Auroral radio emissions at the outer planets: Observations and theories

Abstract

We review both observational and theoretical aspects of the generation of auroral radio emissions at the outer planets, trying to organize the former in a coherent frame set by the latter. Important results have been obtained in the past few years on these radio emissions at the five magnetized planets, from the observations of Ulysses at Jupiter and of Wind and other Global Geospace Science spacecraft in Earth orbit, from the reanalysis of Voyager data about Saturn, Uranus, and Neptune, from ground‐based high frequency‐time resolution and full polarization measurements, and from pioneering multispectral observations of the Jovian and Saturnian aurorae (radio/UV/IR). In parallel, considerable progress has been made in their generation theory (Cyclotron‐Maser operating in small‐scale, laminar, hot‐plasma‐dominated radio source structures), mostly on the basis of in situ observations of terrestrial radio sources. Particle acceleration and precipitation is also better documented, thanks to in situ measurements in the Earth auroral zones and to multispectral studies of Jupiter and Saturn. Finally, the modeling of the planetary magnetic field and magnetospheric plasma at these two planets has also been considerably improved. To organize the wealth of observational results within a coherent theoretical frame, we emphasize unresolved questions (e.g., planetary radio bursts generation) and contradictions and propose ways to answer them. Our ability, already significant, to perform remote sensing of magnetoplasmas at the giant planets and, hopefully, at other distant radio sources (solar, stellar) in the near future, depends on the good understanding of the physical processes underlying the generation of auroral electromagnetic emissions. The question of the existence of exoplanetary radio emissions and the possibility to detect and study them is briefly discussed.

On the Radio Detection of Multiple-Exomoon Systems Due to Plasma Torus Sharing

“…Figure 1, taken from Zarka (1998), shows a summary of all planetary radio emissions found in our solar system. The Io-DAM and non-Io-DAM emissions are shown to the right of the ionospheric cut off, and where it is clear that Io-DAM radio emissions are considerably stronger than non-Io-DAM emissions for most of the applicable frequencies.…”

Control of Jupiter's radio emission and aurorae by the solar wind

“…altitudes (1 to 3 planetary radii) 14,21 by the cyclotron maser mechanism 10,22 . No in situ measurements are available in the highlatitude auroral regions of the jovian magnetosphere, so we use our knowledge of terrestrial auroral processes to help us to understand the processes that are occurring in the jovian magnetosphere.…”

Time-scales of close-in exoplanet radio emission variability

“…Within the solar system, the magnetised planets are all known to be sources of intense non-thermal radio emission (Zarka 2007). The emitted power is proportional to the power incident on the planet's magnetosphere.…”