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9 - Saturn’s Variable Thermosphere

Published online by Cambridge University Press:  13 December 2018

By
Darrell F. Strobel ,
Tommi T. Koskinen  and
Ingo Müller-Wodarg
Edited by
Kevin H. Baines ,
F. Michael Flasar ,
Norbert Krupp  and
Tom Stallard
Kevin H. Baines
Affiliation:
University of Wisconsin, Madison
F. Michael Flasar
Affiliation:
NASA-Goddard Space Flight Center
Norbert Krupp
Affiliation:
Max-Planck-Institut für Sonnensystemforschung, Göttingen
Tom Stallard
Affiliation:
University of Leicester
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Summary

Our knowledge of Saturn’s neutral thermosphere is far superior to that of the other giant planets due to Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of 15 solar occultations and 26 stellar occultations analyzed to date. These measurements yield H2 as the dominant species, with an upper limit on the H mole fraction of 5%. Inferred temperatures near the lower boundary are ~150 K, rising to an asymptotic value of ~400 K at equatorial latitudes and increasing with latitude to polar values in the range of 550–600 K. The latter is consistent with a total estimated auroral power input of ~10 TW generating Joule and energetic particle heating of ~5–6 TW that is more than an order of magnitude greater than solar EUV/FUV heating. This auroral heating would be sufficient to solve the “energy crisis” of Saturn’s thermospheric heating if it can be efficiently redistributed to low latitudes. The inferred structure of the thermosphere yields poleward-directed pressure gradients on equipotential surfaces consistent with auroral heating and poleward increasing temperatures. A gradient wind balance aloft with these pressure gradients implies westward, retrograde winds ~500 m s−1 or Mach number ~0.3 at mid-latitudes. The occultations reveal an expansion of the thermosphere peaking at or slightly after equinox, anti-correlated with solar activity, and apparently driven by lower thermospheric heating of unknown cause. The He mole fraction remains unconstrained, as no Cassini UVIS He 58.4 nm airglow measurements have been published.

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Information
Saturn in the 21st Century , pp. 224 - 250
Publisher: Cambridge University Press
Print publication year: 2018

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Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

Find out more about the Kindle Personal Document Service.

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Save book to Dropbox

To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.

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Save book to Google Drive

To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.

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