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Cassini UVIS Observations of He 584A Airglow at Saturn

Presentation #314.05 in the session “Giant Planets 5: Upper Atmosphere”.

Published onOct 26, 2020
Cassini UVIS Observations of He 584A Airglow at Saturn

The atmosphere of Saturn is mainly composed of H2 and neutral atomic helium. The study of He 584A brightnesses is interesting as the EUV (Extreme UltraViolet) planetary airglow have the potential to yield useful information about mixing and other important parameters in its thermosphere. Resonance scattering of sunlight by He atoms is the principal source of the planetary emission of He 584A. The helium is embedded in an absorbing atmosphere of H2 and since it is heavier than the background atmosphere, its concentration falls off rapidly above the homopause. The scattering region (i.e. where the absorption optical depth in H2 is less than 1) generally lies well above the homopause. As the eddy diffusion coefficient, Kz, increases in the middle atmosphere, relatively more helium is mixed into the scattering region and thus the reflected intensity increases. The principal parameter involved in determining the He 584A albedo are the He volume mixing ratio, fHe, well below the homopause, and Kz, (which will generally be a function of altitude), the solar He 584A flux and line shape, and the atmospheric temperature profile.

He emissions come from above the homopause where optical depth tau = 1 in H2 and therefore the interpretation depends mainly on two parameters: He mixing ratio of the lower atmosphere and Kz. In this paper, we make an estimate of the helium mixing ratio in the lower Saturnian atmosphere. The published stellar occultations give Kz with an accuracy that has never been possible before and the combination of occultations and airglow therefore can provide estimates of the mixing ratio in the lower atmosphere. We made these estimates at several locations that can be reasonably studied with both occultations and airglow and then average the results. Our results will lead to a greatly improved estimate of the mixing ratio of He in the upper atmosphere and below. Once we have an estimate of the He mixing ratio in the lower atmosphere that agrees with both occultations and airglow, helium becomes an effective tracer species as any variations in the Cassini UVIS helium data are direct indicator of changes in Kz i.e., dynamics.

We retrieved the He 584A airglow profiles in Saturn’s thermosphere from 18 observations made by the Cassini/UVIS instrument over a span of time from 2004 until 2013. Using these He brightnesses, we can constrain the dynamics in the atmosphere. These Cassini UVIS data He 584A airglow analyses cover the time span of the observations and allow us to monitor changes in the airglow observations that may correlate with changes in the state of the atmosphere as revealed by the occultations Saturn’s upper thermosphere. We obtain these airglow values using the Laboratory for Atmospheric and Space Physics (LASP) calibration matrix on retrieved PDS data. However, there seems to be a disparity between analyses using the LASP calibration matrix on the He 584A EUV data and the analyses using the forward model technique of Shemansky et al. (2009). We perform a preliminary study to discover the exact nature of the problem and quantify this startling disparity.

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