The chemical effects of auroral oxygen precipitation at Jupiter

Abstract

Energetic oxygen precipitation is one possible explanation for the auroral molecular hydrogen emissions observed by the Voyager ultraviolet spectrometer. These ions deposit their energy in the Jovian thermosphere by means of ionization, dissociation, excitation, and heating processes. The photochemistry of both the neutral gas and the ionosphere is altered by the influx of atomic oxygen. The auroral oxygen flux is about 107 cm-2 sec-1 if all the observed Lyman and Werner band emissions are due to heavy ion precipitation rather than to energetic electron precipitation. The O atoms react with H+ ions forming O+ ions, which then react with H2, starting a chain of chemical reactions involving H2O+ and H3O+ that ultimately leads to the formation of water. The O atoms can also react with CH3 and will eventually form CO deeper in the atmosphere. A numerical model has been developed to study the effects of odd oxygen (i.e., O, OH, and H2O) on the auroral ionosphere of Jupiter. In particular, we show that the electron density is reduced by about a factor of 4 due to the removal of H+ ions by chemistry associated with odd oxygen species, whereas the electron density reduction required for consistency with observations is more than a factor of 20 suggesting that other H+ loss mechanisms must be operating or that sources of oxygen or water other than auroral precipitation are present. The globally averaged flux of oxygen from the aurora is also about a factor of 20 too low to explain the Jovian CO measurements.