Observations of a geomagnetic SI⁺ – SI⁻ pair and associated solar wind fluctuations

Abstract

We report a pair of oppositely directed sudden impulses (SI), in the geomagnetic field (ΔX), at ground stations, called SI⁺ – SI⁻ pairs, that occurred between 1835 UT and 2300 UT on 23 April 1998. The SI⁺ – SI⁻ pair, was well correlated with corresponding variations in the solar wind density, while solar wind velocity and the southward component of the interplanetary magnetic field (Bz) did not show any correspondence. This event had no source on the visible solar disk but was associated with a rear-side fast partial halo coronal mass ejection (CME) and an optically occulted M1.4 class solar flare behind the west limb. This event was unique in that one could clearly identify variations in ΔX at ground stations with solar wind parameters.