| Auteur(s) supplémentaire(s) | P. Zarka, B. Cecconi, R. Prangé, W. Kurth, G. Hospodarsky, A. Persoon, M. Morooka, J.-E. Wahlund, G. Hunt |
Abstract | Understanding the generation of planetary auroral radio emissions, the powerful non-thermal radiations produced by all the magnetized planets explored so far, and actively searched from exoplanets and more massive objects, requires in situ measurements from within their source region. During its early 2008 high-inclination orbits, the Cassini spacecraft unexpectedly sampled at about 10 kHz two local low frequency (LF) sources of Saturn’s Kilometric Radiation (SKR), generally observed over the 1-1000 kHz spectral range, and thus provided the first insights into the underlying physical excitation mechanism at a planet other than Earth. The combined analysis of radio, plasma and magnetic field in situ measurements validated the Cyclotron Maser Instability (CMI), at the origin of Auroral Kilometric Radiation (AKR) at Earth, as being a universal generation mechanism able to operate in different planetary plasma environments. The CMI requires (1) accelerated (out-of-equilibrium) electrons and (2) low density magnetized plasma (where the ratio of the electron plasma frequency fpe to the electron cyclotron frequency fce is much lower than unity) to amplify waves at the frequency fCMI = fce/?+k//v// / 2?, with k the wave vector, v the electron velocity, // the direction parallel to B and ? = (1-v^2/c^2)^(-1/2) the Lorentz factor.
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