What causes geomagnetic activity during sunspot minimum

TL;DR

This paper investigates the causes of geomagnetic activity during sunspot minimum, emphasizing the role of background solar wind components and their relation to the heliospheric current sheet and polar coronal holes.

Contribution

It reveals that geomagnetic activity during sunspot minimum depends on background solar wind parameters, not on interplanetary disturbances like CMEs or high-speed streams.

Findings

Geomagnetic activity correlates with background solar wind parameters.

The thickness of the heliospheric current sheet influences geomagnetic activity.

Background solar wind components vary across solar cycles.

Abstract

The average geomagnetic activity during sunspot minimum has been continuously decreasing in the last four cycles. The geomagnetic activity is caused by both interplanetary disturbances - coronal mass ejections and high speed solar wind streams, and the background solar wind over which these disturbances ride. We show that the geomagnetic activity in cycle minimum does not depend on the number and parameters of coronal mass ejections or high speed solar wind streams, but on the background solar wind. The background solar wind has two components: slower and faster. The source of the slower component is the heliospheric current sheet, and of the faster one the polar coronal holes. It is supposed that the geomagnetic activity in cycle minimum is determined by the thickness of the heliospheric current sheet which is related to the portions of time the Earth spends in slow and in fast solar wind. We demonstrate that it is also determined by the parameters of these two components of the background solar wind which vary from cycle to cycle.