Observations of sunlit N<sub>2</sub><sup>+</sup> aurora at high altitudes during the RENU2 flight

Ellingsen, Pål Gunnar; Lorentzen, Dag; Kenward, David; Hecht, Jams H.; Evans, J. Scott; Sigernes, Fred; Lessard, Marc

doi:https://doi.org/10.5194/angeo-2020-50

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https://doi.org/10.5194/angeo-2020-50

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03 Aug 2020

Observations of sunlit N₂⁺ aurora at high altitudes during the RENU2 flight

Pål Gunnar Ellingsen¹, Dag Lorentzen^2,3, David Kenward⁴, Jams H. Hecht⁵, J. Scott Evans⁶, Fred Sigernes^2,3, and Marc Lessard⁴ Pål Gunnar Ellingsen et al.

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¹Department of Electrical Engineering, UiT The Arctic University of Norway, 8505 Narvik, Norway
²Department of Arctic Geophysics, the University Centre in Svalbard, P.O. Box 156, 9171 Longyearbyen, Norway
³Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen, 5020 Bergen, Norway
⁴The University of New Hampshire, Durham, NH 03824, USA
⁵Space Science Applications Laboratory, The Aerospace Corporation, El Segundo CA, USA
⁶Computational Physics Inc., Springfield, VA, USA

Received: 07 Jul 2020 – Discussion started: 03 Aug 2020

Abstract. We present measurements of sunlit aurora during the launch of the Rocket Experiment for Neutral Upwelling 2 (RENU2) on the 13th of December 2015 at 07:34 UT. The in situ auroral conditions coincide with those of sunlit aurora, and were characterised by the 391.4 nm and 427.8 nm N₂⁺ emissions. A correlation between several auroral wavelengths, as measured by a meridian scanning photometer was used to detect sunlit aurora and indirectly neutral upwelling. These results, based on ground data, agree well with the RENU2 measurements recorded during its pass through the sunlit polar cusp. Using data from RENU2 and the solar photon flux, it was found that sunlit aurora was a major part (≈ 40 %) of the observed 427.8 nm emission.

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Pål Gunnar Ellingsen et al.

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'angeo-2020-50', Anonymous Referee #1, 19 Aug 2020
- AC1: 'Reply to referee #1', Pål G. Ellingsen, 08 Sep 2020
AC1: 'Reply to referee #1', Pål G. Ellingsen, 08 Sep 2020
RC2: 'Review', Anonymous Referee #2, 13 Jan 2021
- AC2: 'Reply on RC2', Pål G. Ellingsen, 13 Jan 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by editor and referees) (11 Feb 2021) by Daniel Whiter

AR by Pål G. Ellingsen on behalf of the Authors (23 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (24 Feb 2021) by Daniel Whiter

RR by Anonymous Referee #1 (07 May 2021)

Suggestions for revision or reasons for rejection

The responses to the previous review have clarified several things, but changes to the paper have not been made in all cases, so that some questions remain.

From line 170: It is still not clear what 'not included' implies and it would be helpful if it was explained more clearly how this ‘omission’ is related to the ionisation rates estimated in this section, and the implications.

Regarding the statement “The exact amount (of resonant scattering of sunlight) is hard to determine…”, a quick search suggests that more recent work than Romick (2001) on this topic is Jokiaho et al. (Ann. Geophys., 27, 3465–3478, 2009), which is a statistical study from Svalbard. It shows how the proportion of scattered N2+ depends very strongly on both the energy of precipitation and shadow height. They state that this energy dependence means that case studies are more relevant than a statistical approach, given the many possible variations in auroral conditions. In the present work, the conditions over Svalbard during the rocket flight were quite variable. There is a discrepancy likely between the measurements (and model) from the rocket, and the brighter and more active auroral conditions within the MSP scans. Why not put the rocket track on the 427.8 nm MSP scans, and use this as a discussion point for the above comparison?

There is still a question (see previous paragraph 3) about the statement: “..it was found that sunlit aurora was a major part (~40%) of the observed 427.8 nm emission.” Section 4.1.3 provides an estimate of the ionisation rates from both photoionization and electron impact, and so it confirms the known fact that their ionization efficiencies are very similar and very hard to distinguish (also stated by Jokiaho, 2009). The real significance of the 40% figure for 427.8 nm emission that is novel and of importance, needs to be made clearer.

Discussion of Fig.6 (wrongly named 4.1.5 in text around line 200) is still hard to follow, not helped by some unintended repetition of a sentence within this paragraph (lines 201 and 204). The statement “The resonance scattering emission vastly exceeds the direct auroral emission” is followed by a sentence with “therefore…” but the logic of this sequence is unclear. The sentence that is repeated is vague - “could affect some of the observations.” I presume the sentence should be in its second position, leading on to the discussion of 630.0 nm. The paragraph needs to be rewritten to be a logical entity.

