Theoretical ideas concerning Jovian magnetospheric phenomena are at least as diverse as the phenomena themselves, and there presently exists no single comprehensive model that encompasses all known phenomena within a unified theoretical framework. We identify here a number of important theoretical concepts, some subset of which (together with perhaps others yet unidentified) will ultimately provide the elements of such a comprehensive model. A number of ideas have been advanced to account for the copious plasma source associated with Io, but none of these has yet accounted satisfactorily for both the magnitude and the morphology of the inferred source. Nevertheless, given the observed fact that Io supplies the bulk of the magnetospheric plasma mass, and the corollary that the net plasma transport is predominantly outward, it follows that the rotational energy of Jupiter is an important if not dominant source of energy for magnetospheric phenomena. This rotational energy is expended in a variety of phenomena, including the electrodynamic Io-Jupiter interaction and associated radio and auroral emissions, the acceleration of charged particles to MeV energies, and the generation of a wide variety of spin-periodic phenomena as observed both remotely and in situ. The spin periodicities observed within the magnetosphere can be explained for the most part as resulting from the diurnal wobble of the magnetospheric current sheet caused by the offset between Jupiter's magnetic dipole axis and its spin axis. However, remotely observed spin periodicities (the “pulsar” phenomena) apparently require the existence of an intrinsic longitudinal asymmetry in the Jovian magnetosphere that corotates with Jupiter.

In the Jovian magnetosphere, electrons, protons, and heavier ions are accelerated to energies well above 10 MeV. These energetic particles constitute a valuable diagnostic tool for studying magnetospheric processes and produce the Jovian radio emissions. In the inner magnetosphere, both the electron and proton fluxes with energies above 1 MeV build up to ~ 108 per cm2 s and constitute a major radiation hazard to spacecraft passing through this region. Surprisingly, high fluxes of energetic oxygen and sulfur (> 7 MeV/nuc) are also found in the inner magnetosphere. Of particular interest are the interactions of these particles with the inner Jovian moons and with the Io plasma torus. Throughout much of the middle magnetosphere and magnetospheric tail, highest fluxes are found in the plasma sheet, which coincides closely with the tilted dipole equator out to 45 Rj (Jupiter radii). This plasma sheet has not been identified beyond 45 Rj in the subsolar hemisphere; however, on the night side, it extends to 200 Rj. On the day side, fluxes near the equator are relatively independent of distance (15 to 45 RJ) and fall into the range 104 to 10 per cm2 s each for protons and electrons above ~ 1 MeV. In the predawn direction, proton and electron fluxes decrease by three orders of magnitude from 20 to 90 Rj (105 to 102 per cm2 s) and then remain relatively constant to the boundary layer near the magnetopause.

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.