DETERMINING EXOSPHERIC HYDROGEN DENSITY BY RECONCILIATION OF H-ALPHA MEASUREMENTS WITH RADIATIVE-TRANSFER THEORY

The singly scattered resonant fluorescent component of the geocoronal Balmer-alpha (H(alpha)) emission is obtained by subtracting the multip ly scattered component of H(alpha), determined by a radiative-transfer (RT) model, from H(alpha) measurements. Exospheric column abundance t hen follows a straightforward single-scattering calculation. The reali ty of that abundance depends upon the validity of the model exosphere used by the RT model, upon the validity of the estimated solar Ly-beta flux, and upon the statistical measurement errors. Iterative adjustme nts of the hydrogen density ([H]) profile input to the RT model, conve rging to a best fit to H(alpha) observations, generate a unique atmosp here best matching model intensity to measured H(alpha) brightnesses. Applying this method to ground-based measurements of the geocoronal H( alpha) emission made at Arecibo, Puerto Rico, in 1988, we find that th e best estimated multiply and singly scattered components of H(alpha) are about 85 and 60%, respectively, in excess of the initial model val ues, while the derived [H] is generally in agreement with the initial model [H] at lower altitudes and about 35% higher than the initial mod el [H] at higher altitudes. Examination of the iteration technique sen sitivity indicates that the neutral temperature prescribed by the MSIS -83 model and the solar Ly-beta flux estimated by a correlation relati on are the parameters with the principal impact on the determined [H]. In addition, the examination shows that these parameters impact the R T-calculated intensity components. Because convergence of the iteratio n technique is insensitive to the initial model guess for most cases, the technique is capable of accurately determining [H] from H(alpha) m easurements, particularly if the temperature is simultaneously measure d and the line center solar Ly-beta flux is known.