Classifying the Magnetosheath Using Local Measurements From MMS

Abstract

The Earth's magnetosheath is a dynamic region of shocked solar wind plasma downstream of the bow shock. Depending on the upstream magnetic field orientation, the magnetosheath usually has one of two distinct configurations: a more variable magnetosheath with strong fluctuations and structures propagating from upstream to downstream, or a more stationary magnetosheath characterized by compression and high ion temperature anisotropy. The more variable magnetosheath is usually observed for quasi-parallel shocks (the angle between the shock normal and the upstream magnetic field ), but the limit can vary for $40°\le {\theta }_{\text{Bn}}\le 70°$. These differences facilitate studies of how different plasma environments affect various processes such as turbulence and heating, and these require an accurate magnetosheath classification. Since ${\theta }_{\text{Bn}}$ can rarely be determined correctly in the absence of upstream monitors, local measurements have been suggested to classify the magnetosheath. However, this has not yet been verified for Magnetospheric Multiscale (MMS) data. We investigate this approach with MMS using locally measured magnetic field variability, ion temperature anisotropy, and suprathermal ion flux. We find the more variable magnetosheath at normalized magnetic fluctuations above 0.29 and ion temperature anisotropy below 0.18. We also find that the suprathermal ion flux can complement the classification given that MMS burst-mode data is used. Our findings provide a method to determine the magnetic connectivity of the magnetosheath with the upstream solar wind in the case of MMS and classify the downstream region into different configurations.