Review Reports - Atmospheric Loss of Energetic Electrons and Protons from the Radiation Belts After the Exceptional Injection of the 11 May 2024 Superstorm Leading to Four Electron Belts

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript provides observations of particle injection and loss during a geomagnetic storm that formed an unprecedented four radiation belts. This event allowed for new measurements of electron and proton lifetimes in the upper atmosphere which, basic science aside, could lead to something of practical use for risk management in low Earth orbit. First approximations are made here, but they are an important step nonetheless. I am overall supportive of quick publication but have some minor comments, particularly about the quality of some figures.

Figures 5 and 11 have some text legibility issues. The fitting values are sometimes superimposed on each other, particularly in the last panel of Figure 5. Please make them more distinct.

In Figure 7 the panel titles are too small. Please make them more legible.

The first sentence in the third paragraph on page 21 says “Our work shows that the electron and proton lifetimes depend on L”. We knew this already, e.g. from Benck et al., 2010, so the authors should probably rephrase it to avoid giving the impression that this is, on its own, a new discovery.

Comments on the Quality of English Language

The English is, for the most part, fine, but there are some small grammar/syntax details which I’m sure the language editor is more qualified than me to comment upon (e.g. missing articles in phrases like "Bottom panels of Fig. 3 illustrate..."); I will just point out one math localization oversight that slipped through, which might not be obvious enough: “EMIC waves (0,1-5 Hz)” at the end of page 11, where the comma should be a period.

Author Response

Summary: The manuscript provides observations of particle injection and loss during a geomagnetic storm that formed an unprecedented four radiation belts. This event allowed for new measurements of electron and proton lifetimes in the upper atmosphere which, basic science aside, could lead to something of practical use for risk management in low Earth orbit. First approximations are made here, but they are an important step nonetheless. I am overall supportive of quick publication but have some minor comments, particularly about the quality of some figures.

Response 1: We thank the reviewer for the useful suggestions to improve the readability of our manuscript. We have modified the manuscript accordingly.

Comment 2: Figures 5 and 11 have some text legibility issues. The fitting values are sometimes superimposed on each other, particularly in the last panel of Figure 5. Please make them more distinct.

Response 2: Thanks. The different panels of Fig. 5 and Fig. 11 have been improved so that the fitting values are well separated. The lifetime values are now indicated (with the standard deviation) alternatively at the bottom and top of each panel, with a colored vertical dashed line that relates the drawn slope to its value at bottom or top of the panel.

Comment 3: In Figure 7 the panel titles are too small. Please make them more legible.

Response 3: Figure 7 has been improved to enlarge the panel titles describing the proton energy channels.

Comment 4: The first sentence in the third paragraph on page 21 says “Our work shows that the electron and proton lifetimes depend on L”. We knew this already, e.g. from Benck et al., 2010, so the authors should probably rephrase it to avoid giving the impression that this is, on its own, a new discovery.

Response 4: We rephrased by “Our work shows the dependence on L of the electron and proton lifetimes obtained using EPT observations during the very active year of 2024”. (see L498 in Track change version).

Comment 5: Comments on the Quality of English Language

Response 5: Thank you. We have corrected the small syntax errors and checked the English text more carefully (see Track changes).

Reviewer 2 Report

Comments and Suggestions for Authors

Review of atmosphere-4181356

Atmospheric loss of energetic electrons and protons from the
radiation belts after the exceptional injection of the 11 May
2024 superstorm leading to four electron belts

by Viviane Pierrard 1,2,* and Alexandre Winant

The authors present a very clear and comprehensive analysis of observtions by the PROBA-V/EPT satellite/instrument of the Earth's radiation belts during and after a major geomagnetic storm in May 2024. I recommend publication.

Minor comments:

Page 2: "... METOP/MEPED ..." not explained and no reference given.

Other pages:

"Such increased proton and electron fluxes ... allow us to determine the lifetimes ..." --> "Such increased proton and electron fluxes ... enable us to determine the lifetimes ..." (no permissions are involved here).

"This allows us to ... " --> "This enables us to... " or "Using this we can ..."

Pse review further usage of "allow" in the manuscript.

"... 5 first channels ..." --> "... all channels except the one with the highest energy (2.4 - 8 MEV) ..." or "... the five lower energy channels ..."?

"... associated with Dst events ..." What is a Dst _event_? Is it an SSC (Storm Sudden Commencement)?

"... pitch angle scattering of electrons in the atmosphere ..." --> "... pitch angle scattering of electrons and their subsequent loss in the atmosphere ...", several occasions?

"... EMIC waves (0,1-5 Hz) ..." --> "... EMIC waves (0.1-5 Hz) ..."?

Author Response

Summary: The authors present a very clear and comprehensive analysis of observtions by the PROBA-V/EPT satellite/instrument of the Earth's radiation belts during and after a major geomagnetic storm in May 2024. I recommend publication.

