Electrostatic waves in the magnetosphere of Uranus

Abstract

During its encounter with Uranus the plasma wave receiver on Voyager 2 observed electrostatic waves similar in many respects to those observed in other planetary magnetospheres. The most prominent type observed was the Bernstein mode emissions between harmonics of the electron cyclotron frequency. As is the case at other planets, the most intense Bernstein waves were observed near the magnetic equator of the planet, even though the tilt of the Uranian magnetic moment with respect to the rotational axis is very large. A small offset in the location of these electrostatic waves from the equator predicted by the offset, tilted dipole magnetic field model suggests some warping of the magnetic equator due to ring currents or external currents flowing on the magnetopause. Other examples of electrostatic Bernstein waves were observed closer to the planet and at higher magnetic latitudes. The energy of resonant electrons is calculated to be a few hundred electron volts, and measurements of electrons with this energy indicate the critical flux required to drive the Bernstein mode is available. The existence of the Bernstein modes near the upper hybrid resonance frequency leads to estimates of the electron density at several locations within the Uranian magnetosphere, and these compare well with densities measured by the plasma science investigation. In addition to the Bernstein modes, a number of highly sporadic emissions were observed in the vicinity of the Miranda L shell. While the absolute determination of the mode of these waves is uncertain, it is likely that some are electrostatic modes. Since this region of the Uranian magnetosphere is very perturbed and interesting, we shall attempt to identify possible modes associated with the waves.