A generally accepted theory of the enigmatic phenomenon of planetary radio emission is not yet available. In this chapter, we direct our attention primarily to the question of how the Jovian decameter radiation might be generated via both direct and indirect mechanisms. Direct mechanisms transform the free energy contained in an electron distribution (typically a loss-cone) directly into electromagnetic waves. Indirect mechanisms transform the free energy contained in an electron beam distribution first into electrostatic waves that can then couple, in some manner, to produce electromagnetic waves. The growth rates for the unstable electromagnetic and electrostatic waves are derived. Nonlinear theories are briefly discussed as they apply to the case of Jupiter's decametric radiation. Because most of the Jovian radio emission seems to be controlled by Io, we describe how Io, through the emission of kinetic Alfven waves, can produce a “beamlike” electron distribution. It is more difficult to understand how Io can enhance or produce a “loss-cone” distribution. Thus we conclude that, at least for Jovian radio phenomena, indirect mechanisms are preferred. We also describe theories and models for the generation of the dynamic spectral arcs that characterize the radio spectrum from hectometric to decametric wavelengths.

Introduction

Jupiter is the most powerful planetary source of nonthermal electromagnetic radiation in the solar system, with a radio spectrum extending from a few kHz to over 100 MHz. The phenomenology of the decimeter component in the GHz range has been discussed in Chapter 7.