The a 3Σg+ – b 3Σu+ Continuum Emission from Electron Impact of Molecular Hydrogen in Saturn’s Atmosphere

Abstract:

Shemansky et al. (2009, Planetary and Space Science 57: 1659-1670) have reported observations of hydrogen atoms flowing out of the top of Saturn’s sunlit thermosphere in a confined, distinct plume of ballistic and escaping orbits, and a continuous distribution of H atoms from the top of Saturn’s atmosphere to at least 45 Saturn radii (R_S) in the satellite orbital plane and to 25 R_S azimuthally above and below the plane. These observations have revealed the importance of the excitation of H₂ by low energy electrons. H₂ is efficiently excited to the triplet states by low energy electrons, and all triplet excitations result in the dissociation of H₂ and the production of hot H atoms. Because of this, the electron impact excitation of H₂ is an important energy deposition mechanism in the upper atmospheres of Saturn and other giant planets. The a ³Σ_g⁺ – b ³Σ_u continuum transition, which dominates all other H₂ transitions in the 168-190 nm region, provides a unique spectral window through which the triplet transition can be observed with the Cassini spacecraft. The excitation and emission cross sections of the a ³Σ_g⁺ state and other triplet states are required for the extraction of the triplet emission and excitation rates from the apparent emission rate measured by the spacecraft. These emission and excitation rates, in turn, help to determine the energy deposition rate by electron impact excitation.

Unfortunately, large discrepancies exist between published measurements of the a ³Σ_g⁺ – b ³Σ_u continuum transition. In order to begin to address this issue, we have recently revisited the problem by measuring electron impact induced a ³Σ_g⁺ – b ³Σ_u emission cross sections. We have also measured direct excitation cross sections of the triplet a ³Σ_g⁺ state. Using these, we are able to partition the excitation function into its direct and cascade components. As stated above, these results will enable improved understanding of phenomena observed in Saturn’s atmosphere.

Acknowledgement: This work was performed at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). Financial support through NASA's PATM program, as well as the NASA Postdoctoral Program (NPP) are gratefully acknowledged.