Modeled and observed N₂Lyman‐Birge‐Hopfield band emissions: A comparison

Abstract

.[1] A thorough understanding of how the N 2 Lyman-Birge-Hopfield (LBH) band emissions vary with altitude is essential to the use of this emission by space-based remote sensors. In this paper, model-to-model comparisons are first performed to elucidate the influence of the solar irradiance spectrum, intrasystem cascade excitation, and O 2 photoabsorption on the limb profile. Next, the observed LBH emissions measured by the High resolution Ionospheric and Thermospheric Spectrograph aboard the Advanced Research and Global Observation Satellite are compared with modeled LBH limb profiles to determine which combination of parameters provides the best agreement. The analysis concentrates on the altitude dependence of the LBH (1,1) band, the brightest LBH emission in the observations. In the analysis, satellite drag data are used to constrain the neutral densities used for the data-to-model comparisons. For the average limb profiles on two of the three days analyzed (28, 29, and 30 July 2001), calculations using direct excitation alone give slightly better agreement with the observations than did calculations with cascading between the singlet electronic N 2 states (a 1 P g , a′S − u , and w 1 D u ); however, the differences between the observed profiles and either model are possibly greater than the differences between the models. Nevertheless, both models give excellent agreement with the observations, indicating that current models provide an adequate description of the altitude variation of the N 2 LBH (1,1) band emissions. Consequently, when using the LBH bands to remotely sense thermospheric temperatures, which can be used to provide an unprecedented view of the thermosphere, the temperatures derived have a negligible dependence on the model used.

Auroral Ionospheric E Region Parameters Obtained From Satellite‐Based Far Ultraviolet and Ground‐Based Ionosonde Observations: Data, Methods, and Comparisons

“…Unlike in Knight et al (2012), the electron N 2 LBH emission cross section used in the generation of our LBHL radiance and LBHS/LBHL ratio values includes cascade from the N 2 (a 0 ) and N 2 (w) states to the N 2 (a) state responsible for LBH emission, using the cross section of Ajello and Shemansky (1985) with a scaling factor of~1.4 to represent cascade, as described in Correira et al (2011; see also Young et al, 2010;Eastes et al, 2011). The inclusion of cascade increases the model electron auroral LBH yields~40%.…”

Remote sensing of neutral temperatures in the Earth's thermosphere using the Lyman‐Birge‐Hopfield bands of N2: Comparisons with satellite drag data

“…This altitude shift is consistent with observations of a bright star (HD108073) on 29 July by both HITS and another instrument aboard ARGOS, the Low-Resolution Airglow and Aurora Spectrograph (LORAAS). For both instruments the discrepancy, 4-5 s or $0.34 degrees, between the time the star was observed and the time expected from the pointing data is consistent with the discrepancy in tangent altitude noted by Eastes et al [2011]. For the current analysis, a shift in tangent altitude equivalent to that calculated by Eastes et al [2011] (16.9 AE 2.4) is assumed when fitting observed spectra to retrieve temperature.…”

“…For both instruments the discrepancy, 4-5 s or $0.34 degrees, between the time the star was observed and the time expected from the pointing data is consistent with the discrepancy in tangent altitude noted by Eastes et al [2011]. For the current analysis, a shift in tangent altitude equivalent to that calculated by Eastes et al [2011] (16.9 AE 2.4) is assumed when fitting observed spectra to retrieve temperature. The tangent altitudes of the HITS observations used in this study vary with latitude from 180 to 195 km (including the shift mentioned above); however, the variation within each bin is only $2-4 km.…”

“…result in all of the HITS temperatures being raised or lowered, depending on the sign of the error. The AE2.4 km uncertainty on the 16.9 km value determined by Eastes et al [2011] translates into an additional, systematic temperature uncertainty of approximately AE6 K when fitting the HITS spectra.…”

“…Unfortunately, due to a GPS failure early in the mission, ARGOS, and consequently HITS, was unable to accurately determine tangent altitudes. However, the analysis of HITS limb scans from July 28-30 [Eastes et al, 2011], which showed good agreement between HITS emission profiles and modeled profiles, suggests that the actual tangent altitudes were 16.9 AE 2.4 km higher than indicated by the pointing data. This altitude shift is consistent with observations of a bright star (HD108073) on 29 July by both HITS and another instrument aboard ARGOS, the Low-Resolution Airglow and Aurora Spectrograph (LORAAS).…”