Electron Distribution Functions Around a Reconnection X-Line Resolved by the FOTE Method

Using data from the MMS mission and the First-Order Taylor Expansion (FOTE) method, here we reveal electron distribution functions around a reconnection X-line at the Earth's magnetopause. We find cigar distribution of electrons in both the magnetosphere-side and magnetosheath-side inflow regions, isotropic distribution of electrons at the separatrix, and loss of high-energy electrons in the antiparallel direction in the magnetosheath-side inflow region. We interpret the formation of cigar distribution in the inflow regions using the Fermi mechanism—as suggested in previous simulations, the loss of high-energy electrons in the magnetosheath side using the parallel electric fields—which evacuate electrons to escape the diffusion region along the antiparallel direction, and the isotropic distribution at the separatrix using the pitch angle scattering by whistler waves—which exist frequently at the separatrix. We also find that the electron distribution functions can change rapidly (within 60 ms) from isotropic to cigar as the spacecraft moves slightly away from the separatrix.