Alfvénic electron acceleration at Jupiter revealed by drifting radio bursts

Abstract The galilean moon Io is known to interact electrodynamically with Jupiter's magnetic field and ionosphere via Alfvén waves [1], producing electromagnetic signatures detectable remotely such as decameter-wave radio emissions generated along the Io Flux Tube (IFT) and UV aurora at the IFT footprints prolongated by a 'tail' emission. These emissions are thought to be produced by electrons having been energized by Alfvénic acceleration [2-4]. Indirect signatures of this acceleration process are the transverse magnetic fluctuations [5-7] and the broadband electron energy spectra [6, 8] observed in situ in the IFT and more generally in Jupiter's auroral regions [9]. A more direct signature is provided by remote measurements of the discrete, fast-drifting, quasi-periodic decameter radio bursts (so-called S-bursts) identified to date only in relation with the Io-Jupiter interaction [10]. S-bursts generation by Alfvén waves has been thoroughly modelled, from electron acceleration to radio emission growth rate [2, 11]. The S-bursts discreteness and quasi-periodicity were correctly reproduced, whereas their time-frequency drift results from the adiabatic motion of accelerated electrons along the IFT. Here, we present the first detection of decameter S-bursts related to the Ganymede-Jupiter interaction and to the main Jovian aurora, revealing the ubiquitous character of Alfvénic electron acceleration in Jupiter's high-latitude regions. We estimate the Alfvén wave periods and the accelerated electrons energy in each case. Two populations of accelerated electrons are found to co-exist, with different energies (a few keV and a few hundred eV).