[Paper Review] Saturn Variable Thermosphere

D. F. Strobel, Tommi Koskinen+1 more|arXiv (Cornell University)|Oct 24, 2016

Astro and Planetary Science104 references3 citations

TL;DR

This paper analyzes Cassini UVIS observations of 15 solar and 26 stellar occultations to characterize Saturn's variable thermosphere, revealing poleward-increasing temperatures up to 550–600 K at the poles, driven primarily by auroral heating (~10 TW) that exceeds solar EUV/FUV input. The inferred westward retrograde winds (~500 m/s) and thermospheric expansion peaking near equinox suggest efficient energy redistribution from high latitudes to low latitudes.

ABSTRACT

Our knowledge of Saturns 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 ~ 400K 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 ~ 10TW generating Joule and energetic particle heating of ~ 5-6TW 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 Saturns 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 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.

Motivation & Objective

• To understand the thermal structure and energy sources of Saturn's thermosphere using in-situ occultation data.
• To resolve the long-standing 'energy crisis' in Saturn's thermospheric heating by quantifying non-solar energy inputs.
• To investigate the role of auroral heating and its latitudinal distribution in driving atmospheric dynamics.
• To determine the thermospheric wind patterns and their implications for atmospheric circulation.
• To examine seasonal and solar cycle variations in thermospheric expansion and temperature profiles.

Proposed method

• Utilized Cassini Ultraviolet Imaging Spectrograph (UVIS) observations of 15 solar and 26 stellar occultations to infer temperature and composition profiles.
• Analyzed temperature profiles from occultation data to derive thermospheric structure, with lower boundary temperatures ~150 K rising to ~400 K at equator and 550–600 K at poles.
• Estimated auroral heating power at ~10 TW, with Joule and energetic particle heating contributing ~5–6 TW, significantly exceeding solar EUV/FUV input.
• Applied gradient wind balance on equipotential surfaces to infer westward retrograde winds of ~500 m/s at mid-latitudes.
• Modeled thermospheric expansion as a function of season and solar activity, finding anti-correlation with solar cycle and peak near equinox.
• Evaluated constraints on H2 and He mole fractions, with H limited to ≤5% and He unconstrained due to lack of 58.4 nm airglow data.

Experimental results

Research questions

• RQ1 What is the dominant energy source responsible for heating Saturn's thermosphere, and how does it compare to solar EUV/FUV input?
• RQ2 How does the thermospheric temperature structure vary with latitude and season, and what drives its expansion?
• RQ3 What are the inferred wind patterns in Saturn's thermosphere, and how do they relate to pressure gradients and energy distribution?
• RQ4 To what extent can auroral heating be redistributed to low latitudes to resolve the thermospheric energy crisis?
• RQ5 Why does the thermosphere expand most strongly near equinox and anti-correlate with solar activity?

Key findings

• Thermospheric temperatures rise from ~150 K at the lower boundary to ~400 K at equatorial latitudes and reach 550–600 K at the poles.
• Auroral heating contributes ~10 TW, with ~5–6 TW from Joule and energetic particle processes, exceeding solar EUV/FUV heating by over an order of magnitude.
• Westward retrograde winds of ~500 m/s (Mach number ~0.3) are inferred from gradient wind balance, consistent with poleward pressure gradients.
• Thermospheric expansion peaks near equinox and is anti-correlated with solar activity, suggesting lower thermospheric heating of unknown origin.
• The H mole fraction is constrained to ≤5%, while the He mole fraction remains unconstrained due to absence of published 58.4 nm airglow measurements.
• The observed poleward temperature increase and pressure gradients are consistent with auroral heating as the primary driver of thermospheric dynamics.