UVIS Saturn Atmosphere Occultation Prospectus D. Shemansky J. Hallett X. Liu 07/14/05

Solar vs Stellar Occultations Stellar occultations can be obtained using the FUV and EUV spectrographs. This allows explorations of the transmission spectrum in the range 91.3 – 190. nm, with sensitivity to H2 and hydrocarbon absorbers. Major disadvantage: Cannot directly measure atomic hydrogen. Solar occultations are obtained in the EUV spectrograph in the spectral range 55. – 115. nm. Major advantage: Obtains absorption in the ionization continuum of all species, including atomic hydrogen. Major disadvantage: Differentiation of hydrocarbon species is difficult in the EUV. Both kinds of occultations at similar latitudes are desirable.

Analytic capability A stellar occultation has been obtained in Rev 6, providing the first direct test of measurement capability for a hydrogen dominated atmosphere for the UVIS experiment. Previous stellar occultations have been obtained at Titan [Shemansky et al., Science, 308, 978, 2005].

The Rev 6 Saturn Occultation This occultation was under reaction wheel control. The data shows no indication of pointing drift, and provides high quality results. The analysis to date has been confined to the region above the hydrocarbon homopause. Analysis at lower impact parameters is deferred until the upper thermospheric structure is fully understood.

Rev 6 EUV transmission spectrum at 1080 km vs H2 model. Model constructed with individual rotation line profiles, of order 10000 absorption lines in this spectrum.

Capability for determination of the state of the hydrogen gas The Rev 6 transmission data contains no detectable absorbers other than hydrogen down to an altitude of 800 km. The absorption spectrum is analyzed using a hydrogen physical model developed at USC. The model includes line shape dependence on perturbations, and has an estimated accuracy of 5% in fundamental parameters [Hallett, Shemansky, and Liu. Ap. J., 624, 448, 2005]. Hydrogen is the most accurately known absorber of all species involved in these occultations.

Extraction of the state of the H2 gas Direct experience with the experimental data shows sensitivity to rotational temperature and vibrational population distribution. This allows two methods of extracting gas kinetic temperature. The reduction process requires iterative modeling of the transmission spectra, and is labor intensive.

Most recent iteration of hydrostatic model fit to Rev 6 occultation data. Strong heating is indicated above 1000 km.

Derived temperature distribution from rev 6 occultation merged into Lindal et al. (1985) below 400 km.

Conclusions to date from Rev 6: Upper thermosphere H2 X(v) is non-LTE by multiple orders of magnitude. Rotation populations are thermal below the exobase. Excitation of H2 X(v) by hot H is required to explain large non-LTE excursion [Hallett, Shemansky, & Liu, in prep]. Strong heating above 1000 km is caused by electron impact dissociation of H2 followed by heat transfer from 60000 K atomic hydrogen product [ Shemansky, Hallett, & Liu, in prep]. Atomic hydrogen from top of Saturn atmosphere is flooding the magnetosphere.

Expected primary results from solar occultation Determination of upper thermosphere atomic hydrogen distribution; Location of diffusive separation. Definitive determination of energy deposition magnitude and distribution. Loss rate of atomic hydrogen into the magnetosphere.

Current limitations Knowledge of hydrocarbon absorption cross section temperature dependence is limited, compromising determination of accurate absolute abundance and abundance profiles [see Shemansky et al., Science, 308, 978, 2005]