ENERGY ESCAPE RATE OF NEUTRALS FROM IO AND THE IMPLICATIONS FOR LOCALMAGNETOSPHERIC INTERACTIONS

The rate at which energy is carried away by atomic oxygen and sulfur e scaping 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 similar to 20 km/s), and (3) fast-velocity charge exchange (centered similar t o 60 km/s). The calculations are based on information for the O and S source rates and their velocity distributions at Io as independently d etermined from the combined results of previous studies for the observ ed column density and/or brightness morphology of the satellite's neut ral corona and extended neutral clouds. The calculated energy escape r ates for the three processes are 7.5 x 10(9) W, 3.3 x 10(10) W, and 6. 04 x 10(11) W and are similar to 11%, 50%, and 900%, respectively, of the upstream ion kinetic energy flow rate of 6.7 x 10(10) W determined for a Voyager corotating plasma flowing through a minimum interaction area of R-pi(Io)2, where R-Io is Io's radius. A larger more physicall y appropriate upstream interaction area of 2(pi)R(Io) = pi(1.414 R-Io) (2) would reduce these percentages by a factors of 2. For incomplete c ollisional cascade, the calculated energy escape rate is expected to b e only similar to 20% of the total energy deposition rate for this pro cess: indicating a heating rate for the atmosphere of 3.0 x 10(10) W ( the remaining similar to 80%). This implies that 56% of the minimum up stream ion kinetic energy flow rate is supplied to the atmosphere thro ugh the collisional cascade process, a factor of 2.8 times larger than the previously adopted value, and that the effective deflection of ma gnetospheric 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 x 10(11) W and is so larg e 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 origina l local corotational and gyration energies by acceleration in the loca l 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 10 will depen d 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 estima tes given here suggest a pickup current in the range of similar to 5 x 10(6) to 2 x 10(7) A and a reduction (Delta B) in the local magnetic field of similar to 450 nT. This estimated reduction of the magnetic f ield is similar to the remaining and unexplained Delta B of similar to 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 fo r Io. Hence the remaining and unexplained similar to 400-nT reduction of the magnetic field measured by Galileo near 10 may be a direct refl ection of the local charge exchange source and need not require an int ernal magnetic field for the satellite.