Two Years of Observations of the Io Plasma Torus by Juno Radio Occultations: Results From Perijoves 1 to 15

Abstract

The Io plasma torus is the primary source for plasma in Jupiter’s magnetosphere. Observations of fundamental properties of the torus place valuable constraints on how the Io plasma torus behaves. Yet measurements of the density structure of the torus that precisely determine its location, total electron content, and scale height are rare. Here, we report measurements of four profiles of the total electron content through the torus by the Juno spacecraft. These profiles were acquired by radio occultation observations during Juno Perijoves 10, 11, 14, and 15. They complement previously reported profiles from Perijoves 1, 3, 6, and 8, for which an initial ad hoc correction has been verified by improved calibration at the ground station. Fits to each profile determined the location, peak total electron content, and scale height of two torus regions: the cold torus and the torus beyond 5.5 RJ. Measurements show the torus locations depend on longitude, but not time. The peak total electron content values vary with time but do not exhibit any clear dependence on longitude. The scale heights may depend on longitude and/or time. There is no apparent dependence of these properties on the Io phase angle. Future analysis of more observations by the Juno spacecraft will clarify the respective roles of longitude, time, and the Io phase angle in shaping the density structure of the Io plasma torus. Observations at previously sampled longitudes will be particularly valuable, as demonstrated here by Perijoves 8 and 15, which sampled the same longitude 1 year apart.