Daily Variability in the Terrestrial UV Airglow

Round 1

Reviewer 1 Report

Dear authors,

thank you for your interesting work. The topic you selected is highly actual and needs extensive research.

The manuscript presents important results and provides a detailed description of the potential physical explanation. The used language is professional and references are provided correctly.

I have only several minor comments to the content:

• I think that the heading of Subsection 1.1. '1.1. Observations of Earth’s Ultraviolet Airglow' is not needed as there is no 1.2. subsection and the whole text in section 1 is consistent. 
• please add a description to the caption of Figure 1, why some spectral lines are highlighted by green color and underscore
• please add highlighting of green part of the curve in Figure 2 (e.g. by changed line width or by an underscore). This will allow recognizing the highlighted parts also in the black & white version. 
• it might be helpful to add the legends (curves' labels) to the panels in the top row of Figure 4
• the word 'Text' in line 310 should be removed

Author Response

Dear reviewer,

  Thank you for your consideration of this research article. I apologize for missing the detailed issues that you found, and have addressed each of them as you directed.

• I think that the heading of Subsection 1.1. '1.1. Observations of Earth’s Ultraviolet Airglow' is not needed as there is no 1.2. subsection and the whole text in section 1 is consistent.  √
• please add a description to the caption of Figure 1, why some spectral lines are highlighted by green color and underscore √
• please add highlighting of green part of the curve in Figure 2 (e.g. by changed line width or by an underscore). This will allow recognizing the highlighted parts also in the black & white version. √
• it might be helpful to add the legends (curves' labels) to the panels in the top row of Figure 4 √
• the word 'Text' in line 310 should be removed √

Each of these items has been corrected. Thank you again for your inputs.

Reviewer 2 Report

General comments

This paper reports measurements of day-to-day variations of UV OI and N2 emissions with the GOLD instrument. The paper is generally well written. The reported observations are interesting and demonstrate the scientific potential of this data set. The analysis itself is a bit weak and no concrete conclusions are drawn. I still think the paper should be published and recommend  accepting the paper subject to moderate revisions (see specific comments below). I hope the true potential of these measurements will be exploited in future studies.  

Specific comments

Line 8: “.. variations in the GOLD response to UV emissions in the monitored regions is determined.”

The use of “reponse” in this context irritates me a little, because “reponse” to me implies “instrumental response function” and corresponds to instrument calibration rather than the scientific measurements. I suggest using a different term here and throughout the manuscript.

Line 9: “We find that oxygen emissions vary by up to 8% a day, relative to an approximately 4% change in N2 LBH”

I suggest mentioning explicitly that these variations correspond to changes on the dayside of the Earth (for similar LT and illumination conditions) from day to day and not to diurnal variations. This wasn’t entirely clear to me when reading the abstract.

Line 36: “Temperature change“ -> „Temperature changes“ or “Temperatures change” ?

Line 52: “The terrestrial albedo is often larger than unity”

Perhaps you can explain briefly what this means or why this is possible? I imagine not all readers will be familiar with the effect.

Line 90: “where the signal in 25 neighboring pixels is co-added”

Co-added in the spectral (or a spatial) domain? Is the detector a 2-D detector? This hasn’t been mentioned yet, I think.

Line 93: “and these can be interpreted”

It’s not entirely clear what “these” refers to. Probably counts?

Line 154: “shown in units of  mean response per pixel”

Use of “reponse” may be misleading (see comments above).

Line 163: “All the photon events produced in the images are represented here, and because these are processing artifacts, they do not move or change during the study period of 2019, nor do the adversely affect the analysis.”

Perhaps you can explain briefly, how these artifacts can/could be corrected or how this will be dealt with in future studies.

Line 172: “that the ionospheric recombination emissions are present”

Perhaps you can provide an example here.

Line 179: “the relative brightness of the thermospheric and ionospheric recombination components is approximately 2-3%”

Do you mean the relative contribution of the recombination component to the overall brightness? The statement is somewhat incomplete.

Line 202: “emission that drive“ -> „emissions that drive” or “emission that drives”? Probably the first makes more sense.

