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

Abstract

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.

Electron‐Driven Dissipation in a Tailward Flow Burst

“…Specifically, we trace and inverse trace a few points around the spacecraft tetrahedron to obtain the magnetic topology, as done in previous studies (e.g., X. H. Chen et al, ; Z. Z. Chen et al, ; Fu et al, ; Fu, Cao, et al, ; Liu, Fu, Cao, et al, ; Z. Wang et al, ). We use the local magnetic field strength as the trace step length.…”

Evidence of Magnetic Nulls in the Reconnection at Bow Shock

“…The method we use is the FOTE method, which was initially developed by Fu et al (), later validated by Fu et al (), and hitherto has been extensively applied to the analyses of reconnection events in the Earth's magnetosphere (e.g., Chen et al, ; Chen, Fu, Liu, et al, ; Chen, Fu, Wang, et al, ; Fu et al, ; Fu, Cao, et al, ; Huang et al, ; Liu, Fu, Cao, et al, ; Liu et al, ; Man et al, ; Wang et al, ; Wang, Fu, et al, ). This method, based on four‐spacecraft measurements, can position a magnetic null both inside and outside the spacecraft tetrahedron and can also reconstruct the magnetic topology of the null.…”

“…We apply the FOTE method to reconstruct the magnetic topology of these three nulls (X‐lines), as done in previous studies (Fu et al, , ; Fu, Cao, et al, ). For better revealing the topological features of these nulls, the eigenvector coordinates ( e 1 , e 2 , e 3 ) obtained from the Jacobian matrix ∇B are used (Fu et al, ; Fu, Cao, et al, ; Liu et al, ; Wang, Fu, et al, ), where e 1 is the eigenvector corresponding to the minimum eigenvalue and e 2 and e 3 are the sum and difference of the other two eigenvectors, respectively. In ( e 1 , e 2 , e 3 ) coordinates, we trace a few points around the null to obtain the topology in Figure .…”

Evidence of Electron Acceleration at a Reconnecting Magnetopause

“…This is also an indication of the local wave‐particle interaction process. The whistler waves may originate from the X‐point (Cao et al, ; Wang et al, ) or from the magnetosphere (Vaivads et al, ); they are initially toward one direction (antiparallel) but later become bidirectional after they accelerate the energetic electrons in this event.…”