Modelling Electrodynamics in Saturn's Upper Atmosphere

Abstract

Although electrodynamics plays an important role in controlling Saturn's thermospheric circulation and energy balance at high latitudes, less is known about its effects at lower latitudes. Recent observations from the Cassini magnetometer instrument near the equator during the Grand Finale tour revealed azimuthal magnetic field perturbations associated with ionospheric electrodynamics at low latitudes. If a significant ionospheric wind dynamo exists in Saturn's ionosphere at low and middle latitudes, it could alter the circulation and energy balance of Saturn's upper atmosphere and could explain the magnetic field perturbations at low latitudes. In this manuscript, we develop several models of thermospheric electrodynamics, including a new formulation for coupling the ionosphere and magnetosphere, and use them to investigate the role ion drag and resistive heating throughout Saturn's thermosphere. Results from our models suggest that electrodynamics can generate substantial currents at low and middle latitudes that can in some cases be comparable to high-latitude currents. We find that the electrodynamics is strongly dependent on the conductivity and wind profiles. In particular, the presence of an equatorial jet is important for driving significant eastward magnetic field perturbations and electrodynamics at low latitudes. Ion drag can help lift an equatorial jet from below to higher altitudes than it could otherwise reach, and may be able to spread a narrow equatorial jet in latitude as well. Because ion drag reduces wind shears that drive electrodynamics, any evidence of significant electrodynamics implies that the winds experience continual forcing by other mechanisms. In some models, ion drag partially opposes the Coriolis force at middle latitudes, slightly weakening the Coriolis barrier and allowing some meridional transport of auroral energy. The resistive heating predicted by our models is insufficient to explain the surprisingly high temperatures at low latitudes. Finally, we discuss directions for future research.