Sheared flows and small-scale Alfvén wave generation in the auroral acceleration region

Coincident particle and optical measurements from the Reimei spacecraft suggest that sheared flows through an inverted-V arc may be unstable to the emission of Alfvén waves. The particle measurements reveal time dispersed field-aligned electron bursts typical of low energy electrons accelerated by inertial Alfvén waves (IAWs). These Alfvén wave accelerated electrons are embedded within multi-keV inverted-V electron structures. The optical measurements at the footprint of the inverted-V structures reveal counter propagating and folded/vortical discrete auroral forms moving with speeds of ̃14-18 km/s. We show that the flow shear inferred from this motion exceeds that required for instability to the emission of Alfvén waves on scales of the order of an electron inertial length. The emission of these waves provides a likely means for driving the low energy dispersed electron bursts we observe.