Effects of vibrational enhancement of N₂ on the cooling rate of ionospheric thermal electrons

This study shows that the cooling rate of ionospheric thermal electrons by molecular nitrogen may be reduced by more than a factor of 3 as a result of the enhanced vibrational excitation of N 2 from a number of chemical sources. Moreover, under conditions of enhanced F region electron densities (N e > 10 6 cm −3 ), N 2 may act as a small net source rather than sink of electron thermal energy. If the most commonly used electron cooling rates are correct, the influence of vibrational enhancement on the electron temperature is small because cooling through fine structure excitation of O is dominant below the altitude of the F 2 peak electron density. However, recent measurements indicate that the cross sections used to evaluate the N 2 cooling rate may have been a factor of 3 too small and other measurements indicate that the O fine structure cooling rate may be too large by a similar factor. Under these circumstances, N 2 vibrational cooling would be dominant, and the neglect of enhanced vibrational excitation of N 2 would lead to an underestimate of about 100°K in the thermal electron temperatures throughout the F region ionosphere at solar minimum. The effect on the electron temperature at solar maximum is small and confined to the region below the F 2 peak density because of the enhanced ion cooling. Two new sources of N 2 vibrational excitation are evaluated. One of these, the photoelectron excitation and subsequent deactivation of the A, B, C, and W triplet states of N 2 is found to be a major source of N 2 vibrational quanta. This source is several times larger than the direct (2.5 eV) photoelectron source above 250 km. The second new source, charge exchange between N 2 + and N 2 , is comparable to the other sources at high altitudes but is small below 200 km.