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 10º 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 ≥ 5. The electron densities are inferred by identifying characteristic
frequencies of the plasma such as the low-frequency cutoff of Z-mode
radiation at fL=0 and the low-frequency cutoff of ordinary mode
radiation at fpe that depend on the electron density. The ’finger’
density ranges from about 0.2 to 65 cm-3 and decreases with increasing
centrifugal latitude. The ’trough’ densities range from 0.05 to
~10 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 10º centrifugal
latitude, we’ve estimated the scale height of the ’finger’ densities as
about 1.17 RJ 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, are elevated off the centrifugal equator. This is confirmed by
a multi-species diffusive equilibrium model.