Collision‐induced transitions between the a¹Π_g, a′ ¹Σ_u⁻, and w¹Δ_u states of N₂: Can they affect auroral N₂ Lyman‐Birge‐Hopfield band emissions?

Abstract

In the Earth's atmosphere bright ultraviolet (UV) emissions from the N2 a 1Πg state (Lyman‐Birge‐Hopfield bands) are seen, but the w 1Δu and a′ 1Σu− states, which have similar excitation cross sections, emit weakly if at all at UV wavelengths. Models of the singlet system (a, a′, and w states) have used radiative cascade, which produces transitions between excited states, and quenching, which removes molecules from them, to explain the lack of emission from the a′ and w states. Another process, which has never been properly included in such calculations, is collision‐induced electronic transitions (CIET) between the singlet states. Recent laboratory measurements of CIET involving N2 indicate that the rate constants are comparable to those for radiation, and recent measurements have yielded enough information to allow a preliminary calculation of CIET between the N2 a 1Πg, a′ 1Σu− and w 1Δu states. These calculations show that CIET can be more important than radiation for transitions between the a, a′, and w states in typical aurorae or up to the peak of typical dayglow emission profiles (∼130 km). The inclusion of CIET may allow self‐consistent modeling of observations of the Lyman‐Birge‐Hopfield (LBH) bands, one of the most prominent ultraviolet molecular emissions from the Earth's atmosphere.

Assessment of sprite initiating electric fields and quenching altitude of a1Πg state of N2 using sprite streamer modeling and ISUAL spectrophotometric measurements

“…The two singlets a ′ 1 Σ u − and w 1 Δ u have a much longer natural lifetimes than a 1 Π g and their quenching rates by atmospheric species N 2 , O 2 , and O are either on the same order or less than those of a 1 Π g state [ Eastes , 2000]. According to modeling studies on UV emissions of dayglow and aurora [ Eastes and Dentamaro , 1996; Eastes , 2000], the total emission from the N 2 LBH band system increases if the radiative and collisional energy transfer between those three singlet states is taken into account. The factor of the increase depends on altitude because the relative direct excitation of the three singlets in dayglow and aurora varies with altitude, and it reaches a maximum 1.67 at altitudes >200 km and drops to about 1.1 at an altitude of 100 km [ Eastes , 2000, Figure 4].…”

Electron and proton excitation of the FUV aurora: Simultaneous IMAGE and NOAA observations

“…In addition, errors can stem from the cross sections used in models [ Kanik et al , 2000]. For example, previous studies [ Eastes and Dentamaro , 1996; Eastes , 2000] indicate that Ajello and Shemansky 's [1985] LBH cross section used in the GLOW model for electrons may be underestimated owing to the presence of three close‐lying states (a 1 Π g , a′ 1 Σ u − and w 1 Δ u ) of N 2 . The a′ 1 Σ u − and w 1 Δ u states are long‐lived and optically forbidden to the ground state but are optically allowed to the a 1 Π g state.…”

Modeled and observed N2Lyman‐Birge‐Hopfield band emissions: A comparison

“…This model include excitation from v = 0–2 of the N 2 ground state (assumes the cross sections for those states are the same as for v = 0); radiative and collisional cascade processes between the a and a′ states and the a and w states; and quenching of the excitation to the a, a′ and w states. Previous modeling of the cascade contributions [ Eastes and Dentamaro , 1996] used the CSD model [ Jasperse , 1976], which makes approximations in order to simplify the calculation of the photoelectron spectrum. In addition to changing the photoelectron model, the cross sections by Young et al [2010], the most recent measurements available, were used when calculating the volume emission rates.…”