Group velocity cones in diverging magnetic reconnection structures

摘要

In a typical geometry of magnetic reconnection, the so-called exhaust regions diverge out of the localized diffusion region. The diverging reconnection structure is conical in shape with its apex near the x-line. Such conical regions have been seen in both MHD and kinetic simulations of reconnection as well as in satellite observations. We demonstrate here that depending on the reconnection regime, the cone angle of the exhaust regions found in numerical simulations and as well as in satellite observations compare well with the maximum angles of group-velocity cones associated with the slow MHD mode, or the whistler mode, or the kinetic Alfvén wave (KAW). For each of these three reconnection regimes, we give a quantitative description of the group velocity cones. In the MHD case the cone angle is small and increases almost linearly with the plasma β < 2. In the whistler regime the cone angle is a constant of 19.5°. In the KAW regime the cone angle depends on plasma β as well as on the timescale associated with the diffusion process in the current sheet. The close agreement between the observed and simulated exhaust cone angles and the group-velocity cone angles in all the three reconnection regimes is highly suggestive that “at least” the geometrical feature of the exhaust region is determined by the group velocity of the disturbances, which propagate out of the diffusion region.