Ionospheric constraints of mesospheric nitric oxide

Abstract

In view of the unreliabilities of γ-band estimates of mesospheric nitric oxide, ionospheric measurements of different types, including some controlled experiments specifically undertaken for this purpose, have been used to define an acceptable range of NO concentration. The ionospheric observations used are (i)the level of ionization reversal with solar activity (Mitra, 1966), (ii) the gradual loss of solar control in the diurnal variation of electron density at levels below 70 km (Mitra, 1969), (iii) magnitude and changes in the ratio of the two molecular ions NO⁺ and O₂⁺. The controlled experiment was the continuous monitoring of changes in mesospheric ionization profiles with the high power wave interaction facility at Pennsylvania State University during selected solar flare events. The flare measurements provide an NO profile similar to, but displaced downwards in height, from that given by Meira's γ-band measurements. The sharp ledge in electron density observed around 85 km requires a sharper valley in NO distribution at this height and the observed electron and positive ion densities at 70 km require that Meira's NO concentration should be increased by a factor between 5 and 7 at 70 km.