Line 213: “variations .. shows” -> “variations .. show”

Line 232: “The RMS of these ratios is 3%, where the RMS values of the LBH ratios is 1.8%.”

I don't understand this statement. Perhaps "of the 135.6 nm IO emissions" is missing in the first half of the sentence?

Same line: I suggest removing “similar” in “The similar RMS values”, because I’m not sure what it refers to (and I think it’s not required).

Line 234: “of of”

Line 234: “initial RMS values of 19% and 54%”

These “initial RMS values” are simply 1./SQRT(S), right? Is this a good measure of the measurement error? This seems like too simple a treatment.

Line 259: “compliment” -> “complement”

Line 273: “An observation of the height profiles“

I suggest mentioning here already that the profiles are limb integrated profiles, not inverted to Volume emission rate profiles.

Line 277: “The data are grouped by AM and PM“

What does this mean exactly? What are the local times?

Lines 288 – 300: the arguments and the discussion are quite general and no real concrete conclusion is drawn. You mention briefly the production/excitation mechanisms of the two emissions. I suggest discussing them in a little more detail. This is possible without additional calculations and analyses. Are the excitation mechanisms a likely candidate to explain the different variabilities of the two emissions?

Caption Fig. 3, line 5: “.. to form images, is identically represented in every image from GOLD,“

I think something is amiss here.

Figure 4: I suggest explaining line colors in figure caption.

Line 310: “Text” ?

Author Response

This paper reports measurements of day-to-day variations of UV OI and N2 emissions with the GOLD instrument. The paper is generally well written. The reported observations are interesting and demonstrate the scientific potential of this data set. The analysis itself is a bit weak and no concrete conclusions are drawn. I still think the paper should be published and recommend  accepting the paper subject to moderate revisions (see specific comments below). I hope the true potential of these measurements will be exploited in future studies.  

 Authors response: Thank you for your detailed review and suggestions for improvements. We implemented all off the suggestions from the referee, specified in their report, as noted below.

Specific comments

Line 8: “.. variations in the GOLD response to UV emissions in the monitored regions is determined.”

The use of “reponse” in this context irritates me a little, because “reponse” to me implies “instrumental response function” and corresponds to instrument calibration rather than the scientific measurements. I suggest using a different term here and throughout the manuscript.

Authors’ response: We *specifically do use* the instrument response in the Level 1 file, measured in events on the detector, in this analysis. All analysis is of the mean counts in regions of interest, or in the case of Figures 1-2, the counts in a 5x5 box as an example of mean counts vs. wavelength. It is important to handle counting statistics of photon events. Conversion of counts to radiance could be done, but is unessential. Also, we do not find the misspelling of “reponse” in this text, if the reviewer was indicating this as problem.

Line 9: “We find that oxygen emissions vary by up to 8% a day, relative to an approximately 4% change in N2 LBH”

I suggest mentioning explicitly that these variations correspond to changes on the dayside of the Earth (for similar LT and illumination conditions) from day to day and not to diurnal variations. This wasn’t entirely clear to me when reading the abstract.

Authors’ response: We understand the concern and the importance to discern periodic vs diurnal variations, and have restated the case that the emissions vary by a particular percent when measured under nearly identical observational geometry on a 24 hour cadence

Line 36: “Temperature change“ -> „Temperature changes“ or “Temperatures change” ?

 Authors’ response : We should be more specific. This sentence has been rewritten.

Line 52: “The terrestrial albedo is often larger than unity”

Perhaps you can explain briefly what this means or why this is possible? I imagine not all readers will be familiar with the effect.

 Authors’ response: It is possible that this discussion is distracting. We remove the sentence discussing albedo, and focus on the discussion of emissions and their properties.

Line 90: “where the signal in 25 neighboring pixels is co-added”

Co-added in the spectral (or a spatial) domain? Is the detector a 2-D detector? This hasn’t been mentioned yet, I think.

  Authors’ response: We apologize for the lack of detail that should clearly have been provided. Detail on the number of pixels in spatial and spectral dimensions is provided in the updated text.

