The Jovian aurora: Electron or ion precipitation?

High signal‐to‐noise spectra of the Jovian aurora at UV wavelengths obtained using the International Ultraviolet Explorer Observatory (including the brightest Jovian aurora observed to date) are used to study the existence of sulfur and oxygen emissions which would be associated with the precipitation of energetic heavy ions in the upper Jovian atmosphere. Model calculations of heavy ion precipitation and corresponding estimates of the associated sulfur and oxygen UV emissions carried out in the preceding companion paper of Horanyi et al. suggest emission values for 1304‐Å O I emission that are at least 50 times larger than the upper limit values set by the IUE observations reported here. On the other hand a possible emission feature of S II at 1256 Å is comparable to the theoretically predicted emission intensity. Earlier X ray observations and in situ plasma observations have indicated the existence of energetic heavy ion precipitation in the Jovian auroral zone. Based on the IUE observations reported here, we suggest a scenario where heavy ion auroral energy deposition is concentrated at altitudes below the homopause (i.e., >300 keV/nucleon) and electrons with energies of 10 to 30 keV are responsible for the bulk of the observable UV and EUV emissions since they deposit their energy above the methane‐absorbing layer defined by the homopause.