A Prolonged Southward IMF-Bz Event of May 02--04, 1998: Solar, Interplanetary Causes and Geomagnetic Consequences

A detailed investigation was carried out to understand a prolonged (~44 hours) weakly southward interplanetary magnetic field (IMF-Bz) condition during May 02--04, 1998. In-situ observations, during the period, showed the passage of an expanding magnetic cloud embedded in an interplanetary coronal mass ejection (ICME), followed up by a shock and an interplanetary discontinuity driven by another ICME. It is the arrival of the ICMEs and the upfront shocks that cause the prolonged southward IMF-Bz condition. The magnetic configuration of the source regions of the IMF associated with the ICME interval were also examined, which showed open magnetic field structures, emanating from a small active region on the north of the heliospheric current sheet (HCS). The structures remained constantly to the north of the HCS, both on April 29 and May 01, suggesting no change in their polarity. The draping of these outward directed radial field lines around the propagating CMEs in the shocked plasma explained the observed polarity changes of the IMF-Bz at 1 AU. In addition, multiple enhancements were also detected in the geomagnetic field variations, which showed a distinct one-to-one correspondence with the density pulses observed at 1 AU, during 0700--1700 UT on May 03. The spectral analysis of both the variations showed the same discrete frequencies of 0.48, 0.65 and 0.75 mHz, demonstrating that the solar wind density enhancements can cause detectable global geomagnetic disturbances. The observations, thus, provide a deeper insight into the possible causes and geomagnetic consequences of a prolonged weakly southward IMF-Bz condition.