Io's Alfvén Wing Electron Density Measured by Juno Radio Occultations

The Jovian magnetosphere, the largest in the Solar System, extends up to 50-100 Jovian radii. Io, Jupiter's innermost Galilean moon, orbits deep within this magnetosphere. Io’s volcanic activity emits 1 ton per second of material into the magnetosphere mainly SO₂. Collisions with magnetospheric particles ionize SO2, forming a torus-shaped plasma cloud called the Io Plasma Torus (IPT) around Jupiter. When Io moves through Jupiter’s magnetic field, Alfvén waves are generated that propagate along Jupiter's magnetic field lines, leading to Alfvén Wings that extend from Io toward Jupiter's poles.

In the past few years, Juno’s radio science instrumentation, which can transmit dual-frequency X-band (8.4 GHz) and Ka-band (32 GHz) radio signals, has been used to conduct radio occultation experiments for the measurements of the ionospheric plasma surrounding Jupiter and its moons, Ganymede and Europa. Recently, the spacecraft performed two close flybys of Io on December 29, 2023 and February 4, 2024, namely I57 and I58. During both encounters, the radio ray path traveled through the Alfvén wings that connect Io to Jupiter. On both Io’s flybys, the downlink plasma was extracted using the dual-link single-uplink multifrequency calibration. Indeed, by performing a linear combination of the X and Ka-band data collected at the Earth’s antenna (namely, the differential frequency depicted in Figure 1), we can directly extract the dispersive contribution to the Doppler shift in the downlink. The extracted Doppler shift induced by the dispersive media indicated a high electron concentration within the Alfvén wings.

Figure 1 Downlink plasma obtained by a linear combination of X and Ka-band data collected during the Io's flybys I57 and I58

These data provide information on the path delay of the signal and, consequently, the Total Electron Content along the line of sight. In order to retrieve the electron density inside these structures from the Doppler measurements, we must introduce some geometrical assumptions on the Alfvén wing's size, orientation, and electron density distribution. Due to their tubular shape, conventional radio occultation inversion methods cannot be used to perform this analysis. Consequently, for these experiments we developed novel techniques tailored to this particular case. We modified an existing ray-tracing-based inversion algorithm, used in the past for the analysis of spherical and oblate ionospheres, by adding the assumption of a cylindrical distribution of electrons within the Alfvén wing. Alternatively, we assumed the electron density to be constant along horizontal tubes parallel to the Juno-Earth line to get an average density along the portion of the line-of-sight traversing the Alfvén wing. Using these approaches led to electron densities up to 17,000-30,000 cm^-3 .