Optical and radar observations of small-scale polar cap auroral structures.

We present ground-based auroral observations from Resolute Bay, Nunavut, Canada (74.73°N, 94.9°W) during January 2011. Two electron-multiplying CCD (EMCCD) imagers were operated at 31 frames per second. One was equipped with an all-sky field of view (FOV) lens and the other with a narrow (19°) FOV lens, centered on the geographic zenith (0° Az., 90° El.), a few degrees away from magnetic zenith (315° Az., 88° El.). The Resolute Incoherent Scatter Radar (RISR) was operating in a mode that enabled common-volume observations with the imagers. Being well inside the polar cap, the magnetic field at Resolute Bay is considered ‘open’ and connects to the lobes of the magnetotail. However, there is no clear consensus on whether polar cap aurorae occur on open or closed field lines. The electron acceleration is likely driven by direct solar wind processes, distant tail lobe processes or plasma sheet processes. One possible mechanism for accelerating the precipitating electrons is the parallel electric field of inertial Alfvén waves. The dynamic nature of the small-scale auroral features, observed on several nights, and the altitude extent of the ionization observed with RISR provide support for this hypothesis. [ABSTRACT FROM AUTHOR]