The effect of solar radiation on molecular nitrogen emissions originating in the sunlit thermosphere of Earth.

Abstract

The vibrational distribution of N₂ triplet states in the sunlit upper thermosphere of Earth is measured and modeled for the first time. A comparison is made between measured and theoretical limb column emission rates for bands originating from each upper vibrational level of C³Πᵤ(v) and A³Σ⁺ᵤ(v). The measured column emission rates for the Second Positive (2PG) bands are 3.2 (± 0.2), 3.2 (± 0.2) and 0.6 (+0.0, -0.4) kRayleighs for bands originating from C³Πᵤ(0 ≤ v ≤ 2) and 13.3 (± 0.2), 10.0 (± 0.2), 3 (+0, -2) and 2 (+0, -2) kRayleighs for Vegard-Kaplan (VK) bands originating from A³Σ⁺ᵤ (0 ≤ v ≤ 2). Predicted limb column emission rates for C³Πᵤ(v) are in excellent agreement with the measured 2PG intensities, but comparisons of predicted A³Σ⁺ᵤ(v) column emissions to measured VK intensities are poor. Despite this discrepancy, the predicted sum of all A³Σ⁺ᵤ(v) emission rates over all v compared well to the sum of measured VK intensities. This implies that the excitation rate into the N₂ triplet states is well understood, but that the cascade mechanisms are not as yet understood sufficiently to use dayglow N₂ band emissions as remote sensing probes of the sunlit thermosphere. The dayglow N₂ emissions are modeled by extending the existing auroral model to include resonance scattering of sunlight and replacing the precipitating auroral electrons with photoelectrons. The effects of solar resonance scattering on the X¹Σ⁺(g), A³Σ⁺ᵤ and B³Π(g) states are presented as a function of A³Σ⁺ᵤ quenching rate. These theoretical predictions have important implications for the analysis of dayglow and auroral emissions. The effect of resonance scattering on the A³Σ⁺ᵤ state is small, and will not be measurable under auroral conditions. This implies that the measured auroral vibrational population of the A³Σ⁺ᵤ state is valid for sunlit aurora. The population B³Π(g)(v = O) relative to other B³Π(g)(v) states is predicted to be enhanced by sunlight. A novel set of computer variables based on tree structures was created to manage the information used. These variables are described in detail and were found to be useful tools for the creation and extension of computer models treating diatomic species.