (GAM), 06:15 (YAP), 12:00 (EWA), 17:00 (SMA), 22:00 (LAQ) and 21:00 (HER). Amplitudes of the Pi 2s were large in the morning sector (> 1 nT) and weak at other local times (< 0.5 nT). We have calculated the directions of the perturbation magnetic vectors during the Bay-like magnetic variations (ΔD, ΔH) and polarizations of the Pi 2s. As a result, we have found the following features. The direction of the polarization shows a consistency with the existence of the current wedge, as suggested by Lester et al. [1984]. On the other hand, the directions of (ΔD, ΔH) were opposite to those of the Pi 2 polarizations in the southern hemisphere, although they were the same in the northern hemisphere. Thus, the Bay-like magnetic variations in this event cannot be interpreted in terms of the field aligned currents (FACs) of the nominal current wedge which produced the Pi 2 pulsations. They are also difficult to explain in terms of the nominal partial ring current, because the low-latitude H-component increased, not decreased, for the events, and because the low-latitude D-component did not change its sign across the equator. We will discuss possible causes of the Bay-like variations.