Impact of CIR Storms on Thermosphere Density Variability during the Solar Minimum of 2008

TL;DR

This study investigates how Corotating Interaction Regions (CIRs) during the 2008 solar minimum affected thermosphere density, revealing significant global responses and variations linked to solar wind streams and coronal hole distributions.

Contribution

It provides new insights into the thermosphere's response to CIRs during an exceptionally quiet solar minimum, highlighting the preconditioning effects and latitudinal variability.

Findings

Thermosphere density changes by 75% on average during CIRs.

Density variability is largest at high latitudes.

Nighttime density response to high-speed streams is greater than daytime.

Abstract

The solar minimum of 2008 was exceptionally quiet, with sunspot numbers at their lowest in 75 years. During this unique solar minimum epoch, however, solar wind high - speed streams emanating from near-equatorial coronal holes occurred frequently and were the primary contributor to the recurrent geomagnetic activity at Earth. These conditions enabled the isolation of forcing by geomagnetic activity on the preconditioned solar minimum state of the upper atmosphere caused by Corotating Interaction Regions (CIRs). Thermosphere density observations around 400 km from the CHAMP satellite are used to study the thermosphere density response to solar wind high - speed streams/CIRs. Superposed epoch results show that thermosphere density responds to high - speed streams globally, and the density at 400 km changes by 75% on average. The relative changes of neutral density are comparable at different latitudes, although its variability is largest at high latitudes. In addition, the response of thermosphere density to high - speed streams is larger at night than in daytime, indicating the preconditioning effect of the thermosphere response to storms. Finally, the thermosphere density variations at the periods of 9 and 13.5 days associated with CIRs are linked to the spatial distribution of low - middle latitude coronal holes on the basis of the EUVI observations from the STEREO.