Sunrise equation

Abstract: Measurements of nitric oxide in the lower E region made by the ultraviolet nitric oxide experiment on Atmosphere Explorer C during 1974 are used to demonstrate diurnal and seasonal effects at low latitudes. At the equator, NO increases by about a factor of 2 between sunrise and the early afternoon: this is followed by a small decline toward sunset. Seasonally, NO shows an asymmetry about the equator with more NO on the summer side than on the winter side; at equinox the asymmetry vanishes. These effects are in qualitative accord with the current theoretical understanding of thermospheric nitric oxide.

Abstract: . It was noticed 60 years ago that at sunrise (i) the ionospheric critical frequency foF2 increases faster in winter than in summer and (ii) the increase begins at a greater solar zenith angle in winter. It was later suggested that this "seasonal sunrise anomaly" is due to a seasonal change of atomic/molecular ratio in the neutral air in the F2-layer. This paper uses the Sheffield University plasmasphere-ionosphere computational model (SUPIM) with the MSIS thermosphere model to examine the relative importance at sunrise of production, loss and diffusion processes, and the effect of neutral air winds. The results show that both (i) and (ii) can be explained in terms of neutral composition changes.

Abstract: The difference in sunrise and sunset low-latitude nitric oxide (NO) mixing ratios in the mesosphere and lower thermosphere (MLT) is shown to be consistent with a perturbation induced by the migrating diurnal tide. The vertical wind of the tide can induce factor of 2 changes over 12 hours at the equator. The vertical, latitudinal and temporal structure of NO perturbations closely matches the structure of vertical winds from a tidal model. In addition, previous observations of the seasonal and interannual variation in the tidal wind appear to correlate with NO variations.