Determining exospheric hydrogen density by reconciliation of Hα measurements with radiative transfer theory

Abstract

The singly scattered resonant fluorescent component of the geocoronal Balmer‐alpha (Hα) emission is obtained by subtracting the multiply scattered component of Hα, determined by a radiative‐transfer (RT) model, from Hα measurements. Exospheric column abundance then follows a straightforward single‐scattering calculation. The reality of that abundance depends upon the validity of the model exosphere used by the RT model, upon the validity of the estimated solar Ly‐β flux, and upon the statistical measurement errors. Iterative adjustments of the hydrogen density ([H]) profile input to the RT model, converging to a best fit to Hα observations, generate a unique atmosphere best matching model intensity to measured Hα, brightnesses. Applying this method to ground‐based measurements of the geocoronal Hα emission made at Arecibo, Puerto Rico, in 1988, we find that the best estimated multiply and singly scattered components of Hα are about 85 and 60%, respectively, in excess of the initial model values, while the derived [H] is generally in agreement with the initial model [H] at lower altitudes and about 35% higher than the initial model [H] at higher altitudes. Examination of the iteration technique sensitivity indicates that the neutral temperature prescribed by the MSIS‐83 model and the solar Ly‐β flux estimated by a correlation relation are the parameters with the principal impact on the determined [H]. In addition, the examination shows that these parameters impact the RT‐calculated intensity components. Because convergence of the iteration technique is insensitive to the initial model guess for most cases, the technique is capable of accurately determining [H] from Hα measurements, particularly if the temperature is simultaneously measured and the line center solar Ly‐β flux is known.