The dependence of modeled OI 1356 and N2 Lyman Birge Hopfield auroral emissions on the neutral atmosphere

Abstract

Images of the entire auroral oval at carefully selected wavelengths contain information on the global energy influx due to energetic particles and some information on the characteristic energy of the precipitating particles. In this paper we investigate the sensitivity of selected auroral emissions to changes in the neutral atmosphere. In particular, we examine the behavior of OI 1356 Å and two Lyman Birge Hopfield (LBH) bands and their ratios to each other with changing atmospheric composition. The two LBH bands are selected so that one lies in the region of strong O2 absorption (1464 Å) and one lies at a wavelength where O2 absorption is effectively negligible (1838 Å). We find that for anticipated average uncertainties in the neutral atmosphere (factor of 2 at auroral altitudes), the resultant change in the modeled intensities is comparable to or less than the uncertainty in the neutral atmosphere. The smallest variations, for example, are for I 1838 (approximately 10 to 20%) while the largest variation is seen in the OI 1356 Å emission which is linear with [O] to within 20%. We have also investigated the dependence of these intensities, and their ratios, to much larger changes in the composition (i.e., [O]/[N2]) such as might be encountered in large magnetic storms, or over seasonal or solar cycle extremes. We find that the variation in the I 1356/I 1838 ratio over the equivalent of a solar cycle is less than 50%. The summer‐to‐winter changes are approximately a factor of 2. The I 1356/I 1838 ratio is a very sensitive indicator of the charactenstic energy, showing a change of 13 over the energy range 200 eV to 10 keV. The corresponding change in the LBH long‐to‐short wavelength ratio is much less (about a factor of 3). However, the latter is insensitive to changes in the neutral atmosphere (<20% changes in LBH emission ratio for large changes in N2). The three emissions therefore potentially provide a most valuable diagnostic of particle characteristic energy and energy flux.