Beaming of Uranian nightside kilometric radio emission and inferred source location

Abstract

Taking advantage of the large range of magnetic latitudes explored by Voyager 2, due to the 60° tilt of the Uranian dipole field with respect to the rotation axis of the planet, we study the beaming pattern of the broadband kilometric radio emission detected over Uranus' nightside. Using the simple physical assumptions of an emission close to the local gyrofrequency and in a time‐stationary average lobe (i.e., assuming that the 10 min‐averaged intensity received does not vary with time for a given observing geometry), we constrain the radiation source to a high south magnetic latitude region (∼80°, or L∼40) magnetically connected to the magnetospheric tail. We discuss the different source locations derived by other authors from different analyses. We determine then the beaming pattern of the main components (smooth and bursty) of the Uranian nightside radio emission and their variations with frequency. It appears to be a hollow cone whose axis is tangent to the magnetic field in the source region, with a large aperture (semi‐apical angle ∼50°) and with an angular thickness (at −3 dB) increasing with decreasing frequency from about 30° at 750 kHz to more than 55° below ∼350 kHz; the bursty emission occurs about 90° away from the magnetic field. The motion of the spacecraft relative to the radiation lobe explains most of the observed emission characteristics. The origin of the bursty emission and its possible link to the smooth component are discussed. The results are compared with the present experimental and theoretical knowledge of planetary radio emission beaming.