A New Method for Inverting Dayside 83.4 nm Nadir Radiance Measurements for Determining the State of the Ionospheric F-Region

We present a new method for inverting the observed nadir radiance of the dayside 83.4 nm ionospheric airglow emission to provide information about the state of the ionospheric F-region. The dayside O II 83.4 nm airglow emission is produced primarily by photoionization excitation of O (and to a lesser extent, photoelectron impact ionization) in the lower thermosphere. Those 83.4 nm photons radiated upward into the ionosphere are scattered several times on average by the O+ ions in the F-region and thus the radiance signature picks up information concerning the spatial distribution of the O+ ions. Inversion of 83.4 nm emission measurements using a detailed radiative transfer model has been demonstrated as a means of accurately specifying the dayside ionospheric state when the radiance measurements are made on the Earth's limb from low Earth orbiting satellites or from rockets. We have extended the inversion technique in a novel manner to include nadir measurements and thus to infer information on the horizontal distribution of the O+ ions. This makes it possible to image the dayside ionosphere from a geostationary platform.