The effect of resonance absorption on the determination of the height of airglow layers

Abstract

The theory of the self-absorption of resonance photons within a luminescent airglow layer is developed and expressions obtained analogous to the Van Rhijn equation for the variation of intensity as a function of zenith angle. The measured flux is shown to be a function of L0/T, where L0 is the vertical thickness in atoms/cm2 and T the absolute temperature of the layer. The attenuation at large zenith angles in the case of the sodium D lines is shown to be large enough when L0 is between 1 and 2×1010 cm−2 and T between 160° and 220° to cause a layer at 70 km to simulate a non-absorbing (Van Rhijn) layer between 200 and 300 km. If self-absorption is important, it is shown that the ratio of D1 to D2 intensity should vary with zenith angle and different apparent heights found for the two components. Accurate measurements of L0 and T are shown to be necessary for even good approximate height deductions where resonance radiation (for example, Na and OI) is concerned.