The Acceleration of Electrons to High Energies Over the Jovian Polar Cap via Whistler Mode Wave-Particle Interactions

Upward traveling electrons with energies from tens of kiloelectron volts to several megaelectron volts have been observed over the Jovian polar regions in association with intense upward propagating whistler mode waves. The electrons have a power law-like energy distribution, indicative of a stochastic acceleration process. The energy flux of the upward propagating whistler mode waves is comparable to and strongly correlated with the energy flux of the upward traveling energetic electrons, suggesting that the whistler mode waves may be accelerating the electrons. We propose that a downward field-aligned current over the polar cap generates strong downward parallel electric fields and associated upward electron beams in the low-density regions of Jupiter's upper ionosphere, a mechanism similar to the formation of inverted-Vs in Earth's auroral regions. At Jupiter, the upward-traveling electron beams produce intense upward propagating whistler mode emissions over a broad frequency range, similar to upward propagating auroral hiss at Earth. As the whistler mode waves propagate upward out of the inverted-V source region, the waves are absorbed by the plasma, thereby accelerating the electrons. We attribute the stochastic power law-like energy spectrum of the accelerated electrons to the development of Hamiltonian chaos (velocity space diffusion), the signature of which is indicated by the occurrence of spiky soliton-like variations in the whistler mode electric fields. Acceleration to high energies may be facilitated by the rapid increase in the phase velocity of the whistler mode waves with increasing altitude, which could accelerate some of the electrons trapped in the wavefield to very high relativistic energies.