  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.
