Study of Extreme Magnetopause Distortions Under Varying Solar Wind Conditions

To first order, the magnetopause (MP) is defined by a pressure balance between the solar wind and the magnetosphere. The boundary moves under the influence of varying solar wind conditions and transient foreshock phenomena, reaching unusually large and small distances from the Earth. We investigate under which solar wind conditions such extreme MP distortions occur. Therefore, we construct a database of magnetopause crossings (MPCs) observed by the THEMIS spacecraft in the years 2007 to mid-2022 using a simple Random Forest Classifier. Roughly 7% of the found crossing events deviate beyond reported errors in the stand-off distance from the Shue et al. (1998, ) MP model and thus are termed extreme distortions. We find the occurrence of these extreme events in terms of expansion or compression of the MP to be linked to different solar wind parameters, most notably to the IMF magnitude, cone angle, velocity, Alfven Mach number and temperature. Foreshock transients like hot-flow anomalies and foreshock bubbles could be responsible for extreme magnetospheric expansions. The results should be incorporated into future magnetopause models and may be helpful for the reconstruction of the MP locations out of soft x-ray images, relevant for the upcoming SMILE mission.

Automated summary

The magnetopause (MP) is primarily determined by the pressure balance between the solar wind and the magnetosphere. The boundary can move due to different solar wind conditions and transient foreshock phenomena, resulting in unusually large or small distances from the Earth. In this study, we investigate the occurrence of extreme MP distortions under specific solar wind conditions and identify several parameters, such as IMF magnitude, cone angle, velocity, Alfven Mach number, and temperature, that are linked to these extreme events. The findings have implications for future magnetopause models and the reconstruction of MP locations using soft X-ray images for missions like SMILE.