During November--December 1983 the first spectral images of the N2+ first negative bands in the dayglow were measured by the Imaging Spectrometric Observatory on the Spacelab 1 shuttle mission. These data contained two unexpected characteristics: the intensities were considerably higher than current photochemistry predicts (by factors of 3--5), and the bands showed larger than expected populations in the higher vibrational levels. Both of these characteristics persisted throughout the mission and were independent of vehicle attitude. The spectra were imaged for the second time in the dayglow by the same instrument from the ATLAS-1 shuttle mission in March 1992, providing an opportunity to re-examine these issues. The vibrational distributions measured from ATLAS-1 are found to be rather similar to those measured from Spacelab 1, but with somewhat lower populations in levels, &ngr;'=2 and 3. The Spacelab 1 and the ATLAS-1 missions were the first opportunities to image the Δ&ngr;=+1 progression at 3584 ¿, allowing the first determinations of the populations in levels higher than 2, which earlier studies were not able to address. The integrated column intensities of the 0-0 band emission at 3914 ¿ as measured from ATLAS-1 are found to be in good agreement with model values, while the Spacelab 1 intensities appear to have been significantly affected by vehicle-induced effects.

It would appear that the Spacelab 1 mission was highly anomalous in this regard, and the emission spectra from that mission are contaminated by non-ambient features, many of which are from the same species as are found in the thermosphere. The ion environment, in particular, appears to have been very perturbed on that mission. By contrast, the ATLAS-1 mission spectra appear to be very clean and do not show obvious evidence of vehicle effects. It appears that the absence of the large pressurized double-Spacelab module that occupied much of the payload bay for Spacelab 1, and the revised payload and orbiter processing procedures, have resulted in an environment that is substantially improved for remote sensing purposes. Studies of natural emissions, such as this one of the N2+ bands, are feasible and provide good data for modeling of the region. ¿ American Geophysical Union 1993