Distribution of Plasma in the Io Plasma Torus as Seen by Radio Occultation During Juno Perijove 1

Abstract

The moon Io is the dominant plasma source for the Jupiter magnetosphere. The plasma is distributed into a torus of material around Jupiter, called the Io plasma torus. The Juno spacecraft performed its first perijove on 27 August 2016. During this time the spacecraft's X and Ka-band radio signals passed through the Io plasma torus. From the differential Doppler shift of the X and Ka-band frequencies we are able to determine the Io plasma torus total electron content. From the total electron content, we determine that the electron densities are larger than predicted from Voyager-based models by around 35 ± 14% in the cold torus and 38 ± 14% in the torus beyond 5.5 RJ. The ion temperatures were greater than predicted from the models by 44 ± 15% in the cold torus but consistent with models in the torus beyond 5.5 RJ. From the time of maximum total electron content, which is sensitive to the torus location, we also find the Io plasma torus equatorial plane appears to be tilted by about 1.5° more than the nominal centrifugal equator tilt based on the tilt of a dipole magnetic field approximation. Different tilts were found for the cold torus and torus beyond 5.5 RJ.