Structure and Dynamics of the Jovian Magnetodisk Seen by Juno

The Jovian magnetodisk refers to a thin disc of plasma and electric current near the equatorial plane of Jupiter’s magnetosphere. It serves as an “energy converter” within Jupiter’s planetary system, receiving rotational energy from Jupiter’s interior and upper atmosphere and converting it into the thermal and kinetic energy of the magnetospheric plasma. Juno’s in-situ observations of fields (MAG) and charged particles (JADE and JEDI) provide valuable data for revealing the structure and dynamics of the magnetodisk. 

By examining this dataset, we developed an empirical model of the magnetodisk, capturing the distributions of its magnetic field, electric current, and plasma. The results show that: (a) heavy ions dominate both the number density and pressure; (b) the number density and pressure of all species decrease with radial distance; (c) the temperature increases with radial distance for electrons and heavy ions, but decreases for protons; (d) on average, the parallel pressure exceeds the perpendicular pressure for all species; and (e) the magnetodisk exhibits strong local time asymmetry.

Based on this model, we investigated the equilibrium and stability of the magnetodisk. On average, the magnetodisk is in radial force balance, with the dominant forces being the inward magnetic stress and the outward plasma anisotropy force. However, signatures of ongoing non-equilibrium activity are also detected, ranging from global-scale magnetic dipolarization down to ion- and electron-scale plasma waves. A detailed data survey indicates that anisotropy in the pitch-angle distributions of charged particles plays a key role in these processes, both in determining marginal equilibrium states and in controlling the growth of plasma instabilities. A theoretical model incorporating this aspect could provide a better description of the magnetodisk and its role in the dynamics of the magnetosphere.