Temporal Variations of Jupiter’s Plasma Disk Observed by Juno

Jupiter’s magnetosphere features internal mass loading from its innermost moon Io. The neutral gases from Io’s escaping atmosphere are ionized to become the plasma torus, which mainly consists of sulfur and oxygen ions. Under centrifugal force, plasma in the torus is transported outward and forms a thin plasma disk near the equator, while the transport mechanism and timescale remain unclear. Since 2016, the plasma disk between 10 and 50 RJ has been continuously observed by the Juno mission. Using multi-year thermal plasma measurements from the JADE ion detector, we perform an analysis that reveals significant temporal variation of plasma disk from a long-term perspective. For different Juno orbits, the plasma disk observations are categorized as either enhanced or depleted based on plasma density. Extreme cases indicate vastly different states of the plasma disk, with variations exceeding one order of magnitude. Further analysis of multiple plasma disk crossings by Juno reveals correlations between density enhancements and fluctuations in plasma density and magnetic field profiles, which are typical features of flux tube interchange. This suggests that flux tube interchange is triggered by an increase in the plasma source and is considered the primary mechanism for outward plasma transport. Finally, Juno’s in-situ measurements also show a correlation with remotely sensed Io’s torus ribbon brightness from the ground-based IoIO observatory, lagged by about 30 to 50 days. This suggests that the temporal variation of the plasma disk is modulated by changes in Io’s torus and that the average plasma transport time from the torus to the plasma disk is around 40 days.