Statistical study of extreme magnetopause locations

Abstract

The magnetopause (MP) is the boundary that separates the solar wind plasma from the Earth’s (inner) magnetosphere. To first order, its equilibrium position is defined by the pressure balance across it. The boundary moves under the influence of varying solar wind conditions and transient foreshock phenomena, thereby sometimes reaching unusually large and small distances from Earth. We investigate the occurrence of such extreme MP distortions. Therefore, we construct a database of magnetopause crossings observed by the THEMIS spacecraft in the years 2007 to mid-2022 using machine learning techniques. Crossing events deviating from the Shue et al. (1998) MP model by more than the reported uncertainties are denoted as extreme distortions. The occurrences of these extreme events in terms of expansion or compression of the magnetosphere are linked to different solar wind parameters. The results should be applied to future magnetopause models and may be validated by MP observations in soft x-ray images by the upcoming SMILE mission.