Gold Level L2 exospheric temperature

Algorithm theoretical basis Limb profiles of thermospheric airglow emissions depend fundamentally on temperature, particularly the decay rate with altitude above the peak of the emission. This has been exploited in retrieval algorithms for analyzing far-ultraviolet limb emissions from low-Earth orbit (e.g., Picone and Meier [2000]). For GOLD, the low spatial resolution on the limb mandates that, rather than attempting to fit an entire temperature profile, we only infer a single parameter, the exospheric temperature (TLIMB), defined as the temperature of the atmosphere when in diffusion equilibrium.

We use daytime, non-auroral N2 LBH emission limb brightness profiles where the only excitation mechanism is photoelectron impact on N2. LBH emission bands in the 137-160 nm range are integrated spectrally, excluding the N I 149.3 nm line. The GOLD limb scan measurements are done in one hemisphere at a time, and the L1C LIMB data covers a latitude range from the equator to ~20 degree.

The specific steps involved in the TLIMB retrieval are as follows: • Filter data using topside tangent height range (~100-300 km). • Fit a Chapman function to the emission brightness profile. • Obtain the N2 scale height H (Zo) from the Chapman fit. • Obtain T∞ from H (Zo) = kT/Mg, where k is Boltzmann’s constant, M is the molecular mass of N2, and g is the gravitational acceleration. Note that this fit is independent of the absolute brightness calibration of the airglow intensity, it depends only on the shape of the radiance profile.

For this reason, it is necessary to detect stars in the field-of-view, since the emission from stars can produce a profile shape that can be very different from a profile produced solely by thermospheric airglow. References Picone and Meier (2000), Similarity transformations for fitting of geophysical properties: Application to altitude profiles of upper atmospheric species, J. Geophys. Res., 105, 18599, doi:10.1029/1999JA000385].

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Snowden, D., R. V. Yelle, J. Cui, J.-E. Wahlund, N. J. T. Edberg, and K. Ågren (2013), The thermal structure of Titan’s upper atmosphere, I: Temperature profiles from Cassini INMS observations, Icarus, 226, 52–582.