TIME-RESOLVED N-2 TRIPLET-STATE VIBRATIONAL POPULATIONS AND EMISSIONSASSOCIATED WITH RED SPRITES

The results of a quasi-electrostatic electron heating model were combi ned with a time dependent N-2 vibrational level population model to si mulate the spectral distributions and absolute intensities observed in red sprites. The results include both N-2 excited state vibrational l evel populations and time profiles of excited electronic state emissio n. Due to the long atmospheric paths associated with red sprite observ ations, atmospheric attenuation has a strong impact on the observed sp ectrum. We present model results showing the effect of atmospheric att enuation as a function of wavelength for various conditions relevant t o sprite observations. in addition, our model results estimate the var iation in the relative intensities of a number of specific Nz emission s in sprites (1PG, 2PG, and VK) in response to changes in observationa l geometry. A recent sprite spectrum, measured from the Wyoming Infrar ed Observatory (WIRO) on Jelm Mountain, during July, 1996, has been an alyzed and includes N-2 IPG bands down to v' = 1. In addition to N-2 1 PG, our analysis of this spectrum indicates the presence of spectral f eatures which are attributable to N-2(+) Meinel emission. However, due to the low intensity in the observed spectrum and experimental uncert ainties, the presence of the N-2(+)(A(2)II(u)) should be considered pr eliminary. The importance of both the populations of the lower levels of the N-2((BIIg)-I-3) and the N-2((BIIg)-I-3)/N-2(+)(A(2)II(g)) popul ation ratio in the diagnosis of the electron energies present in red s prites is discussed. While the current spectral analysis yields a vibr ational distribution of the N-2((BIIy)-I-3) which requires an average electron energy of only 1-2 eV, model results do indicate that the pop ulations of the lower levels of the N-2((BIIg)-I-3) will increase with increases in the electron energy primarily due to cascade. Considerin g the importance of the populations of the lower vibrational levels, w e are beginning to analyze additional sprite spectra, measured at high er resolution, which contain further information on the population of B(v = 1). (C) 1998 Elsevier Science Ltd. All rights reserved.