Structures of Alfvén shocks: S‐shaped magnetic hodogram observed at the magnetopause

The Alfvén shock structure is obtained by a two‐fluid simulation code including dissipations due to resistivity, viscosity and heat conductivity. The hodogram of the tangential magnetic field component of the simulated Alfvén shock is S‐shaped with a 180° angular rotation. This is the first simulation result to show the S‐shaped hodogram associated with Alfvén shocks. The S‐shaped hodograms have been observed at the magnetopause [Berchem and Russell, 1982]. On the basis of this result, we propose that the magnetopause is an Alfvén shock where and when the hodogram is S‐shaped and rotates 180°. The upstream portion of the hodogram is always right‐hand polarized; the downstream portion is always left‐handed. The thickness of the Alfvén shock transition layer is about a few ion inertial lengths, which tends to increase with decreasing shock normal angle θ_bn. For θ_bn = 80°, the thickness is estimated to be about 5 c / ω_pi.