The kinetic structure of the electron diffusion region observed by MMS during asymmetric reconnection*

NASA’s MMS spacecraft mission has recently explored asymmetric reconnection at the Earth’s dayside magnetopause [1]. Supported by a kinetic simulation, we derive an exclusion energy parameter, providing a lower kinetic energy bound for an electron to cross from one inflow region to the other. Only high-energy electrons are permitted to cross the inner reconnection region, setting the electron distribution function observed along the low-density side separatrix during asymmetric reconnection [2]. The analytic model accounts for the two distinct flavors of crescent-shaped electron distributions observed by spacecraft in a thin boundary layer along the low-density separatrix [1]. Consistent with the MMS observations, the fully kinetic simulation also display beams of electrons flowing toward the topological magnetic x-line. Within the ~ d_e electron diffusion region, the beams become oblique to the local magnetic field, providing a unique signature of the electron-diffusion region where the electron frozen-in law is broken [3].

[1] Burch JL, et al., (2016), Science 352, aaf2939.

[2] Egedal J, et al., (2016) Phys. Rev. Lett, 117, 185101.

[3] Egedal J, et al., (2018) Phys. Rev. Lett, 120, 055101.