DOI: 10.48322/9m3m-tg87
title: Gold Level L2 exospheric temperature

description: 
	  GOLD retrieves the exospheric temperature from the integrated N2 LBH radiance profile 
	  obtained from atmospheric limb scans (LIM observation mode). This data product, referred to as TLIMB, 
	  is derived from limb scan data by fitting the shape of the LBH radiance profile between 100 and 300 km.
	  
	  Algorithm heritage
      An approach similar to the GOLD TLIMB retrieval has been used to analyze data from TIMED/GUVI, 
	  Cassini/UVIS and MAVEN/IUVS. The GOLD TLIMB algorithm is most similar to the operational algorithm 
	  used to retrieve exospheric temperature on Mars from MAVEN/IUVS CO2 density retrievals. The GOLD 
	  algorithm follows the procedure outlined in Lo et al. [2015] as originally applied to the atmosphere of 
	  Mars. The operational code is implemented in IDL and has been generalized to be used with any species in 
	  any planetary atmosphere.
      
	  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].
      
	  Lo, D. Y., et al. (2015), Nonmigrating tides in the Martian atmosphere as observed by MAVEN IUVS, Geophys. 
	  Res. Lett., 42, 9057–9063, doi:10.1002/2015GL066268.
	  
      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.