A Persistent Depletion of Plasma Ions within Jupiter’s Auroral Polar Caps

Abstract:

The interior of the luminous auroral ovals at Jupiter are largely though not completely devoid of ions in the plasma energy range (0.01 keV/q – 40 keV/q) at Jovicentric distances up to at least 6 R_J. Here we report statistical ion measurements from the Jovian Auroral Distribution Experiment - Ion sensor (JADE-I) on the Juno spacecraft in polar orbit around Jupiter to characterize the presence or absence of structured plasma ion fluxes in the Jovian auroral polar caps. We identify nominal geometric centers of published north and south Jovian main luminous ovals based on recently published optical measurements. We develop a probability metric for the observation of plasma ions (more specifically, the lack thereof in this case) as a function of angular distance of the magnetic foot point of the Juno observation point from these nominal centers. This analysis is performed in the north and south separately owing to the fact that the nominal centers of the north and south optical patterns are not conjugate in either SIII or Jovian magnetic coordinates. Applying data from Juno’s first 26 perijoves, our 'depletion metric' shows a systematic, though not monotonic depletion in the probability of observing structured plasma ions with distance from the auroral oval centers. In both the north and the south, values of this depletion metric attain a large fraction of unity near the chosen poles, decrease to lesser values at intermediate angular distances (~ 10 degrees) as structured auroral zone ion fluxes are encountered and then rise again with increasing distance from the nominal auroral centers owing to large fluxes of penetrating radiation (low values of correlation) as the radiation belts are approached. These results are consistent with previous evidentiary reports of large electrostatic potential drops along polar cap Jovi-magnetic field lines.