The Role of Magnetospheric Coupling in Saturn's Thermosphere

Abstract In a rotationally dominated magnetosphere, plasma dynamics exert persistent drags on the planet's thermosphere via electromagnetic coupling that exerts a torque and produces persistent, steady departures from corotation. Two categories of mechanisms have been proposed to resupply angular momentum to the thermosphere: vertical transport via eddy diffusion and horizontal advection from other latitudes. For the latter, Smith and Aylward’s (2008) atmospheric circulation model applied a fixed function for the magnetospheric angular velocity profile. This fixes the convection electric field for the thermosphere, which then evolves toward a steady state governed by the magnetospheric velocity. We develop a model for the angular velocity of the neutral thermosphere that neglects all physical factors except for three factors: angular momentum transport by poleward advection; magnetospheric torque; and drag from the lower atmosphere, replacing the earlier invocation of eddy diffusion as a mechanism for transporting angular momentum upward. We follow Hill's (1979) derivation for radial plasma transport driven by a rigidly rotating atmosphere, but we reverse the logic and solve the thermosphere's angular velocity as constrained by the prescribed magnetospheric plasma velocity and the lower atmosphere. Our results reproduce the qualitative features of Smith and Aylward's results. However, the thermosphere should control the dynamics of the magnetosphere rather than the reverse.