NUMERICAL SIMULATIONS OF BURSTY RADIO EMISSIONS FROM PLANETARY MAGNETOSPHERES

The Voyager spacecraft observed both smooth and bursty radio emissions from Uranus and Neptune. These emissions arc known to be freely propagating primarily in the right-hand circularly polarized (RCP) mode with the bursty emissions having burst periods as short as a few tenths of a second and the smooth emissions being observed over periods of a few hours. While the smooth emission is probably due to the electron cyclotron maser instability, some other processes must be at work to produce the bursty emissions. It is proposed that one important difference in mechanisms is that the smooth emissions are associated with continuous injection of electrons while the bursty emissions arc associated with impulsive injection. In the latter case, the electron distribution can develop a beam feature with a temperature anisotropy. It is shown via one-dimensional (three velocity) relativistic particle simulations that while the beam may be initially unstable to an electrostatic instability, this instability quickly saturates and eventually the beam is unstable to a strong electromagnetic beam instability which utilizes the temperature anisotropy as free energy; a beam feature is not explicitly needed for the growth of this electromagnetic instability. The radiation generated by the instability is able to convert particle energy to wave energy at similar levels as the maser instability. For omega(pe)/OMEGA(e) greater than or similar to 1, most of the wave energy is LCP and is trapped. However, for omega(pe)/OMEGA(e) less than or similar to 1 the dominant mode is RCP. The induced waves are in the form of modified whistlers when the beam speed is low and omega(pe)/OMEGA(e) congruent-to 1 but are in the freely propagating x mode branch when omega(pe)/OMEGA(e) congruent-to 0.4. The amount of radiation with frequencies above the local x mode cutoff increases with beam speed. It is also postulated that some of the radiation generated below the local x mode cutoff may also be able to escape the plasma and be detected remotely via mode conversion between regions where field-aligned currents produce local perturbations in the magnetic field.