Line 93: “and these can be interpreted”

It’s not entirely clear what “these” refers to. Probably counts?

 Authors’ response: Yes, you are right: detector counts, and this is now made explicitly clear.

Line 154: “shown in units of  mean response per pixel”

Use of “reponse” may be misleading (see comments above).

 Authors’ response: We truly are using raw detector counts. The GOLD spectrometer provides photon events that are used exclusively in this analysis.

Line 163: “All the photon events produced in the images are represented here, and because these are processing artifacts, they do not move or change during the study period of 2019, nor do the adversely affect the analysis.”

Perhaps you can explain briefly, how these artifacts can/could be corrected or how this will be dealt with in future studies.

Authors’ response: The reconstruction of the 2D image from events gathered from a constantly scanning slit introduces this pattern. It is simply a matter of organizing events, and at the level 1 step the artifact is introduced, and corrected in conversion radiances. However, radiances in these low-count pixels would have different statistics (higher uncertainties), that would significantly complicate the analysis – we instead stick to the instrument response and work to explain this better here.

Line 172: “that the ionospheric recombination emissions are present”

Perhaps you can provide an example here.

Authors’ response: Yes, we have cited a recent paper and describe the fact that this is only a concern in the 135.6-nm range of observations.

Line 179: “the relative brightness of the thermospheric and ionospheric recombination components is approximately 2-3%”

Do you mean the relative contribution of the recombination component to the overall brightness? The statement is somewhat incomplete.

 Authors’ response: We agree, this was not made clear. This section has been updated.

Line 202: “emission that drive“ -> „emissions that drive” or “emission that drives”? Probably the first makes more sense.

 Authors’ response: yes indeed, we adopt emissions that drive as you suggest.

Line 213: “variations .. shows” -> “variations .. show”

 Authors’ response: We apologize for the grammatical error.

Line 232: “The RMS of these ratios is 3%, where the RMS values of the LBH ratios is 1.8%.”

I don't understand this statement. Perhaps "of the 135.6 nm IO emissions" is missing in the first half of the sentence?

 Authors’ response: Yes, this was left out. The reference to 135.6 is now included.

Same line: I suggest removing “similar” in “The similar RMS values”, because I’m not sure what it refers to (and I think it’s not required).

 Authors’ response: We clarified that these are for the same 135.6-nm and LBH data in the northern hemisphere.

Line 234: “of of”

  Authors’ response : fixed this typographical error

Line 234: “initial RMS values of 19% and 54%”

These “initial RMS values” are simply 1./SQRT(S), right? Is this a good measure of the measurement error? This seems like too simple a treatment.

 Authors’ response: The referee is right, and we clarified the source of that calculations. GOLD has a photon-counting detector, so Poisson statistics specifically do apply. 

Line 259: “compliment” -> “complement”

  Authors’ response: Thank you for this correction

Line 273: “An observation of the height profiles“

I suggest mentioning here already that the profiles are limb integrated profiles, not inverted to Volume emission rate profiles.

  Authors’ response: Thank you, this is now noted.

Line 277: “The data are grouped by AM and PM“

What does this mean exactly? What are the local times?

 Authors’ response: We provide more detail in the report.

Lines 288 – 300: the arguments and the discussion are quite general and no real concrete conclusion is drawn. You mention briefly the production/excitation mechanisms of the two emissions. I suggest discussing them in a little more detail. This is possible without additional calculations and analyses. Are the excitation mechanisms a likely candidate to explain the different variabilities of the two emissions?

 Authors’ response: Thank you for this constructive critique. We provide more detail in the report.

Caption Fig. 3, line 5: “.. to form images, is identically represented in every image from GOLD,“

I think something is amiss here.

  Authors’ response: We have updated the captions

Figure 4: I suggest explaining line colors in figure caption.

    Authors’ response: This is now done in the caption and the figure.

Line 310: “Text” ?

  Authors’ response: An outlier. Now fixed.

