The contribution of singly scattered photons to the optically thick resonance radiation field of the helium geocorona

Abstract

Analysis of the geocoronal He-I 58.4-nm resonance radiation field with rocket borne gas absorption cell photometers has revealed singly scattered photons to attain steadily increasing importance in the radiation signals if photons close to the line center are effectively suppressed. In order quantitatively to confirm this, we have reinvestigated the radiation transport problem and have given the spectral intensity in terms of contributions from different scattering orders. In the formulation of these contributions we have used the concept of angle-dependent partial frequency redistribution in comparison with that of complete redistribution. The first and second order intensity contributions are studied in detail here with respect to their spectral profile, to their local and solar zenith angle dependence, and to their height profile. The second and higher order intensity contributions are shown to decrease steeply with increasing distance from the line center, such that for a specific observation a critical wavelength distance can be calculated beyond which only singly scattered photons contribute to the total intensity. The intensity due only to singly scattered photons is proven to be proportional to the helium column density on the line of sight within clearly determinable accuracies. Thus its height profile enables immediate deduction of local atmospheric parameters, as the atmospheric helium density and temperature, even under optically thick conditions. Fahr and $eidl; 1978, H. J. Fahr et al., unpublished manuscript, 1981]. This is due to the fact that resonance gas absorption cell photometers can selectively suppress photons that are within a specific distance A Xc from the center of the resonance line. A broadband photometer with a pressurized resonance cell in front is thus able to register only those photons of the incomi ng radiation intensity that are beyond this specific wavelength distance. 1977;The interesting fact with these transmitted intensities, as obtained in the case of He 1 58.4 nm observations in the He geocorona, is that their logarithmic plot versus height above the planetary surface, in very good approximation, yields a redistribution process (see section 5) the above conclusion is not reached. For the purpose of a theoretical confirmation of the above mentioned conclusion we have reinvestigated the problem of the resonance radiation transport in a threedimensional planetary atmosphere under the scope of representing separately radiation contributions from different scattering orders. Basic treatments of the radiation transport problem applied to plane parallel atmospheres have been published by Chandrasekhar [1960] and $obolev [1963]. More realistic three-dimensional structures of the planetary atmosphere cannot be treated analytically, but can only be carried out by numerical computations. A first calculation of the geocoronal Lyman-Alpha resonance field was carried out by Thomas [ 1963] with the help of an iterative procedure to determine alternatingly source function and radiation...