the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 May 2025
Effect of Ionospheric Variability on the Electron Energy Spectrum estimated from Incoherent Scatter Radar Measurements
Abstract. The ion composition in the E-region is modified by auroral precipitation. This affects the inversion of electron density profiles from field-aligned incoherent scatter radar measurements to differential energy spectra of precipitating electrons. Here a fully dynamic ionospheric chemistry model (IonChem) is developed that integrates the coupled continuity equations for 6 ion and 9 neutral species, modeling the rapid ionospheric variability during active aurora. IonChem is used to produce accurate, time-dependent recombination rates for ELSPEC to improve the inversion of electron density profiles to primary electron energy spectra. The improvement of the dynamic recombination rates on the inversion is compared with static recombination rates from the International Reference Ionosphere (IRI) and the steady-state recombination rates from a ionospheric chemistry model, FlipChem. A systematic overestimation at high electron energies can be removed using a dynamic model. The comparison with FlipChem shows that short-timescale density variations are missed in a steady-state chemistry model
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RC1: 'Comment on egusphere-2025-2340', Anonymous Referee #1, 01 Jul 2025
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This manuscript presents the incorporation of time-dependent chemistry into the ELSPEC inversion technique for estimating precipitating particle information from incoherent scatter radar data. The ability to include time-dependent chemistry is a substantial improvement for ELSPEC with significant utility for future auroral research. The inverse methods are well described and clearly presented. The chemistry underlying the model has several weaknesses, and most of my critiques have to do with the details of the chemical model.
Major Comments:
1. The manuscript does not adequately explain how the concentrations of the minor neutral constituents (NO, N(4S), N(2D), H, O(1D), O(1S)) are initialized in the chemical model. Section 2.4 discusses the initial conditions for the ions in detail, but sensitivity to the initial concentration of the minor neutral constituents are not examined. I am particularly concerned about how NO is handled. The paper cites NRLMSIS2.0 (Emmert et al., 2021), which does not provide NO densities. If the more recent NRLMSIS2.1 was used instead, that version adds NO densities. See Emmert et al. 2022Emmert, J. T., Jones, M. Jr., Siskind, D. E., Drob, D. P., Picone, J. M., Stevens, M. H., et al. (2022). NRLMSIS 2.1: An empirical model of nitric oxide incorporated into MSIS. Journal of Geophysical Research: Space Physics, 127, e2022JA030896. https://doi.org/10.1029/2022JA030896
2. The manuscript does not demonstrate that the time-dependent chemistry is appropriate and reaching equilibrium for all the minor neutral constituents included, particularly NO and N(4S). NO and N(4S) are relatively long-lived species whose concentrations should be effected by vertical diffusion (both molecular and eddy diffusion) in addition to chemical production and loss. The evolution of NO in particular has been studied in depth, for example, by Bailey et al. (2002) and Barth (1992)
Bailey, S. M., C. A. Barth, and S. C. Solomon, A model of nitric oxide in the lower thermosphere, J. Geophys. Res., 107(A8), doi:10.1029/2001JA000258, 2002.
Barth, Charles A. (1992), Nitric oxide in the lower thermosphere, Planetary and Space Science, Volume 40, Issues 2–3, Pages 315-336, ISSN 0032-0633, https://doi.org/10.1016/0032-0633(92)90067-X.
Both of these models are 1-D vertical models describing production, loss, and vertical diffusion by molecular and eddy diffusion. NO takes days to reach equilibrium in these models. Barth (1992) describes running the model for 5 days for it to completely settle. Bailey et al. (2002) writes the following
"The lifetime of an NO molecule to chemical destruction (or e-folding time in the NO density) under illuminated conditions is 19 hours [Barth et al., 2001]. The lifetime of the NO molecule to diffusive transport is approximately one day [Barth, 1992]. Given that the solar illumination varies throughout the day, the abundance of NO at any one time is then representative of the level of solar energy deposition (solar irradiance and auroral energy) over the past day."
For the ground state of atomic nitrogen ( N(4S) ), the principal sink is the reaction
N(4S) + NO -> N2 + O
Therefore, I also expect N(4S) to be similar to NO and take days to equilibrate.
3. The model comparisons between IonChem and FlipChem are using inconsistent reaction rates, which confuses the contrast between time-dependent effects and effects of different rates. The manuscript presents the contrast between IonChem and FlipChem as being primarily due to the time-independent assumptions in FlipChem. The reaction rates in appendix A are provided without a citation to their source. Nonetheless, they appear to be copied from Appendix 5 of the Rees (1989) textbook. FlipChem, however, uses the reaction rates from Richards and Voglozin (2011), which include updates from more recent laboratory measurements.Richards, P. G., and Voglozin, D. (2011), Reexamination of ionospheric photochemistry, J. Geophys. Res., 116, A08307, doi:10.1029/2011JA016613.
If you compare appendix A to the table in Richards and Voglozin (2011), many of the rates do not match. The most meaningful way to compare the effects of time-dependent versus time-independent chemistry would be to use two models where the rates exactly match. This could be achieved by making a version of IonChem using the rates from Richards and Voglozin (2011).Citation: https://doi.org/10.5194/egusphere-2025-2340-RC1
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