Abstract Non‐gyrotropic distribution functions are often observed in thin current sheets prior to magnetic reconnection. This study uses NASA's Magnetospheric Multiscale mission data to confirm a novel source of agyrotropy in compressed current sheets and highlights its significance in reconnection. 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 agyrotropy is consistent with theory that includes the effect of a localized transverse electric field on the distribution function. The electric field affects the gyro‐plane asymmetrically through the term , where is the spatial gradient of the velocity and is the cyclotron frequency. This asymmetry causes the agyrotropy, which is confirmed by data analysis. For compression such that , the electron distribution function is stretched in the direction of the drift. Conventional methods for quantifying agyrotropy, based on pressure tensor contributions that are uncorrelated to the electric field, are inadequate in the current sheet center where reconnection is likely to be initiated. The perpendicular ambipolar electric field provides a measure of agyrotropy in distribution functions, which may be the most relevant indicator of reconnection.
DuBois et al. (Thu,) studied this question.