The source location and beaming of broadband bursty radio emissions from Uranus

Abstract

Among the radio emissions detected with Voyager 2 at Uranus was a so‐called broadband bursty or “b‐bursty” component at frequencies of 300‐700 kHz, which was first observed✓ just before closest approach and then during subsequent planetary rotations about 1‐6 hours before the more prominent smooth component. Although not obvious for individual rotations because of the bursty nature of these signals, a systematic occurrence pattern for these emissions has been detected in a new composite radio spectrogram where the bursts from different rotations were superimposed. Consisting of symmetrical emissions flanking a quiet region, with their peak frequencies occurring about an hour and a half on either side, this pattern is attributed to Voyager intercepting a hollow emission cone from the nightside of Uranus which tipped past its direction and back during successive planetary rotations. On the assumption that this cone was circular and aligned with the magnetic field at the cyclotron frequency, its source location and beam angles were determined by minimizing the apparent cone‐angle deviations at the highest frequencies. The source was thus found to occur at a southern Uranian latitude of 45°, west longitude 235°, and at a height of 1.43 RU, in excellent agreement with previous studies. The beam angle at the highest frequencies, with respect to the source magnetic field, was found to be about 85°, and that for the entire emission pattern about 80°‐87°.