Comparison of Global Ultraviolet Imager limb and disk observations of column O/N2 during a geomagnetic storm

We compare column O/N-2 values obtained from the analysis of limb profiles of oxygen 135.6 nm and N-2 Lyman-Birge-Hopfield emissions with those obtained from nadir disk images of these same FUV dayglow features. This study represents the first direct comparison of the same metric using these two different techniques and thus is a means to compare the fidelity of each. Both data sets were measured by the Global Ultraviolet Imager (GUVI) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite during a series of geomagnetic storm events in late July 2004. We present daily maps obtained during the geomagnetically quiet time prior to the storm onset as well as similar maps obtained during the peak storm activity. These images show a strong reduction in column O/N-2 at high latitudes and a corresponding lower-latitude enhancement during the peak of the storm. Although the general morphology of these observations is expected and reflected in the NRLMSISE-00 neutral atmosphere model, the strength of the storm-induced changes is underrepresented. The similarity of the GUVI results obtained from both the limb-and disk-imaging techniques and their dissimilarities from current model outputs is strong confirmation of the quantitative results of each. The airglow data show a 40% increase in O/N-2 compared to NRLMSISE-00 near the geomagnetic equator near 1200 UT on 27 July. The peak impact of the storm at these latitudes also occurs at an earlier time than predicted by the model.