Vibrationally excited H2 in the outer planets thermosphere: Fluorescence in the Lyman and Werner bands

Abstract We have considered the impact of fluorescence of ground state H 2 on the distribution of the vibrational levels of Ha in the upper atmospheres of Jupiter and Saturn for non-auroral latitudes. For v ⩾ 3, for the conditions studied, this is the most important source of vibrationally excited H 2 compared with other sources, such as photoelectron induced fluorescence, dissociative recombination of H + 3 ions, and direct vibrational excitation of H 2 by photoelectron impact. Combining the Voyager limb observations of H 2 band emissions on Saturn, theoretical calculations of the H 2 fluoresence distribution, and column constraints of Jovian H 2 airglow, we estimate that some of the higher vibrational levels may have effective temperatures > 3000 K on both Jupiter and Saturn. In turn, the vibrational population of v ⩾ 4 levels are sufficiently increased by the fluorescence source that the chemical sink for the ionization is enhanced. As a result, ionospheric densities may be greatly affected. We also show that the vertical ion flows induced by horizontal neutral winds or dynamo electric fields must play some role in maintaining the plasma peaks at higher altitudes.

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