Gyro-resonant electron acceleration at Jupiter

According to existing theory, electrons are accelerated up to ultra-relativistic energies(1) inside Jupiter's magnetic field by betatron and Fermi processes as a result of radial diffusion towards the planet and conservation of the first two adiabatic invariants(2-4). Recently, it has been shown that gyro-resonant electron acceleration by whistler-mode waves(5,6) is a major, if not dominant(7), process for accelerating electrons inside the Earth's outer radiation zone, and has redefined our concept for producing the Van Allen radiation belts(8). Here, we present a survey of data from the Galileo spacecraft at Jupiter, which shows that intense whistler-mode waves are observed outside the orbit of the moon Io and, using Fokker-Planck simulations, are strong enough to accelerate electrons to relativistic energies on timescales comparable to that for electron transport. Gyroresonant acceleration is most effective between 6 and 12 jovian radii (R-j) and provides the missing step in the production of intense synchrotron radiation from Jupiter(1,9).