N2(A) in the Terrestrial Thermosphere. Issue 1 (16th January 2020)

• Record Type: Journal Article
• Authors: Yonker, Justin D.; Bailey, Scott M.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 1(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-01-16
• Subjects: Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2019JA026508
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: Recent work has indicated the presence of a nitric oxide (NO) product channel in the reaction between the higher vibrational levels of the first electronically excited state of molecular nitrogen, N2 (A 3 Σ u + ), and atomic oxygen. A steady‐state model for the N2 (A) vibrational distribution in the terrestrial thermosphere is here described and validated by comparison with N2 A‐X, Vegard‐Kaplan dayglow spectra from the Ionospheric Spectroscopy and Atmospheric Chemistry spectrograph. A computationally cheaper method is needed for implementation of the N2 (A) chemistry into time‐dependent thermospheric models. It is shown that by scaling the photoelectron impact production of ionized N2 by a Gaussian centered near 100 km, the level‐specific N2 (A) production rates between 100 and 200 km can be reproduced to within an average of 5%. This scaling, and thus the N2 electron impact ionization/excitation ratio, is nearly independent of existing uncertainties in the 2–20 nm solar soft X‐ray irradiance. To investigate this independence, the N2 electron‐impact excitation cross sections in the GLOW photoelectron model are replaced with the results of Johnson et al. (2005, https://doi.org/10.1029/2005JA011295 ) and the multipart work of Malone et al. (2009 https://doi.org/10.1103/PhysRevA.79.032704 ) (Malone, Johnson, Young, et al., 2009, https://doi.org/10.1088/0953-4075/42/22/225202 ; Malone, Johnson, Kanik, et al., 2009, https://doi.org/10.1103/PhysRevA.79.032705 ; Malone et al., 2009, https://doi.org/10.1103/PhysRevA.79.032704 ), together denoted J 05 M 09. Upon updating these cross sections it is found that (1) the total N2 triplet excitation rate remains nearly constant; (2) the steady state N2 (A) vibrational distribution is shifted to higher levels; (3) the total N2 singlet excitation rate responsible for the Lyman‐Birge‐Hopfield emission is reduced by 33%. It is argued that adopting the J 05 M 09 cross sections supports (1) the larger X‐ray fluxes measured by the Student Nitric Oxide Explorer (SNOE) and (2) a temperature‐independent N2 (A)+O reaction rate coefficient. Plain Language Summary: Plain Language Summary Theoretical modeling of nitric oxide (NO) in the thermosphere has historically been underestimated in comparison with measurements. A new chemical source of NO has been proposed, but to accurately incorporate it into existing models requires a fast way of calculating the electronic and vibrational temperature of the reacting nitrogen gas. It is shown in this work that this can be done by using the N2 ionization rate as a proxy and that this has the added benefit of being independent of existing unknowns regarding the solar flux at X‐ray wavelengths. These results are further discussed in light of recent measurements by the atomic and molecular physics community concerning standard thermospheric diagnostic emissions, particularly the N2 Lyman‐Birge‐Hopfield emission. Use of these new cross sections in thermospheric models is found to have significant implications for our understanding of the energy budget of the upper atmosphere. Key Points: The N2 (A) vibrational distribution in the thermosphere can be determined rapidly by scaling the electron impact ionization rate of N2 The N2 ionization/excitation ratio is independent of existing uncertainties in the solar soft X‐ray irradiance Reevaluation of the N2 electron impact cross sections in use by the aeronomy community is needed