Reviewer 3 Report

The manuscript submitted by Immel et al. reports on observations of the terrestrial UV airglow in the frame of the NASA GOLD mission. The manuscripts is quite well written and suitable for the Atmosphere journal. I have just few minor comments specified below. My recommendation is: ‘minor revision’.

Comments/questions to be considered:

1. If I understand well, the GOLD tracks temporal variations of the temperature and composition in the thermosphere. Please underline the progress with respect to the past approaches/results.
2. Section ‘2. Results’: Right after the Introduction, the authors make a jump to provide results without describing details of the experimental setup. Just few sentences in the section 2. Results related to their hyperspectral imager. I would suggest improving this point (provide more details on the spectrograph, detector and spectra acquisition/processing), maybe in the dedicated section.
3. Section ‘3.Discussion’: Section 3 starts with ‘The fact that the variance in the OI emissions changes over the period is the third remarkable fact.’ Consider discussing all remarkable facts in one (this) section.
4. Section 4. Conclusions: The authors write ‘The GOLD observations provide a unique new view’. In the last sentence they claim ‘These types of observations offer an avenue for new insights …’. However, what are these new insights except of ‘the emissions of thermospheric atomic oxygen vary from one day to the next to a much greater degree than the thermospheric N2 emissions’ provided through this work ? Any new physics/chemistry relevant to processes responsible for the OI and N2-LBH emissions? I would suggest developing better this point in order to make clear what is new here.

Author Response

Comments/questions to be considered:

1. If I understand well, the GOLD tracks temporal variations of the temperature and composition in the thermosphere. Please underline the progress with respect to the past approaches/results.

Authors’ reply: We have expanded the discussion of previous efforts and the compelling aspects of GOLD’s observations.

1. Section ‘2. Results’: Right after the Introduction, the authors make a jump to provide results without describing details of the experimental setup. Just few sentences in the section 2. Results related to their hyperspectral imager. I would suggest improving this point (provide more details on the spectrograph, detector and spectra acquisition/processing), maybe in the dedicated section.

Authors’ reply: We have added additional detail regarding the performance and operation of the GOLD instrument.

1. Section ‘3.Discussion’: Section 3 starts with ‘The fact that the variance in the OI emissions changes over the period is the third remarkable fact.’ Consider discussing all remarkable facts in one (this) section.

Authors’ reply: We have rearranged this section to hold the three observations out of the discussion section.

1. Section 4. Conclusions: The authors write ‘The GOLD observations provide a unique new view’. In the last sentence they claim ‘These types of observations offer an avenue for new insights …’. However, what are these new insights except of ‘the emissions of thermospheric atomic oxygen vary from one day to the next to a much greater degree than the thermospheric N2 emissions’ provided through this work ? Any new physics/chemistry relevant to processes responsible for the OI and N2-LBH emissions? I would suggest developing better this point in order to make clear what is new here.

Authors’ reply: Thank you for this suggestion. We have responded by clarifying the conclusion, and being more definite in the discussion about the mechanisms that may be at work to drive the observed changes in airglow.

Reviewer 4 Report

Review of "Daily variability in the terrestrial UV airglow" by Immel et al.

The paper presents new observations from the GOLD and ICON satellites that reveal that the variability in atomic oxygen (OI) is greater than the variability in molecular nitrogen (N2). The authors conclude that there are sources of variability in OI that are not currently accounted for. The work has important consequences for studies of the thermosphere-ionosphere. However, I have one major concern that I would like to see addressed before recommending the paper for publication.

