Instabilities in the separatrices of asymmetric reconnection at the dayside magnetopause

Abstract:

Magnetic reconnection is a crucial process in space plasmas. At Earth's dayside magnetopause, where the cold dense magnetosheath plasma reconnects with the hot tenuous magnetospheric plasma, reconnection is highly asymmetric. Asymmetric reconnection differs significantly from symmetric reconnection and is not well understood. During magnetic reconnection the separatrix regions, which separate the inflowing and outflowing plasmas, are potentially unstable to a variety of instabilities. Here we report observations of the separatrices of asymmetric reconnection as Cluster crosses the magnetopause close to the subsolar point. The relative motion between the spacecraft and plasma is small enough to resolve spatial changes of the electron distribution in the separatrices over multiple spacecraft spin periods. The electron distributions are shown to be unstable to the electrostatic beam mode and electromagnetic whistler mode. The predicted wave properties are shown to be consistent with observations. The whistler waves are driven by the temperature anistropy caused by a loss of magnetospheric electrons propagating away from the X line, and can be enhanced by the presence of magnetosheath electrons. The whistler waves propagate toward the X line, while the beam mode waves, which potentially lead to the formation of electron solitary waves, propagate away from the X line. The implications for asymmetric reconnection are discussed.