The Cause of Non-Gyrotropic Distribution Functions in Compressed Magnetotail Current Sheets

Non-gyrotropic distribution functions are often observed in thin current sheets prior to magnetic reconnection. We use MMS data to confirm a novel source of agyrotropy in compressed current sheets. Data analysis reveals a strong correlation between agyrotropy at the current sheet center and the perpendicular ambipolar electric field, which develops to maintain quasi-neutrality as the current sheet is compressed to sub-ion gyro-radius scales. This is consistent with theory that includes a localized transverse electric field in the distribution function. The electric field affects the gyro-plane asymmetrically, η=1+V'_ExB/Ω. This asymmetry causes the agyrotropy, which is confirmed by data analysis. For compression such that Ω_iExB<Ω_e, the ion distribution function is stretched in the direction of the ambipolar electric field while that of the electrons in the ExB direction. This is due to different magnetization levels of electrons and ions produced by η through the renormalized gyro-frequency. Conventional methods for quantifying agyrotropy, based on pressure tensor contributions, are compared to η, which is shown to be a more reliable measurement of agyrotropy in the current sheet center where reconnection is likely to be initiated.