N I [3466 Å] and N I [5200 Å] emissions from various, nighttime and daytime auroras

Abstract

N I [2p³ 2P → 4S, 3466 Å] and N I [2P³, ²D → 4S, ∼ 5200 Å] multiplet radiations were monitored simultaneously from various nighttime and daytime auroras by using two large through‐put co‐aligned spectrophotometers. Measurements were made around winter solstice periods at Longyearbyen (Λ = 75°N), Svalbard, where large (>20°) solar depression angles, even during local midday, permitted continuous round the clock observations. Average energy of particles precipitating in auroras sighted over Longyearbyen ranged from a few keV at night to less than a hundred eV in the midday cusp section of the auroral oval; the corresponding atmospheric heights of peak auroral‐energy deposition varied from around 110 km, where molecular species are the major constituents of the atmosphere, to above 200 km, where atomic species are more dominant. Because of the difference in radiative lifetimes of N I [²P] and [²D] states, as well as difference in atmospheric extinction around 3466 Å and 5200 Å, the intensity ratio I [5200 Å]/I [3466 Å] varied in dynamic auroras associated with large changes in auroral energy flux, and with the extent of clouds or haze in the sky. Using only measurements from clear sky during steady auroras, we find that in all auroras the average value of the intensity ratio I [5200 Å]/I [3466 Å] = 1.8±0.4.