Asymmetric magnetic reconnection in the presence of a guide field

The properties of asymmetric magnetic reconnection in the presence of a guide magnetic field are investigated using two‐dimensional particle‐in‐cell simulations. The reconnection process is initiated by applying a spatially localized and temporally steady convection electric field at the high‐density/low‐magnetic‐field (magnetosheath) side of the current layer. The in‐plane Hall currents are dominated by the electron flows along the separatrices from the high‐density to low‐density side of the layer, and they strongly enhance the out‐of‐plane magnetic field in one hemisphere and decrease it in the other. On the enhanced magnetic field side are situated a bipolar pair of parallel electric fields and an electron velocity shear flow layer, both of which extend several ion inertia lengths ( d i ) away from the X line. The shear flow layer is unstable to the generation of small‐scale (≪ d i ) electron vortices which propagate away from the X line and produce a reduction of the order of 30% in the magnitude of the B y field. An example of such a large‐amplitude, short‐duration depression in B y is identified in a magnetopause crossing by the THEMIS spacecraft. The Ohm's law ( E + U e × B / c ) y = 0 is violated in this parallel field/velocity shear region, and the deviation arises predominantly from the divergence of the electron pressure tensor. Criteria based on the demagnetization of the electrons (large values of the electron agyrotropy and the Lorentz ratio) are found to characterize neither the immediate electron scale region around the X line nor the larger electron shear flow region.