Structure of the dayside reconnection layer in resistive MHD and hybrid models

Abstract

The structure of reconnection layer at the Earth's dayside magnetopause is studied based on one‐dimensional simulations for the evolution of an initial current sheet. A resistive MHD code and a hybrid code are used in simulations. Initially, the magnetopause current sheet separates two uniform plasma regions: magnetosheath (high plasma density and low magnetic field) and magnetosphere (low plasma density and high magnetic field). In MHD simulations, it is found that steady intermediate shock, time‐dependent intermediate shock (TDIS), Alfvén wave pulse, slow shock, slow expansion wave, or contact discontinuity can be formed as a result of magnetic reconnection. The current density in the magnetopause‐boundary layer region is mainly concentrated at the intermediate shock on the magnetosheath side. High‐speed flow is observed in the boundary layer. The Alfvén wave pulse, which propagates to the magnetosphere, is found to have a large amplitude in plasma flow speed. In hybrid simulations, the reconnection layer has striking differences from that in MHD simulations: (1) Because of the mixing of hot magnetospheric plasma and cold magnetosheath plasma along magnetic field lines, the contact discontinuity does not appear in the reconnection layer. (2) In general cases, the TDIS evolves quickly to a steady rotational discontinuity with a constant width. In the singular case in which the magnetic fields in the magnetosheath and in the magnetosphere are exactly antiparallel, a weak steady intermediate shock is formed. (3) No clear shock front can be identified for the slow shock, and the slow expansion wave is strongly modified as compared with that in the MHD simulations. (4) The boundary layer is a transition region between the magnetosheath and the magnetosphere with a mixing of plasmas from the two sides; the plasma density decreases monotonically from the magnetosheath side to the magnetosphere side, while the total temperature and magnetic field strength increase monotonically toward the magnetosphere side. (5) A decrease in T⊥/T∥ is observed in the boundary layer. Our results indicate that in general cases rotational discontinuities bound the reconnection layer at the dayside magnetopause.