Impulsive solar wind‐driven emission from Uranus

Abstract

Several days prior to the Voyager spacecraft encounter with Uranus, the plasma wave and radio astronomy receivers detected extraordinarily narrowband bursty signals, the first indication of any radio emission from the planet. The characteristics of these signals were so unusual that their identity as a natural planetary emission was questioned at first. Subsequent analysis has shown, however, that the n bursts are modulated at the 17.24‐hour Uranus rotation period and are therefore planetary in origin. We show, in addition, that the typical bandwidth and time scale for the bursts are about 5 kHz and 250 ms, respectively. The phase of the rotation modulation suggests a probable source for these events in the vicinity of the north (weak) magnetic pole. The waves are right‐hand polarized and are therefore emitted in the extraordinary magnetoionic mode if the emission in fact originates above the north magnetic pole. In the context of the electron cyclotron maser mechanism, inferred upper limit electron densities range from 15 cm −3 at 0.9 R U altitude to 0.4 cm −3 at 2.4 R U altitude. Discovery of events predating the encounter identifications by up to 1 month indicates the n bursts were organized into two major ≃10‐day‐long episodes. These episodes were simultaneous with times of enhanced solar wind levels at Uranus, leading to the first evidence of a solar wind‐driven radiation at Uranus.