Unique Observations of a Geomagnetic SI^+ -- SI^- Pair: Solar Sources and Associated Solar Wind Fluctuations

TL;DR

This study documents a rare geomagnetic pair of opposite sudden impulses linked to solar wind density variations, with evidence suggesting a rear-side solar event can influence Earth's geomagnetic environment.

Contribution

It provides the first convincing evidence connecting a specific solar event, including rear-side CME and flare, to observed geomagnetic SI pairs, highlighting new solar-terrestrial interaction insights.

Findings

SI${^{+}}$ -- SI${^{-}}$ pair correlates with solar wind density changes

Rear-side solar flare can generate shocks affecting Earth

Event lacks direct solar disk source, indicating complex solar influence

Abstract

The paper describes the occurrence of a pair of oppositely directed sudden impulses (SI), in the geomagnetic field ($\Delta$X), at ground stations, called SI${^{+}}$ -- SI${^{-}}$ pairs, that occurred between 1835 UT and 2300 UT on 23 April 1998. The SI${^{+}}$ -- SI${^{-}}$ pair, was closely 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. Further, this event had no source on the visible solar disk. However, a rear-side partial halo coronal mass ejection (CME) and an M1.4 class solar flare behind the west limb, took place on 20 April 1998, the date corresponding to the traceback location of the solar wind flows. This event presents empirical evidence, which to our knowledge, is the best convincing evidence for the association of specific solar events to the observations of an SI${^{+}}$ -- SI${^{-}}$ pair. In addition, it shows that it is possible for a rear side solar flare to propagate a shock towards the earth.