Major Point
1) Variability of signals.
1a)The centerpiece of the analysis is the comparison of the relative variability of the OI and N2 signals. Based on Poisson statistics the RMS variability in the raw OI signal of mean 28 counts is 19% and in the raw N2 signal of 3.4 counts is 54%. This is cited by the authors. If 200 pixels are averaged to yield the daily signal, then the RMS variability decreases by a factor of 14 and the RMS variability in the daily signal is 1.4% and 3.8%. The experimental RMS values are quoted as 3.0-3.1% for OI and and 1.5-1.8% for N2. Thus the experimental RMS variability in N2 appears less than the minimum expected from Poisson statistics. Can the authors clarify the statistical analysis?
1b) The OI and N2 show considerable variability in the amplitude of the fluctuations. In period from about day 50 to day 120 the OI signal and the N2 signal have larger fluctuations than in the rest of the 200 day period. Do the authors have any explanation for this increase in variability. Is the relative variability during this period similar to the variability averaged over the whole 200 day period?
1c) A correlation analysis of the relative variability signals may provide further insight into the relative variation of the OI and N2 signals. Can the authors conduct a correlation analysis and examine the value and lag in the correlation.
2) Description of two systems
There is extensive presentation of the GOLD satellite and instruments. Given the use of ICON data, the authors might add a succinct description of the ICON instrument in section 1.1.

Minor Points
Line 23: typo "countries"
Line 27: "above 98 km" seems over precise, about 100 km.
Line 33: "for the fact" might better read "due to".
Line 95: "instrumental precision" is not the only issue, I agree that absolute accuracy is not required in this analysis. The authors might reword this.
Line 221: The three remarkable behaviors are presented in two different sections of the paper. They might better be presented in the discussion section.
Figure 1: Information would be easier to read if plotted on a logarithmic vertical scale.
Figure 2: The figure where N2 is appears to reverse the color coding in Figure 1 (N2 green, OI black). Is this correct. The authors may wish to consistently color code the Oi and N2 features of interest in both figures.

Author Response

The paper presents new observations from the GOLD and ICON satellites that reveal that the variability in atomic oxygen (OI) is greater than the variability in molecular nitrogen (N2). The authors conclude that there are sources of variability in OI that are not currently accounted for. The work has important consequences for studies of the thermosphere-ionosphere. However, I have one major concern that I would like to see addressed before recommending the paper for publication.

Major Point
1) Variability of signals.
1a)The centerpiece of the analysis is the comparison of the relative variability of the OI and N2 signals. Based on Poisson statistics the RMS variability in the raw OI signal of mean 28 counts is 19% and in the raw N2 signal of 3.4 counts is 54%. This is cited by the authors. If 200 pixels are averaged to yield the daily signal, then the RMS variability decreases by a factor of 14 and the RMS variability in the daily signal is 1.4% and 3.8%. The experimental RMS values are quoted as 3.0-3.1% for OI and and 1.5-1.8% for N2. Thus the experimental RMS variability in N2 appears less than the minimum expected from Poisson statistics. Can the authors clarify the statistical analysis?

Authors’ response:

  Thank you for this incisive comment. The referee is quite right in identifying this issue. The problem came from an error in collecting the counts from the detector in the 16 OI regions and the 120 LBH regions. An average of those counts was calculated, effectively reducing the standard deviation of the counts to a standard deviation of the mean of the counts, thereby reducing the errors by factors of 4 and 11, below any level that could normally be expected. Figures 3 and 4 now reflect a revised analysis of the counting rates of 135.6 and LBH. We find that the observed variability is now above the theoretical minimum for Poisson statistics in all cases.

1b) The OI and N2 show considerable variability in the amplitude of the fluctuations. In period from about day 50 to day 120 the OI signal and the N2 signal have larger fluctuations than in the rest of the 200 day period. Do the authors have any explanation for this increase in variability. Is the relative variability during this period similar to the variability averaged over the whole 200 day period?

Authors’ response: The revised analysis shows a different trend in the counting statistics, from which similar overall conclusions can be drawn. With the correction required to address the reviewer’s highest concern (above) no marked change in the statistics over the 200-day period now, so that discussion is removed.

1c) A correlation analysis of the relative variability signals may provide further insight into the relative variation of the OI and N2 signals. Can the authors conduct a correlation analysis and examine the value and lag in the correlation.

Authors’ reply: A correlation analysis is done in the OI and N2 counts, compared to each other in each analysis box, and between the analysis boxes at the same wavelength. The results are now discussed in the paper.

2) Description of two systems
There is extensive presentation of the GOLD satellite and instruments. Given the use of ICON data, the authors might add a succinct description of the ICON instrument in section 1.1.

