Hydrogen Balmer alpha intensity distributions and line profiles from multiple scattering theory using realistic geocoronal models

Abstract

The H Balmer α nightglow is investigated by using Monte Carlo models of asymmetric geocoronal atomic hydrogen distributions as input to a radiative transfer model of solar Lyman β radiation in the thermosphere and exosphere. The radiative transfer model includes all orders of scattering, temperature variation with altitude and solar zenith angle, and anisotropic velocity distributions. The influences of multiple scattering of Lyman β radiation and of observing geometry on the H Balmer α intensity and effective temperature are evaluated in detail. Morning and evening hydrogen distributions for minimum, medium, and maximum solar activity are used in calculations of nightglow emission rates and line profiles. For each of the hydrogen models the H Balmer α intensity and effective temperature are displayed as a function of solar depression angle, observation zenith angle, and azimuth relative to the sun, to make possible detailed comparison to observations. Except for rather unique observing conditions, multiple scattering effects cannot be ignored for the determination of either intensity or temperature. Observed effective temperatures which are significantly less than the exobase temperature may be due either to low‐altitude thermospheric hydrogen or to high‐altitude exospheric hydrogen and depend on the observing conditions and the level of solar activity. Previous (selected) observations of the H Balmer α nightglow and effective temperatures are compared to models which most closely represent the solar conditions of the observations. Results of the theoretical investigation of H Balmer α emissions and of model‐data comparisons indicate that predicted morning‐evening concentration variations yield small intensity variations for solar depression angles greater than 20°. The variations with solar depression angle of observed intensities and temperatures both show discrepancies in comparison with model results. A substantial disagreement occurs in absolute intensity: the long‐standing result that the observations are a factor of 2 higher than model results is reaffirmed with recent observational data and improved estimates of the solar H Lyman β flux. Observing schemes are discussed which lead to first approximations to hydrogen content independent of solar flux and to the high‐altitude atomic hydrogen distribution.

Non-thermal hydrogen atoms in the terrestrial upper thermosphere

“…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.…”

“…The Chamberlain theory, despite its unphysical collisionless assumption, has been widely used, for its relative simplicity, as the standard model in the last few decades to analyse remote sensing measurements of the hydrogen geocorona14151617, as well as to study magnetospheric energy dissipation and the resultant precipitation of energetic neutral atoms onto the thermosphere29. It is widely recognized that by relying on the Chamberlain model, the expected significant variation of exobase density and temperature with declining solar activity cannot be obtained from the analysis of geocoronal emission observed by the Dynamics Explorer mission15.…”

Neutral hydrogen density profiles derived from geocoronal imaging

“…The geocorona is produced when solar Lyman α radiation is resonance scattered by exospheric neutral hydrogen. Since rocket‐borne experiments [e.g., Kupperian et al , 1959; Donahue , 1966] provided the first measurements of the geocoronal Lyman α in the late 1950s, efforts have been made to model the neutral density profiles [ Chamberlain , 1963; Tinsley et al , 1986; Bishop , 1991; Hodges , 1994] and to predict the Lyman α intensities that can be observed from space [ Anderson and Hord , 1977; Anderson et al , 1987; Bishop , 1999].…”

Particle tomography of the inner magnetosphere

“…The early Chamberlain [1963] model, analytically derived from kinetic theory, described only the radial profile of the hydrogen distribution. More recent models utilize computationally intensive Monte Carlo simulations of the exosphere [ Tinsley et al , 1986; Anderson et al , 1987; Hodges , 1994]. Tracing a large number of test particles, these models also consider diurnal and solar cycle dependence in determining the neutral distributions.…”