HIGH ROTATIONAL-EXCITATION OF NO INFRARED THERMOSPHERIC AIRGLOW - A SIGNATURE OF SUPERTHERMAL NITROGEN-ATOMS

The reaction between superthermal N(S-4) atoms produced by exothermic processes and O-2 has been proposed to explain observations of highly rotationally excited nitric oxide in the sunlit thermosphere. We exami ne the importance of this mechanism using a detailed calculation of th e fast N(S-4) atoms energy distribution. It is shown that the hot ther mal N atoms are able to produce rotationally excited NO in the upper t hermosphere through the reaction of O-2 with N(S-4). By contrast, near the NO peak at 110 km, the Maxwellian nitrogen atoms produce substant ially less rovibrationally excited NO than the superthermal component. Consequently, the non Maxwellian N(S-4) atoms show a clear spectral s ignature in the (1-0) and (2-1) bandheads at this altitude. The calcul ated rovibrationally excited NO concentration at 140 km is shown to be consistent with the value derived from the analysis of infrared airgl ow spectra.