The radiation belt origin of Uranus' Nightside radio emission

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Abstract
From the location of the source field lines of the smooth nightside component of Uranus kilometric radiation (UKR) (Kaiser et al., this issue) the most likely free energy source is the outer radiation belts. As the terminator sweeps over the magnetic north polar region, precipitation of electrons produced by solar heating of the upper atmosphere and consequent submergence of the electron mirror points deeper in the atmosphere will create a backscattered electron distribution with an enhanced population at large pitch angles. This distribution will propagate to the conjugate magnetic south pole of Uranus, where the converging magnetic field lines will produce a mirroring electron distribution moving upward from the south pole, which will drive a cyclotron maser instability. The upper frequency limit to the UKR emissions corresponds closely to the electron gyrofrequency of the mirror point of the weaker northern pole and is consistent with a backscattered electron population originating from precipitation near the northern pole and hence mirroring at higher altitudes at the conjugate point in the southern magnetic hemisphere. The rise in emission intensity is shown to scale directly with the terminator's speed across the northern pole; the smooth emission result from sunrise over the northern pole and collision‐induced precipitation. Strong radio emissions are limited to the continually dark south pole, since only there is the electron plasma frequency to gyrofrequency ratio sufficiently small. The clocklike radio emission is shown to be a direct consequence of the terminator's control of the emission process. Hence solar‐driven upper atmospheric tides underlie the Uranian radio clock. Since the radiation belt population is most likely only weakly dependent on solar wind conditions, strong solar wind control of smooth UKR is not expected.