These results indicate that perhaps protons play a larger role in the torus than was previously thought. Since a review of the literature and databases shows no measured photon emission cross sections for protons colliding with SO2, we have initiated a determination of these cross sections. We present here spectra and preliminary cross section data for proton-SO2 collisions over a range of proton energies from 50 to 250 keV. This range of energies dominates the proton energy flux. Experimental Details General DescriptionMore information on the general nature of the experimental techniques employed by this laboratory can be found in previous publications [Monce, 1988[Monce, , 1990Freeman et al., 1992]. However, as this is the first publication utilizing a CCDspectrometer system for detection of the photons, we will describe the procedure in greater detail.The The target gas cell is aligned along the beam path with an adjustable viewing slit along this length. The target gas enters the cell through a tube in the top. A tube at the bottom is connected to a capacitance manometer for pressure measurement. The gas pressure is regulated with a precision needle valve. Also in the target gas cell is a thermistor for temperature measurement. The thermistor was calibrated using a 0.1øC mercury thermometer.The photons emitted 90 ø to the beam path are collected and 26,147

[1] Observations of atomic emissions from the Io plasma torus and previously published laboratory work suggest that proton collisions with SO 2 may be a source of some of the spectral lines. We present an analysis of Balmer series lines seen in the spectra of collisions of protons with SO 2 over the range of 50 to 250 keV. Absolute emission cross sections for these lines are measured to be on the order of 10 À19 cm 2 and are in rough agreement with a parametric model for electron capture which supports charge-transfer ionization of the SO 2 target.

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