A Durable Electron Density Profile Near the Inner Edge of the Io Torus

Abstract

The Juno Waves instrument can be used to accurately determine the electron density inside Io's orbit, the inner Io torus. These observations have revealed a local peak in the electron density just inside M = 5 and at centrifugal latitudes above about 10$°$ that is likely the ’cold torus' as identified in Earth-based observations of $S^{+}$ emissions. This peak or “finger” is separated from the more dense Io torus by a local minimum or ’trough’ at M $\ge$ 5. The electron densities are inferred by identifying characteristic frequencies of the plasma such as the low-frequency cutoff of Z-mode radiation at $f_{L=0}$ and the low-frequency cutoff of ordinary mode radiation at $f_{pe}$ that depend on the electron density. The “finger” density ranges from about 0.2 to 65 ${\text{cm}}^{-3}$ and decreases with increasing centrifugal latitude. The “trough” densities range from 0.05 to $\sim$10 ${\text{cm}}^{-3}$. This pattern of a density “trough” followed by the “finger” closer to Jupiter is found on repeated passes through the inner Io torus over a range of centrifugal latitudes. Using a simple model for the electron densities measured above about 10$°$ centrifugal latitude, we've estimated the scale height of the “finger” densities as about 1.17 $R_J$ with respect to the centrifugal equator, which is somewhat surprising given the expected cold temperature of the cold torus. The larger scale height suggests a population of light ions, such as protons, is elevated off the centrifugal equator. This is confirmed by a multi-species diffusive equilibrium model.