Jovian cusp processes: Implications for the polar aurora

Recently, high temporal and spatial resolution Hubble Space Telescope (HST) imaging in the ultraviolet (UV) has revealed a new feature in Jupiter's polar aurora. Of highly variable intensity, while remaining persistently near local noon, a polar "spot" of aurora has been suggested to be associated with the jovian cusp. The main jovian X-ray source also appears to be colocated with this UV emission and is characteristically pulsed with a period of ∼45 min. Here we take up the suggestion that these emissions correspond to the cusp, i.e., where magnetosheath plasma gains entry to the jovian magnetosphere. We first observe that without acceleration, the largest precipitating energy fluxes are associated with magnetosheath protons, but these produce only ∼0.4-4 kR of UV emission, well below that which is observed (>100 kR). Therefore acceleration must be present. We thus investigate the possibility that the UV and X-ray emissions are associated with pulsed reconnection at the dayside magnetopause, with an interpulse period estimated to be 40-50 min, hence providing a possible explanation for the pulsed X-ray signatures. Based upon the approximate bimodality of solar wind conditions at Jupiter, we have developed two conceptual models, "slow" and "fast" flow models corresponding to low and high values of the interplanetary magnetic field strength, respectively, of the twin-vortical flows which occur as a result of pulsed dayside reconnection, and the bipolar field-aligned currents which are also present. We find that peak field-aligned currents due to the vortical flows in the two models are ∼1.15 and 8.13 μA m^-2, respectively, for the slow and fast models. We subsequently consider conditions under which magnetospheric and cusp particle populations can carry these field-aligned currents, by estimating the field-aligned voltages which will be present and the energy fluxes and auroral output which results. Although the details depend upon the model assumptions, for the slow flow model the reconnection pulses produce two adjacent bright (>100 kR) "spots" or "arcs" of emission of order a few 1000 km along the open-closed field line boundary and a few 100 km wide, separated by a few 1000 km, comparable with observations. The X-ray emission in this slow flow scenario is negligible due to low voltages associated with the flow vortices. In the fast flow model, overall, the emission takes the form of an intense ∼10 MR arc extending ∼10,000 km along the open-closed field line boundary, with a "gap" between electron and ion contributions. We also find that in this case magnetospheric O²⁺ in the region of the downward currents is accelerated sufficiently to produce X-rays, with intensities ∼few kR and peak power outputs of a few GW, in good agreement with observations.