The northern auroral region as observed in nitric oxide

The distribution of thermospheric nitric oxide in the northern polar region has been measured from the Student; Nitric Oxide Explorer (SNOE). A polar image of a one-year average at 106 km shows the maximum density lies betwee n 60 degrees and 70 degrees N geomagnetic latitude. The density around this auroral oval varies as a function of longitude with the larger densities o ccurring at western geomagnetic longitudes. A polar image of a second year of observations demonstrates the same latitudinal-longitudinal density dist ribution indicating that the asymmetry is a persistent property of the auro ral region. For a period of high geomagnetic activity, the nitric oxide obs ervations indicate that the flux of precipitating electrons at 90 degrees W geomagnetic longitude was 75% larger than the flux at 90 degrees E geomagn etic longitude. We suggest that this asymmetry is caused by the longitudina l variation in the magnetic field strength around the auroral oval.