The comparison of the data and the model is still not easy using this figure. The answers to the previous review do not attempt to help this situation. Having to compare the presented results with those from the published Hecht paper is not ideal. I wonder if some helpful labels or shaded regions could be included to aid the eye. If I have followed correctly, it is claimed that there is a good comparison with the model between 550-600 from precipitation induced ions which have scattered emission, but between 500-640 the ‘plateau’ is not seen in the model, hence it must be from extra N2+ ions from upflows in the precipitation region. If I am correct in my interpretation, then these features would be easier to see if they were shaded or marked in some way, and then the text made more clear. I have had to read the two paragraphs many times while examining the figure to check I was seeing the features as described.

In summary, the results of this paper are still not presented well enough, and their significance could be made more relevant to existing understanding of the role of resonant scattering in the cusp region.

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ED: Publish subject to minor revisions (review by editor) (07 Jun 2021) by Daniel Whiter

AR by Pål G. Ellingsen on behalf of the Authors (25 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (04 Aug 2021) by Daniel Whiter

Thank you for submitting the updated version of your manuscript. I think there are still a few things (listed below) that need addressing before the paper can be published, so I am returning it for minor corrections with further review by myself as editor.

Line numbers here refer to the version of the manuscript with tracked changes shown.

Line 120: 428.7 should be 427.8.

There is still a date with an ordinal suffix, on line 139 ("13th"). Please convert this to a date (with month and year) both to avoid ambiguity and to follow the journal guidelines.

Line 176: I assume "comprehensible" should be "comprehensive".

Figure 6 caption: Please add "(dashed line)" and "(blue line)" into the caption somewhere to make things easier for the reader and to avoid ambiguity.

Figure 6: Please mark on the start and end times of the "plateau", e.g. with shading, vertical lines, or arrows, as requested by the reviewer. This plateau is unclear to the reviewer, so is likely to also be unclear to other readers.

Line 206, sentence starting "However": I suggest rewriting this sentence to make it easier to read. Perhaps this is an accurate rewrite: "However, during this period the AURIC/B3C modelling shows that the resonance scattering component of the 391.4 nm emission vastly exceeds the direct auroral emission component, and is enhanced due to the increased production of N2+ ions by the auroral precipitation."

Line 216, sentence starting "The data": I'm sorry but I don't understand what point you are trying to make with this sentence, and I don't think the reviewer did either. Are you saying the model predicts that emission from auroral precipitation should be observed from 450s onwards? If so, what point does the "therefore" follow from?

Line 269: "is closely followed by" - to me this reads as if there is a lag between the correlation coefficients, but I don't think that's what you mean. Probably "closely follows" is better.

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AR by Pål G. Ellingsen on behalf of the Authors (09 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (09 Aug 2021) by Daniel Whiter

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

- Printer-friendly version

- Supplement

RC1: 'angeo-2020-50', Anonymous Referee #1, 19 Aug 2020
- AC1: 'Reply to referee #1', Pål G. Ellingsen, 08 Sep 2020
AC1: 'Reply to referee #1', Pål G. Ellingsen, 08 Sep 2020
RC2: 'Review', Anonymous Referee #2, 13 Jan 2021
- AC2: 'Reply on RC2', Pål G. Ellingsen, 13 Jan 2021

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by editor and referees) (11 Feb 2021) by Daniel Whiter

AR by Pål G. Ellingsen on behalf of the Authors (23 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (24 Feb 2021) by Daniel Whiter

RR by Anonymous Referee #1 (07 May 2021)

Suggestions for revision or reasons for rejection

Hide

ED: Publish subject to minor revisions (review by editor) (07 Jun 2021) by Daniel Whiter

AR by Pål G. Ellingsen on behalf of the Authors (25 Jun 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (04 Aug 2021) by Daniel Whiter

Hide

AR by Pål G. Ellingsen on behalf of the Authors (09 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (09 Aug 2021) by Daniel Whiter

Journal article(s) based on this preprint

Pål Gunnar Ellingsen et al.

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https://doi.org/10.5194/angeo-2020-50-supplement

Pål Gunnar Ellingsen et al.

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Short summary

Using the RENU2 rocket and ground based instruments, we show that significant parts of the blue aurora above Svalbard at the time of launch was sunlit aurora. Sunlit aurora occurs when nitrogen molecules are ionised by extreme UV sunlight and subsequently hit by electrons from the Sun, resulting in blue/violet emissions. Understanding the source of an auroral emission gives insight into the interaction between the Sun and the Earths upper atmosphere.


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Observations of sunlit N2+ aurora at high altitudes during the RENU2 flight

Journal article(s) based on this preprint

Interactive discussion

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Interactive discussion

Peer-review completion

Journal article(s) based on this preprint

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Observations of sunlit N₂⁺ aurora at high altitudes during the RENU2 flight