Radio occultation experiments of the Io plasma torus: from Juno to JUICE

The volcanic activity of Io, the innermost of the Jupiter’s Galilean moons, is the main source of plasma in the Jupiter’s magnetosphere: the neutral particles ejected by Io’s volcanos are indeed ionized via electron collision and charge exchange processes (Thomas, N., et al., 2004). The produced ions are then affected by the electromagnetic force and by the gravitational and centrifugal forces, becoming confined in a torus around Io’s orbit called the Io plasma torus (IPT).

The IPT affects any radio signal travelling across this region, causing an additional frequency shift and a delay in, respectively, Doppler and range measurements between a deep space probe and the Earth. This effect can be exploited to analyze the IPT, in particular radio science data allow to study electron density models.

Radio occultation experiments have been performed with the Juno mission (Phipps, P.H., et al., 2021). The radio-tracking system enables a two-link configuration in X and Ka band but only Doppler measurements are performed, in two-way coherent mode. The two possible dual-link configurations are X/X + Ka/Ka and X/X + X/Ka and they allow isolating either the uplink or the downlink plasma contribution, which can be related to the total electron content and so to the electron density N_e. The Doppler plasma contribution can be integrated to derive the path delay which can be expressed in terms of the total electron content.

In this way it is possible to study electron density models for the IPT; in particular, we consider the empirical model proposed by (Phipps, P. H., and Withers, P., 2017): it divides the IPT into three main regions, the cold torus, the ribbon and the warm torus, plus the extended torus;  in each region N_e is modeled with a Gaussian-like distribution, and it is expressed as a function of the radial distance r from Jupiter in the centrifugal plane and the distance z away from the plane of the centrifugal equator. This is an axisymmetric model but there may be dependences of the density with the longitude or with time (due to Io’s volcanic activity). These dependences may be expressed with a Fourier expansion for the terms N_i and H_i, the central density and the scale height, respectively, for each region (Moirano, A., et al., 2021).

Juno performed many perijoves with occultation of the IPT allowing us to estimate the model parameters using a Markov Chain Monte Carlo (MCMC) algorithm; however, the model could be potentially further improved with future measurements from the JUICE mission (Grasset, O., et al., 2013).

The JUICE spacecraft is equipped with a radio-tracking system similar to BepiColombo (Iess, L., et al., 2021; Cappuccio, P., et al., 2025): a Deep Space Transponder can establish simultaneously X/X and X/Ka two-way coherent links, while a Ka-Transponder ensures an additional Ka/Ka one. In this way three links are simultaneously established, and the multi-frequency calibration scheme allows isolating the plasma contribution on both the uplink and downlink legs. Differently from Juno, JUICE can collect both Doppler and range data (in a coherent two-way mode) which give access to the absolute value of the total electron content. JUICE also hosts an Ultra Stable Oscillator (USO) (Shapira, A., et al., 2016) which can be used to perform dual frequency X-Ka IPT observations in non-coherent one-way downlink mode.

We report on an analysis performed to find the future optimal opportunities for the occultation of the IPT with the JUICE spacecraft. The occultation opportunities are identified using the SPICE kernels of the mission, then the path delay and the path delay rate are simulated.

The IPT morphology is not yet well understood, and the rich dataset collected by Juno together with the future torus occultations of JUICE gives the opportunity to study in more detail this plasma region and in particular to perform analyses on the electron plasma density.