Whistler emission in the separatrix regions of asymmetric magnetic reconnection

At Earth's dayside magnetopause asymmetric magnetic reconnection occurs between the cold dense magnetosheath plasma and the hot tenuous magnetospheric plasma, which differs significantly from symmetric reconnection. During magnetic reconnection the separatrix regions are potentially unstable to a variety of instabilities. In this paper observations of the separatrix regions of asymmetric reconnection are reported as Cluster crossed the magnetopause near the subsolar point. The small relative motion between the spacecraft and plasma allows spatial changes of electron distributions within the separatrix regions to be resolved over multiple spacecraft spins. The electron distributions are shown to be unstable to the electromagnetic whistler mode and the electrostatic beam mode. Large-amplitude whistler waves are observed in the magnetospheric and magnetosheath separatrix regions, and outflow region. In the magnetospheric separatrix regions the observed whistler waves propagate toward the X line, which are shown to be driven by the loss in magnetospheric electrons propagating away from the X line and are enhanced by the presence of magnetosheath electrons. The beam mode waves are predicted to be produced by beams of magnetosheath electrons propagating away from the X line and potentially account for some of the electrostatic fluctuations observed in the magnetospheric separatrix regions.