Implications for Magnetosphere-Ionosphere Coupling From Jupiter's System IV Quasi-Period

The Io plasma torus exhibits a fundamental rotational periodicity distinct from the rotation of the magnetic field that is easily observed in physical components such as ultraviolet emission or ionic mixing ratios, yet this distinct periodicity has no widely accepted explanation. To seek the origins of this additional periodicity, called System IV, a two-dimensional (radial and azimuthal) physical chemistry model (Copper et al., 2016), is used to investigate the torus response to a prescribed System III hot electron modulation following Steffl et al. (2008) and an additional hot electron modulation that we relate to various transport and mass loading parameters. The model thus self-consistently generates a radially independent periodicity consistent with that of System IV. We argue that the origin of this periodicity is the interplay between chemical and transport time scales working to prevent an azimuthal shearing of the hot electron modulation due to the physical subcorotation profile. We also examine recent observations (Tsuchiya et al., 2019,) that volcanic eruptions return the System IV periodicity toward rigid corotation with System III, counter to previous magnetosphere-ionosphere(MI) coupling models that examined mass loading (Pontius & Hill, 1982, ). Instead, the return to corotation implies improved MI coupling, possibly due to a transport-related increase in hot electrons following the eruption. Thus, we argue that the System IV periodicity is a diagnostic of MI coupling efficiency at Jupiter and the ultimate result of energy dissipation associated with Alfvénic coupling to the planet.