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Seasonal variation of Saturn’s Lyman alpha airglow and upper atmospheric hydrogen

Presentation #325.08 in the session Origin and Evolution of Giant Planet Systems I (Poster)

Published onOct 23, 2023
Seasonal variation of Saturn’s Lyman alpha airglow and upper atmospheric hydrogen

Saturn’s Lyman alpha airglow, emitted from upper atmospheric atomic hydrogen, was observed by Cassini’s Ultraviolet Imaging Spectrograph (UVIS) from 2003 to 2017. These airglow emission observations provide measurements across the Saturn disk over short timespans (5-17 hours), allowing for the examination of latitudinal structure and seasonal variations in the atomic hydrogen distribution. Lyman alpha emissions are strongly absorbed below the methane homopause, which serves as the lower boundary of the emitting column. In this study, we have analyzed Lyman alpha brightness data covering the entirety of the Cassini mission, away from auroral latitudes.

In order to further examine the calibration of the Cassini/UVIS instrument at Lyman alpha, we compare background interplanetary hydrogen (IPH) observations with a model of the interaction between the solar wind and interstellar plasma. This model, informed by SWAN and BepiColombo/PHEBUS observations, demonstrates a close agreement with Cassini/UVIS measurements of the IPH Lyman alpha.

A multivariate regression analysis of dayside Lyman alpha brightnesses confirms that resonance scattering is the primary driver of emissions in Saturn’s upper atmosphere. The incident solar flux, latitude, emission angle, and incidence angle are key drivers of the observed Lyman alpha brightness.

Furthermore, we compare the Lyman alpha brightnesses with a radiative transfer model based on the doubling and adding of thin atmospheric layers. This approach allowed us to retrieve the effective optical depth of the atmospheric hydrogen column above the methane homopause. We compare the optical depths to the predictions from our photochemical model for Saturn’s stratosphere and thermosphere based on UVIS stellar occultations and CIRS limb scans. The retrieved optical depths agree well with the magnitude and latitudinal structures predicted by the photochemical model, including a reduction in optical depth observed near the ring shadow in the winter hemisphere. Additionally, we identified seasonal variations in the hydrogen distribution, with a peak in upper atmospheric hydrogen observed between 10-20 degrees N/S during their respective summer seasons.

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