the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Mar 2025
Effects of geomagnetic mirror force and pitch angles of precipitating electrons on ionization of the polar upper atmosphere
Abstract. We studied the effects of the geomagnetic mirror force on electron density enhancements in the polar atmosphere due to energetic electron precipitation. Using pitch angle and energy distribution of electrons observed by the low-altitude Electron Losses and Fields INvestigation (ELFIN) satellites as initial conditions, the electron density in the atmosphere caused by precipitating electrons was calculated by a simulation with two different methods: a traditional method that does not include the effect of the mirror force and a recently developed method that includes the effect. From a simultaneous observation event of the ELFIN satellite and the European Incoherent SCATter scientific radar system (EISCAT) Tromsø radar, it was found that the method with the effect of the mirror force reduces electron density by about 40 % at an altitude of 80 km compared to the traditional method. This decrease was pronounced when the pitch angle distribution of high-energy electrons was concentrated in the trapped and boundary regions. The maximum decrease was 50 %. It was verified that electron density distribution estimated using the method with the effect of mirror force showed good agreement with an electron density profile derived from the EISCAT radar. The validation of simulation results based on these observation data contributes to the establishment and improvement of atmospheric ionization models using various types of precipitating electrons.
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Status: open (until 17 May 2025)
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RC1: 'Comment on egusphere-2025-768', Anonymous Referee #1, 23 Apr 2025
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Overview:
The paper titled: “Effects of geomagnetic mirror force and pitch angles of precipitating electrons on ionization of the polar upper atmosphere” utilizes ELFIN observations along with simulations to estimate and compare electron density profiles with the EISCAT radar. The authors find 5 conjunctions between ELFIN and EISCAT and perform analysis to determine the impacts of including mirror force into numerical simulations has on electron density profiles. I think this paper shows some very interesting results concerning the overestimation of electron density profiles in the D-region ionosphere when not including the mirror force from trapped electrons. However, some incomplete reasoning and a clearer presentation is needed prior to publication. Below I’ve listed some general comments along with more specific comments that should be addressed.
General Comments:
- Why use the auroral electrojet index in place of Kp or a more robust index for this energy range of electrons? The AE index is primarily an index for aurora as it measures ionospheric currents in the auroral oval. Additionally, as you mention, they are sourced from the radiation belts where the precipitation mechanisms are often different from more general auroral electrons. It would be more useful to choose time periods with varying Kp or maybe even DST instead of AE for this study.
- All five of the events that are discussed take place in mostly undisturbed times. It is mentioned that the density ratio does not appear to trend with AE, but I disagree with this because none of the events that are shown are during geomagnetically disturbed conditions. Even the event with 350 AE shows virtually no Kp enhancement indicating very little geomagnetic activity. I do not think you can claim the density ratio does not depend on activity (in the text you claim AE) when the events chosen are not very different in terms of enhancements. I would reword the conclusion to state that these times represent mostly undisturbed time periods.
- I would suggest showing an additional panel with four other plots of the density profiles for the other four events instead of just for one event. I think the most compelling is the plot that shows the simulation that includes the mirror force agrees better with EISCAT than the one without.
- On that same note, are there any other events using EISCAT and ELFIN that are during more geomagnetically active time periods? If not have you considered adding additional radars such as HARRP or the Canadian ISRs? I think having a range of density profiles based on geomagnetic activity would be compelling. It could also be a very good follow-on paper as well.
Specific Comments:
Line 27: The first sentence doesn’t make grammatical sense. Consider revising.
Line 32: There are other references that should be added here that discuss chemical changes from EEP. Some to consider are: …
Line 35: “magnetic field” not “magnet field”
Line 47: “electrons’” should be “electron”
Line 50: Remove “between”
Line 70: It may be worth adding the equation here on how you defined the loss cone mathematically.
Line 79: Grammar. “data of an electron” doesn’t make sense. Please revise.
Line 119-121: I’m confused about Figure 2b. The sentence claims that the < 500 keV electrons have larger pitch angles north of the radar than south. However, Figure 2b shows the opposite. I see more electrons north of the radar (according to Figure 1 before crossing the EISCAT instrument) with pitch angles below the loss cone angle (preassembly the black line in the plot). This tells me that more electrons are precipitating north of the radar than south according to the figure. Is the figure incorrect or is the explanation incorrect?
Line 123: change ‘major’ to ‘the majority’
Line 210: I think I might understand this sentence, but it should be changed to be clearer.
“For instance, precipitating electrons from the radiation belt due to the wave-particle interaction are thought to have a relatively small pre/tra ratio because they originally had a large pitch angle to maintain the region”
Are you saying that the large pitch angles for radiation belt electrons required for the radiation belts to exist? Because otherwise, if all the electrons were at low pitch angles, would they precipitate? I find the wording of this confusing.
Citation: https://doi.org/10.5194/egusphere-2025-768-RC1
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