Effect of Field-Aligned Potentials on Magnetospheric Dynamics at Jupiter and Saturn

We present a time-independent model of Jupiter’s rotationally driven aurora based on torque balance between the ionosphere and magnetosphere, including the effects of a field-aligned potential and a variable Pedersen conductivity. The field-aligned potential arises from field-aligned current limitation at high-latitudes and changes the mapping of the electric fields between the ionosphere and the magnetosphere. We apply a Vlasov study to determine the location and extent of the field-aligned potential drop along the flux tube. We then adjust the Pedersen conductivity to vary with electron precipitation energy and incident energy flux consistent with the study by Millward et al. (2002). The primary effect of the field-aligned potential is to enhance coupling by increasing angular momentum transfer from the ionosphere to the magnetosphere. The net result is an equatorial mapping location for the main auroral oval at ~25 RJ. Our model reproduces many of the observed characteristics of Jupiter’s main auroral oval including the energy flux into the ionosphere (2 - 30 mW/m2), the width of the aurora at the ionosphere (1000 km) and field-aligned potentials consistent with observed electron energies ( 30 - 200 keV). We apply our model to Saturn and find that the magnetospheric plasma at Saturn may make a significant contribution to Saturn’s main auroral oval. Our model at Saturn reproduces the angular velocity profiles determined from Cassini and Voyager data.