Effect of hot N(4S) atoms on the NO solar cycle variation in the lower thermosphere

The variation of the nitric oxide peak density near 110 km with solar activity is calculated using a photochemical diffusive model of thermospheric odd nitrogen. This model includes the reaction of translationally excited ('hot') nitrogen atoms with O2 as a source of nitric oxide, in addition to the classical photochemistry. It is confirmed that the dissociation of N2 by energetic photoelectrons due to the ionization of atmospheric constituents by solar soft X-rays is an important source of atomic nitrogen which controls the observed NO maximum near 110 km. The consideration of the hot N(4S) source increases the NO peak density by 45 to 60 percent dependent on the solar activity level considered. The calculated NO peak density increases by a factor of about 3.5 from low to high solar activity conditions, in agreement with the Solar Mesosphere Explorer satellite observations. The absolute concentrations calculated in the model with an N(2D) effective yield of 54 percent from N2 electron impact dissociation are midway between the two sets of solar cycle NO variation measurements currently available.