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onomy of the thermosphere not only require knowledge of the ions present, but also of the relative populations of the different vibrational and rotational states.3 Lidar techniques offer the ability to measure distinct molecular species precisely and to differentiate between the different vibrational and rotational popu-lations that are present. The potential of lasers to probe thermospheric molecular species has been rec-ognized since the 1960’s.4 Interest in radio iono-spheric modification experiments has prompted further analysis of the problem.5 Recently, a detailed analysis of auroral lidar systems, employing current laser technology and a 1.5-km-thick layer of aurorally excited molecules and ions at 100 km has been pre-sented.6 Here we extend the earlier analyses to consider set of observations for a night with typical auroral activity is presented and the performance of the lidar system is evaluated. 2. Resonance Lidar in the Upper Atmosphere For a lidar system employing a photon-counting de-tection system, we can express the expected photon count from the altitude range ~z 2 Dzy2, z 1 Dzy2! in a time interval Dt as the sum of three terms: N~z! 5 NS~z! 1 NB 1 ND, (1) where NS~z! represents the signal detected from the scatterers at altitude z, NB represents the photon counts that are due to background skylight condi-tions, and ND represents the detector noise. These terms can be expressed a