Radio and plasma waves at the outer planets

Abstract

We review our present knowledge of plasma waves and non-thermal radio emissions at the outer planets. The review mainly concerns waves linked to electron dynamics. After a summary of the basics of radio and plasma wave modes as derived from the theory and from observations in the Earth's vicinity, we discuss the counterpart of these waves as observed in outer planets' magnetospheres. Plasma wave spectra display a remarkable overall similarity at all magnetized planets in spite of the very diverse magnetospheric configurations, emphasizing the importance of similar microphysics processes at all magnetized planets, in particular the outer giant planets. But the magnetospheres of the outer planets also give birth to a large diversity of radiosources, reflecting their complex structure (plasma reservoirs, gradients, regimes of f pe/ f ce, etc.) and dynamics (locations and distributions of energetic electrons). We discuss in more details these radio components, including their generation mechanisms. Emphasis is put on the direct generation of the very intense high latitude radio emissions, but conversion processes also occur in low latitude regions. The Earth stands as a reference in our comparative approach, but the study of radio and plasma waves at the outer planets allows to apply the concepts developed at Earth in different plasma environments, broadening the regime of plasma parameters involved in the generation of these waves. One specific Jovian radio component is identified (made of sporadic bursts), whose study could bring significant conceptual advances in the physics of the generation process involved. Finally, radio waves, because they can freely propagate far from their source regions, are of utmost importance as they allow for remote sensing of magnetospheric plasmas.