NUMERICAL SIMULATIONS OF BURSTY PLANETARY RADIO EMISSIONS R. M. Winglee * , J. D. Menietti * , and H. K. Wong * Abstract The Voyager spacecraft observed both smooth and bursty radio emissions from Uranus and Neptune. These emissions are known to be freely propagating primarily in the right hand circularly polarized mode (RCP) 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 are associated with impulsive injection. In the latter case, the electron distribution can develop a beam feature with a temperature anisotropy which is unstable to an electromagnetic beam instability that can gen- erate bursty emissions. The characteristics of this radiation is determined via one dimensional (three velocity) particle simulations. It is shown that the electromag- netic beam instability anisotropy is as efficient as the maser instability in converting particle energy to wave energy. For ω pe /Ω e > ∼ 1, most of the wave energy is LCP but is trapped. However, for ω pe /Ω e < ∼ 1 the dominant mode is RCP and freely propagating. Furthermore, the radiation is bursty in nature due to the competition between different modes and is most intense when 0.4 < ∼ ω pe /Ω e < ∼ 1. This regime complements that of the maser emission which is expected to be most intense when ω pe /Ω e < ∼ 0.3. 1 Introduction Observations of planetary radio emissions from Uranus and Neptune obtained by the Voyager 2 spacecraft have shown clear evidence of two distinct classes of radiation char- acterized as smooth and bursty. At Uranus, the smooth emission is seen as broadband continuous emissions, from a few kilohertz to about 900 kHz and continuous in time over periods of a few hours. Many studies have suggested the b–smooth emission is gener- ated by the cyclotron–maser instability [e.g. Kaiser et al., 1987; Menietti et al., 1990]. * Department of Space Sciences, Southwest Research Institute, San Antonio, TX 78228–0510 317