The Io plasma torus observed by Juno between 2016 and 2022

Results from the Io footprint position and the Io plasma torus radio occultations

¹ Laboratory for Planetary and Atmospheric Physics, Space Sciences, Technologies and Astrophysical Research Institute, University of Liège, Liège, Belgium
² Institute for Space Astrophysics and Planetology, National Institute for Astrophysics (INAF-IAPS), Rome, Italy
³ Department of Industrial Engineering, Alma Mater Studiorum – Università di Bologna, Italy
⁴ Centro Interdipartimentale di Ricerca Industriale Aerospaziale, Alma Mater Studiorum – Università di Bologna, Italy
⁵ Aix-Marseille Université, CNRS, CNES, Institut Origines, LAM, Marseille, France

^★ Corresponding author: alessandro.moirano@uliege.be

Received: 22 December 2024
Accepted: 29 April 2025

Abstract

Context. The Io plasma torus (IPT) is a dense, toroidal plasma cloud around Jupiter, approximately centered on Io’s orbit. Iogenic volcanic activity supplies material to the IPT, mainly through direct outgassing or sublimation of frozen volcanic sulfur dioxide. Material from the IPT diffuses outward into an extended plasma disk, whose electric currents generate a strong magnetic field. This field, together with the internally driven planetary magnetic field, contributes to a powerful and highly variable magnetospheric environment. Because the plasma disk is supplied by the IPT, monitoring its condition and variability is essential for understanding the dynamics of the magnetosphere and to estimate potential hazards for deep space missions.

Aims. This study aims to constrain the average IPT conditions between 2016 and 2022 using observations of the auroral footprint of Io and radio occultations of the IPT conducted by the Juno orbiter. In addition, we investigate density and temperature variations occurring over timescales of weeks to a few months.

Methods. We computed the IPT’s plasma distribution using a diffusive equilibrium, force-balance model. This model simulated both the position of the Io footprint observed in the infrared and ultraviolet and the induced path delay in the radio-tracking data recorded by the Deep Space Network. The advantage of this approach is the ability to break the parameter-space degeneracy that arises when analyzing the two types of observations individually.

Results. We conclude that, between 2016 and 2022, the IPT temperature was 80₋₅₀⁺¹⁰⁰ eV, and the electron density was 3000±1400 cm⁻³.