Intersystem collisional transfer of excitation in low altitude aurora

Abstract

The characteristics of auroral optical emission as a function of altitude are exploited as a source of data for the investigation of the intersystem collisional transfer (ICT) of excitation in the nitrogen molecule. The procedure is based on a recently proposed model for the generation of the red lower border of type B auroras as resulting from the effect of the increased collision frequency at lower altitudes on the distribution of population within the excited molecules. In particular, the 85 km turn-on altitude for the red lower border of auroral arcs provides an indicator for the determination of the density and pressure required for the onset of the ICT process at mesopause temperatures. Values are obtained for collision cross sections and rate constants for the coupling of neighboring vibrational levels in adjacent electronic states in nitrogen. The results are compared with the findings of several laboratory studies on similar collisional transfers among nested electronic states. A two-part process is suggested to account for the observed features of intersystem collisional transfer phenomena.