The latitudinal gradient of the NO peak density

Abstract

The latitudinal gradients of the maximum nitric oxide densities near 110 km are presented for solstice and equinox periods from 1982 through 1985 as observed by the Solar Mesosphere Explorer satellite. The data indicate that the response of the maximum NO densities to the declining level of solar activity is latitudinally and seasonally dependent: the polar regions exhibit little sensitivity to solar activity, while the low latitude NO responds strongly. The data also reveal marked asymmetries in the latitudinal structure of the two hemispheres for each season. During June solstice periods, the latitudinal distribution is fairly flat, unlike December solstice periods which tend to show a definite minimum near 30° N. Similarly, March data show very little latitudinal variation in the NO peak density between about ±40°, while the September data show marked gradients for the later years. The SME data further indicate that the nitric oxide densities vary considerably from day to day, even during very quiet geomagnetic periods, suggesting that the concept of an “average” distribution is of limited usefulness in understanding nitric oxide. A two‐dimensional model is used to simulate the June solar cycle minimum data. The latitudinally averaged magnitudes of the observed NO peak densities are reproduced reasonably well by the model, but the shape of the latitudinal variation is not. Sensitivity studies which varied the N(²D)+ O quenching rate and the eddy diffusion coefficient are presented. The model simulations support a quenching coefficient of 0.5–1×1012 cm³ s−1.