Electric potential difference due to MHD slow shocks propagating along the Io flux tube

Abstract

Many ionospheric and magnetospheric phenomena, e.g., the northern lights, require the existence of accelerated particle populations. One possible explanation for the development of such particles is an electric field directed along magnetic field lines. The main aim of this paper is to investigate the physical mechanisms leading to an electric potential difference along the Jo flux tube with special emphasis on the processes acting in the outer ionosphere of Jupiter. As a starting point, we assume a pressure perturbation at the position of Ιo and follow the evolution of this pressure perturbation from To towards Jupiter. Initially, the pressure pulse produces two slow mode waves propagating along the Ιo flux tube. These slow mode waves are converted into slow shocks traveling towards Jupiter, and are accompanied by a supersonic flow behind the shock front. The crucial point is now that due to the propagation into a more narrow flux tube, the flow velocity behind the shock increases, in particular fast near the surface of Jupiter. Such a strong plasma flow generates an electric potential difference along the magnetic field. We estimate this potential difference using well-known techniques of kinetic theory. It turns out that the strength of the potential drop is directly proportional to the flow energy of ions. Thus, the very heavy ion populations in the Ιo torus plasma provide an appropriate environment in order to generate an electric potential difference of the order of 1 kV. Therefore, the pressure pulse mechanism can contribute to the explanation of aurora and planetary radio emissions together with the generally accepted Alfven wings model.