Perpendicular heating of electrons by upper hybrid waves generated by a ring distribution

Satellite observations of electron conical distributions with enhanced fluxes just outside the loss cone suggest that electrons have been heated perpendicularly to the magnetic field in the mid‐altitude polar magnetosphere. To understand electron conical distributions, plasma simulations are conducted to examine an upper hybrid wave instability of a ring electron distribution perpendicular to the magnetic field in a cold electron background. The simulations indicate that both the cold and ring distributions are heated perpendicularly during the saturation stage. From the plasma data, a ring distribution can be identified as a trapped distribution function with an enhancement near 90° pitch angle in the phase space density plot. It is suggested that the ring distribution might provide an additional free energy source for generating upper hybrid waves associated with electron conical events.