The prolonged southward IMF- B sub(z) event of 2-4 May 1998: Solar, interplanetary causes and geomagnetic consequences

A detailed investigation is carried out to understand the prolonged (44 h) weakly southward interplanetary magnetic field (IMF-B sub(z)) condition during 2-4 May 1998. In situ observations, during the period, show 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 caused the prolonged southward IMF-B sub(z) condition. The magnetic configuration of the source regions of the IMF associated with the ICME interval was 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 29 April and 1 May, suggesting no change in their polarity. The draping of these outward directed radial field lines around the propagating CMEs in the shocked plasma explains the observed polarity changes of the IMF-B sub(z) 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 3 May. The spectral analyses 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-B sub(z) condition. Key Points * A 44 h prolonged southward IMF-B sub(z) * The draping of the radial field lines around the CMEs in the shocked plasma explains the polarity changes of the IMF-B sub(z) at 1 AU * The solar wind density pulses show a distinct one-to-one correspondence with global geomagnetic response