Effect of solar flux versus compositional variations on the variability of daytime oxygen optical emission rates over low- and mid-latitudes

Abstract

Daytime oxygen airglow emission rate variability provides us with a means of remote investigations of the upper atmospheric behavior. Dayglow emission rates typically show a diurnal pattern with a peak at around noon, especially during geomagnetic quiet conditions. The photochemical and empirical models show that the emission rates typically vary as a function of solar zenith angle (SZA) and solar flux. Thus, both, larger solar flux magnitudes and smaller solar zenith angles, contribute to larger dayglow emission rates as the yield of excited oxygen atoms is expected to be greater in those conditions. However, variability in the annual dayglow emission rates obtained from measurements at low-latitudes (Hyderabad, India; 170 N, 800 E; 8.70 N Mag. Lat.) and mid-latitudes (Boston, USA; 42.20 N, 710 W; 48.30 N Mag. Lat.) show different behavior with regard to solar flux variation. The variability in the low-latitude emission rate is primarily dependent on that of the solar flux. However, such primary dependence on the solar flux is not reflected on the emission rate variability seen over mid-latitudes. This discrepancy is understood to be due to the relative effects of solar flux and the globally changing compositional variability, characterized by the oxygen to molecular density ratios (O/N2), that show varying behavior from mid- to low-latitudes. Seasonal compositional variation due to transport processes is attributed to be the cause for the observed discrepancy which is also confirmed by independent satellite measurements. We also present an empirical model of O/N2 as a function of day of the year and latitude obtained using satellite based ultraviolet measurements to quantify the compositional variation.