Dynamics of Thermal Electron Anisotropy in the Magnetotail Current Sheet During Substorm Growth Phase

The growth phase of the magnetospheric substorm is accompanied by the formation of a thin magnetotail current sheet, which is subsequently destroyed due to magnetic reconnection. The configuration and kinetic properties of this current sheet determine its stability and are important in the context of reconnection onset. This study focuses on the electron component of such thin current sheets. Observations in the near-Earth magnetotail show that electrons are predominantly field-aligned anisotropic. However, this anisotropy decreases as the current sheet becomes thinner. The observed isotropization cannot be explained by electron scattering, as anisotropy is provided by the subthermal electron population. We propose an adiabatic model for such isotropization based on observations from the Time History of Events and Macroscale Interactions during Substorms mission, and demonstrate that the inclusion of a self-consistent electrostatic field (current sheet polarization) can explain the decrease in electron anisotropy. Our findings highlight the importance of the role of electron temperature anisotropy and current sheet polarization in regulating the magnetotail dynamics during the substorm growth phase.