Time resolved N2 triplet state vibrational populations and emissions associated with red sprites

The results of a quasi-electrostatic electron heating model were combined with a time dependent N2 vibrational level population model to simulate the spectral distributions and absolute intensities observed in red sprites. The results include both N2 excited state vibrational level populations and time profiles of excited electronic state emission. Due to the long atmospheric paths associated with red sprite observations, atmospheric attenuation has a strong impact on the observed spectrum. We present model results showing the effect of atmospheric attenuation as a function of wavelength for various conditions relevant to sprite observations. In addition, our model results estimate the variation in the relative intensities of a number of specific N2 emissions in sprites (1PG, 2PG, and VK) in response to changes in observational geometry. A recent sprite spectrum, measured from the Wyoming Infrared Observatory (WIRO) on Jelm Mountain, during July, 1996, has been analyzed and includes N2 1PG bands down to v′ = 1. In addition to N2 1PG, our analysis of this spectrum indicates the presence of spectral features which are attributable to N+2 Meinel emission. However, due to the low intensity in the observed spectrum and experimental uncertainties, the presence of the N+2(A2Πu) should be considered preliminary. The importance of both the populations of the lower levels of the N2(B3Πg) and the N2(B3Πg)/N+2(A2Πg) population ratio in the diagnosis of the electron energies present in red sprites is discussed. While the current spectral analysis yields a vibrational distribution of the N2(B3Πg) which requires an average electron energy of only 1–2 eV, model results do indicate that the populations of the lower levels of the N2(B3Πg) will increase with increases in the electron energy primarily due to cascade. Considering the importance of the populations of the lower vibrational levels, we are beginning to analyze additional sprite spectra, measured at higher resolution, which contain further information on the population of B(v = 1).