Atmospheric absorption The emissions studied by ALIS are absorbed mainly in the lower atmosphere. Further the emission lines observed by ALIS at $4278, 5577, 6300, 8446$ Å are all optically thin in the ionosphere, i.e. photons emitted in the ionosphere are not absorbed in the region of emission. The absorption is mainly by the stratospheric ozone. The amount of absorption depends on the length of the light path through the stratosphere. The smallest amount of absorption is obtained for a light path with zero zenith angle. With increasing zenith angle the light path through the stratosphere increases as does the absorption. For moderate zenith angles, $\theta_z<70$, the curvature of the earth can be ignored and with a zenith optical depth $\alpha(\lambda)$ that is dependent on wavelength, the absorption is: $$e^{-\alpha(\lambda)/\cos\theta_z}$$ (5.15) Table 5.1: Optical depths for the strongest auroral lines The optical depths by Arnoldy and Lewis (1976) that have been used in the current forward model for auroral lines measured by ALIS are given in table 5.1.