Energy escape rate of neutrals from Io and the implications for local magnetospheric interactions
Authors: Smyth William H. 
Published: February 2004

The rate at which energy is carried away by atomic oxygen and sulfur escaping from Io because of the interaction of its atmosphere with the corotating magnetospheric plasma is calculated for three different ion‐neutral collisional processes: (1) incomplete collisional cascade, (2) slow‐velocity charge exchange and direct ejection (centered ∼20 km/s), and (3) fast‐velocity charge exchange (centered ∼60 km/s). The calculations are based on information for the O and S source rates and their velocity distributions at Io as independently determined from the combined results of previous studies for the observed column density and/or brightness morphology of the satellite's neutral corona and extended neutral clouds. The calculated energy escape rates for the three processes are 7.5 × 109 W, 3.3 × 1010 W, and 6.04 × 1011 W and are ∼11%, 50%, and 900%, respectively, of the upstream ion kinetic energy flow rate of 6.7 × 1010 W determined for a Voyager corotating plasma flowing through a minimum interaction area of πRIo2, where RIo is Io's radius. A larger more physically appropriate upstream interaction area of 2πRIo2 = π(1.414 RIo)2 would reduce these percentages by a factors of 2. For incomplete collisional cascade, the calculated energy escape rate is expected to be only ∼20% of the total energy deposition rate for this process, indicating a heating rate for the atmosphere of 3.0 × 1010 W (the remaining ∼80%). This implies that 56% of the minimum upstream ion kinetic energy flow rate is supplied to the atmosphere through the collisional cascade process, a factor of 2.8 times larger than the previously adopted value, and that the effective deflection of magnetospheric plasma out of the interaction region near Io is less than previously estimated. The total estimated neutral energy rate for all three processes (including heating) is 6.75 × 1011 W and is so large that it can only be supplied by the magnetic field energy, which is partially converted near Io to kinetic energy for the neutrals by the ion pickup current created by these processes. This is possible since an ion after a collision with a neutral can rapidly regain its original local corotational and gyration energies by acceleration in the local corotational electric field and magnetic field and may undergo many collisions in the interaction region and hence transfer many times its initial kinetic energy to the atmospheric neutrals. The magnitude of the pickup current and its magnetic field reduction near Io will depend critically upon the volume of the interaction region established by the solution of the three‐dimensional magnetospheric flow problem past Io, including these complex plasma‐neutral interactions. Rough estimates given here suggest a pickup current in the range of ∼5 × 106 to 2 × 107 A and a reduction (ΔB) in the local magnetic field of ∼450 nT. This estimated reduction of the magnetic field is similar to the remaining and unexplained ΔB of ∼400 nT determined in a recent analysis [Khurana et al., 1997] of the magnetic field depression measured near Io by the Galileo magnetometer [Kivelson et al., 1996a, b] and attributed in their treatments (where pickup current was neglected) to an internal magnetic dipole field for Io. Hence the remaining and unexplained ∼400‐nT reduction of the magnetic field measured by Galileo near Io may be a direct reflection of the local charge exchange source and need not require an internal magnetic field for the satellite.