Sub-ion-scale Dynamics of the Ion Diffusion Region in the Magnetotail: MMS Observations

摘要

This paper reports magnetospheric multiscale (MMS) observations of the sub-ion-scale dynamics within the ion diffusion region (IDR) in the Earth's magnetotail. MMS crossed the IDR from the southern to the northern hemisphere, at about two ion inertial length earthward of the X line with a small guide field. Electrons were anisotropic in the inflow region of the IDR and turned into isotropic within the IDR. The isotropization of the electrons was probably due to the pitch angle scattering in highly curved magnetic field lines. We suggest that the thickness of the electron isotropic region strongly depends on the horizontal distance to the X line. The out-of-plane current bifurcated in the IDR. It peaked at the boundaries between the inflow and outflow electrons around the separatrices. Magnetic energy conversion and dissipation predominantly occurred at the peak of the out-of-plane current instead of at the neutral sheet center where B-L = 0. Both the energy dissipation and normal electric field E-N exhibited evident asymmetry with respect to the neutral sheet. The energy dissipation was larger around the northern separatrix than around the southern separatrix. The electric field E-N showed a tripolar variation across the neutral sheet, that is, a unipolar E-N around the southern separatrix and a bipolar E-N around the northern separatrix. The reasons and implications of these asymmetries are discussed.