Response 1: We thank the reviewer for the review and the positive recommendation.

Comment 2: Minor comments:

Page 2: "... METOP/MEPED ..." not explained and no reference given.

Response 2: Short explanations and references have been added (see L49): Medium Energy Proton and Electron Detector (MEPED) on board METOP satellite circulating on the same polar orbit at 820 km [4, 5].

Comment 3: Other pages: "Such increased proton and electron fluxes ... allow us to determine the lifetimes ..." --> "Such increased proton and electron fluxes ... enable us to determine the lifetimes ..." (no permissions are involved here).

"This allows us to ... " --> "This enables us to... " or "Using this we can ..."

Pse review further usage of "allow" in the manuscript.

Response 3: Thanks for this English improvement. We have corrected each time it appeared.

Comment 4: "... 5 first channels ..." --> "... all channels except the one with the highest energy (2.4 - 8 MEV) ..." or "... the five lower energy channels ..."? Corrected

"... associated with Dst events ..." What is a Dst _event_? Is it an SSC (Storm Sudden Commencement)?

Response 4: Geomagnetic storms are often called Dst (Disturbance Storm Time) events because this Dst index strongly decreases during such events. We replaced by geomagnetic storm to avoid any confusion (L240).

"... pitch angle scattering of electrons in the atmosphere ..." --> "... pitch angle scattering of electrons and their subsequent loss in the atmosphere ...", several occasions? Corrected

"... EMIC waves (0,1-5 Hz) ..." --> "... EMIC waves (0.1-5 Hz) ..."? Corrected

Reviewer 3 Report

Comments and Suggestions for Authors

In May (10-11) and October (10) the Earth was impacted by an exceptionally strong geomagnetic storm injected new energetic protons and electrons in the terrestrial radiation belts, which could be studied by the Energetic Particle Telescope EPT on ESA's PROBA-V satellite. The data was extraordinary revealing never previously observed electron belts which were observed for several weeks after the event. This paper presents these observations and compares them with more quiescent data. The authors then proceed to analyse the data and suggest that these observations may be ascribed to energy loss mechanisms caused by scattering in the atmosphere after the storm. The paper is mainly a report of data with a brief discussion and can be published in its present form. However it would perhaps be useful to define the mean electron temperature in these belts. A summary table of lifetimes for the different particles as a function of the position and energy ranges shown in Figure 5 would be useful in addition to the listing in the text. How are he errors in Figure 10 derived? If 'Linear regressions are not the most appropriate fits to determine the evolution times of flux variations in logarithmic scale' how may these be refined?

Author Response

Summary: In May (10-11) and October (10) the Earth was impacted by an exceptionally strong geomagnetic storm injected new energetic protons and electrons in the terrestrial radiation belts, which could be studied by the Energetic Particle Telescope EPT on ESA's PROBA-V satellite. The data was extraordinary revealing never previously observed electron belts which were observed for several weeks after the event. This paper presents these observations and compares them with more quiescent data. The authors then proceed to analyse the data and suggest that these observations may be ascribed to energy loss mechanisms caused by scattering in the atmosphere after the storm. The paper is mainly a report of data with a brief discussion and can be published in its present form.

Response 1: We thank the reviewer for the attentive reading, useful suggestions and positive recommendation.

Comment 2: However it would perhaps be useful to define the mean electron temperature in these belts.

Response 2: In Van Allen radiation belts, the density is very low, while the temperatures are very high (ranging from roughly 2,000K up to millions of degrees Kelvin (K)). These temperatures refer in fact to the kinetic energy of the charged particles (ions and electrons) trapped in the plasma. That is why in such low-density regions where thermometer can not give measurements, the physicists characterize the particles by their energy (in MeV) more than by their temperature.

Comment 3: A summary table of lifetimes for the different particles as a function of the position and energy ranges shown in Figure 5 would be useful in addition to the listing in the text.

Response 3: This is a good idea. We have added such a Table 1 after Figure 5 where we show the mean lifetimes compared to the lifetimes observed after the 11 May 2024 event.

Comment 4: How are the errors in Figure 10 derived?

Response 4: We added in the text more specifications about the errors illustrated Fig. 10:” The shaded red region corresponds to the lifetime standard error of the linear regression fits. The black dashed line illustrates the relative error (in percentage) obtained by dividing the standard error by the mean lifetime at each L value.“

Comment 5: If 'Linear regressions are not the most appropriate fits to determine the evolution times of flux variations in logarithmic scale' how may these be refined?

Response 5: An exponential decrease or a power law could be useful. Nevertheless, to be able to compare with previous results and because linear regressions work often very well, except for some very strong storms, the slope of the linear regression remains the most interesting parameter to determine. This is discussed L243 of Track change version.