Extension of the Electron Diffusion Region in a Guide Field Magnetic Reconnection at Magnetopause

The structure of the electron diffusion region (EDR) in different plasma regimes is an outstanding question related to magnetic reconnection. Here we report a long EDR that extended at least 20 ion inertial lengths downstream of an X line at the Earth's magnetopause, which was observed by the Magnetospheric Multiscale mission. This EDR was detected in the exhaust of an asymmetric magnetic reconnection with a moderate guide field, the reconnection rate of which was ∼0.1. It corresponds to strong positive energy dissipation ($J\cdot E^{\prime }>0$) and enhancement of electron nongyrotropy. The energy dissipation was contributed by the electron jet and non-ideal electric field along the outflow direction, which suggests that the EDR probably plays more important roles in the energy conversion in magnetic reconnection than previously thought. Our result could be a significant step toward fully understanding the structure of the EDR.