Where Is the Io Plasma Torus? A Comparison of Observations by Juno Radio Occultations to Predictions From Jovian Magnetic Field Models

The Io plasma torus is thought to lie in Jupiter's centrifugal equator, a location that depends on Jupiter's rotation and magnetic field. Yet previous observations and predictions of the location of the Io plasma torus are inconsistent. Here we test the hypothesis that the Io plasma torus lies in the centrifugal equator by comparison of observations by Juno radio occultations to predictions derived from Juno‐era magnetic field models. These observations determine the locations of two torus components: The cold torus is centered near 5.3 Jovian radii (RJ), and the “torus beyond 5.5 RJ,” dominated by the warm torus, is centered near 5.9 RJ. The observations deviate by 1–2° from the planar centrifugal equator expected for a Voyager epoch dipolar magnetic field. In each observation, the locations of distinct torus regions differ by as much as 1° indicating significant radial structure. The root‐mean‐square error between observation and prediction is smaller for predictions from the JRM09 magnetic field model than for predictions from the VIP4 magnetic field model, confirming the JRM09 model is an improvement over the VIP4 model. Agreement between observations and predictions improves for the warm torus if the magnetic field contributions of a nominal magnetospheric current sheet model are included but worsens for the cold torus. Magnetic field conditions around 5.3 RJ are adequately represented by an internal field model without the current sheet. Conditions around 5.9 RJ require internal and external contributions. These results place constraints on properties of the magnetospheric current sheet during the Juno epoch. Key Points The JRM09 magnetic field model performs better at matching the radio occultation data than the VIP4 magnetic field model The magnetic field induced by magnetospheric currents affects the Io plasma torus location The Io plasma torus location may constrain properties of the magnetospheric current sheet