Differences in near UV (∼3400–4300 Å) optical emissions from midday cusp and nighttime auroras

Abstract

Our ground‐based spectrophotometric observations of nighttime auroras over Fairbanks (Λ = 64.6°N), Alaska, and from dayside cusp auroras over Longyearbyen (Λ = 75°N), Svalbard, show that while molecular bands contribute most of optical radiation from nighttime auroras, atomic line emissions, particularly from metastable atmospheric species, dominate midday cusp auroral spectra. In the near UV (∼3400–4300 Å) nighttime auroral optical emissions (excited by electrons with average energy of a few keV) consist mostly of N2+ first negative group (1 NG), N2 2P and N2 Vegard‐Kaplan (VK) bands; the only prominent atomic feature is [N I] 3466A line and its intensity is about 1/300 of N2+ 1 NG (0, 0) band; the [O II] 3726‐3729 Å lines are very weak (<2R). In midday cusp regions, where magnetosheath electrons with average energy <100 eV precipitate, the N2 2P and the N2 VK bands are extremely weak (<2R), on the other hand [N I] 3466 Å and [O II] 3726–3727 Å lines are relatively intense. N2+ 1 NG bands are observed in midday cusp auroras, but unlike night auroras, where these bands are excited by electron impact ionization of N2, in cusp auroras most of the N2+ 1 NG band emissions come from resonant scattering of sunlight by N2+ ions; these ions are formed mostly through charge exchange reaction: N2(X¹Σu+) + O+[²D]→ N2+ (A²π) + O (³P). The vibrational rotational distributions of these bands vary with shadow height but on the average they are approximately similar to bands excited by electron impact ionization of N2 thermalized at ∼2500°K. In midday cusp auroras, the ratio of [N I] 3466‐Å line intensity to particle‐excited component of N2+ 1 NG (0, 0) band intensity is ∼1/10. Relatively more energetic (average energy ∼1 keV) electrons precipitate equatorward of the cusp, exciting both atomic and molecular emissions; the optical spectra of these auroras, which occur temporally coincident with but spatially separated from the cusp auroras, are similar to those observed on the nightside. Even in the cusp region, the average energy of precipitating particles can be higher during pre and post local magnetic noon periods; cusp auroral spectra observed during these periods contain molecular bands. Hence, major differences in auroral optical emissions are most noticeable when nightside spectroscopic observations are compared with similar measurements from the cusp region made around (±1 hour) local magnetic noon.