Lyman α airglow emission: Implications for atomic hydrogen geocorona variability with solar cycle

Abstract

[1] Satellite-based measurements of geocoronal Lyman˛(Ly˛) emission at 121.6 nm, created through multiple scattering of solar Ly˛photons by atomic hydrogen, offer a valuable means of inferring the hydrogen abundance, [H], in the terrestrial thermosphere and exosphere on a global, long-term basis. We present initial results from an analysis of Ly˛radiance measurements acquired across the Earth's limb from 2002 to 2007 by the Global UltraViolet Imager (GUVI) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft. This data spans nearly half of a solar cycle, and both the absolute Ly˛radiance as well as its relative variation across the limb are shown to exhibit a significant dependence on solar activity. We describe sensitivities of a forward radiative transport (RT) model to key parameters governing the [H] distribution in order to assess implications for [H] estimation from the GUVI limb scan data throughout the solar cycle. Based on data-model comparisons, we conclude that the observed solar cycle variability is indicative of a decrease in dayside H density at the exobase with increasing solar activity. These results, along with additional forward RT modeling based on NRLMSISE-00 model specification of [H], are also used to assess contemporary semiempirical model accuracy.

Non-thermal hydrogen atoms in the terrestrial upper thermosphere

“…As described in Methods, the observed Ly α radiances are binned and averaged both spatially, over solar zenith angle (SZA), and climatologically, over the daily solar 10.7 cm radio flux, a proxy for solar activity. With SZA bin sizes of 2° and F 10.7 bins of 10 flux units, the number of scans included in each bin ranges from several hundreds near solar maximum, to several thousands under moderate solar activity, to more than 10 4 during solar minimum23.…”

“…2a,d. The radiances are normalized to the peak radiance of each case, since the hydrogen density can be quantified from the shape of the Ly α radiances across the limb without requiring knowledge of the absolute instrumental calibration or the solar Ly α flux at line centre, which linearly scale the measurements1623. The four cases, labelled F 10.7 =200, 170, 120 and 80, indicate a clear trend of the Ly α radiances varying with solar activity.…”

“…Detailed information about the GUVI instrument and the data processing algorithm used in this work has been documented previously23 and is briefly reviewed as follows. The 11.78° field-of-view of the GUVI instrument is mapped into 14 spatial pixels along the spacecraft orbital track and 160 spectral bins spanning 115–180 nm.…”

“…An alternative hydrogen density parameterization that has been used to invert observed Ly α radiances is the modified Chamberlain model developed by Bishop25, with extension to the thermosphere and mesosphere using the same parameterization technique mentioned above2326. The modified Chamberlain model accounts for the effects of solar radiation pressure, and more importantly, the effects of charge exchange processes in the plasmasphere, by introducing two additional free parameters, namely the effective exobase density, n s , and the effective exobase temperature, T s , for the satellite populations.…”

“…The modified Chamberlain model accounts for the effects of solar radiation pressure, and more importantly, the effects of charge exchange processes in the plasmasphere, by introducing two additional free parameters, namely the effective exobase density, n s , and the effective exobase temperature, T s , for the satellite populations. However, because this model maintains a discrete transition between the thermosphere and exosphere, the hydrogen density profiles reconstructed from GUVI data exhibit an unphysical structure at the nominal exobase (specifically a discontinuity in the density gradient) and the radiative transfer model cannot reproduce the observed minimum in the relative radiance near the 109° look angle23. Parameterization of the satellite population using an effective exobase temperature, on one hand, leads to a more accurate description of the hydrogen density in the exosphere, but on the other hand, allows an abrupt change of the hydrogen temperature from the ambient thermospheric temperature to the effective temperature, resulting in two significantly different scale heights above and below the exobase.…”