Authors’ reply: Additional description of the ICON instrument is now provided.

Minor Points

-Authors' responses are interspersed below
Line 23: typo "countries"

-corrected

Line 27: "above 98 km" seems over precise, about 100 km.

-accepted this change

Line 33: "for the fact" might better read "due to".

-accepted this change

Line 95: "instrumental precision" is not the only issue, I agree that absolute accuracy is not required in this analysis. The authors might reword this.

-This is a good comment and observation. We require that the instrument response not vary on periods shorter than a week, at least not more than the mean RMS values in the regions of interest, which are about 1%. We have provided a statement that addresses this point. This is, in the end, instrumental precision, but it is more correct to derive precision requirement than state simply that it is needed.

Line 221: The three remarkable behaviors are presented in two different sections of the paper. They might better be presented in the discussion section.

-accepted this change

Figure 1: Information would be easier to read if plotted on a logarithmic vertical scale.

-We looked at this and the logarithmic plot mainly highlights lower brightness features in the spectrum that we exclude. We accentuated the different features in the linear plot that we would like the reader to see, and reduced the range to let the OI emissions extend beyond the box. Please see the updated plot.

Figure 2: The figure where N2 is appears to reverse the color coding in Figure 1 (N2 green, OI black). Is this correct. The authors may wish to consistently color code the Oi and N2 features of interest in both figures.

-Accepted. We have colored OI emission red.

Round 2

Reviewer 2 Report

The paper is now more or less acceptable in my opinion, but there are still a number of little issues that should be corrected:

Line 8/9: ‘variations .. is determined‘ -> ‘variations .. are determined‘

Line 10: ‘variations .. is‘ -> ‚‘variations .. are‘

Line 92: ‘in numerous studies in including by‘; Something is wrong/missing here.

Line 144: ‘1356‘ -> ‘135.6‘

Line 157: ‘GOLD counts every photon event ..‘; Please split this sentence into two parts. I read it several times and I'm not sure I fully understand it. It contains too many individual statements " .. to combine..  to provide .. that provides .. to provide .. "

Line 242: ‘where there samples from 4 scans‘; Something is missing/wrong here.

Line 255: ‘(as shown in 3‘ -> ‘(as shown in Fig. 3‘   and closing parenthesis missing.

Line 277: ‘around day 80‘; looking at the Figure it’s more like around day 100.

Line 360: ‘with altitude that OI‘; should it read ‘than OI‘?

Line 408:  ‘emission are‘ -> ‘emissions are‘ or ‘emission is‘

Caption Fig. 1, line 3: ‘at 135 nm is the 1S-1D oxygen doublet‘; are these really the correct electronic states?

Caption Fig. 2, line 4: ‘green‘ -> ‘red‘

Author Response

We thank the referee for an additional detailed reading of the paper. We have reviewed and accepted all of their suggestions. 

Line 8/9: ‘variations .. is determined‘ -> ‘variations .. are determined‘

 Authors’ response : corrected to “Variations … are determined

Line 10: ‘variations .. is‘ -> ‚‘variations .. are‘

Line 92: ‘in numerous studies in including by‘; Something is wrong/missing here.

 Authors’ response : This sentence is rewritten. Numerous studies have demonstrated a particular capability to retrieve the ratio of OI and N$_{2}$ from remote observations  \cite{strickland95a,strickland95b}, \cite{zhang04}, \cite{meier05}.

Line 144: ‘1356‘ -> ‘135.6‘

Authors’ response: corrected

Line 157: ‘GOLD counts every photon event ..‘; Please split this sentence into two parts. I read it several times and I'm not sure I fully understand it. It contains too many individual statements " .. to combine..  to provide .. that provides .. to provide .. "

Authors’ response : Agreed, improvement needed. This is rewritten.

Line 242: ‘where there samples from 4 scans‘; Something is missing/wrong here.

Authors’ response: Yes that was a problem. Fixed. 

Line 255: ‘(as shown in 3‘ -> ‘(as shown in Fig. 3‘   and closing parenthesis missing.

 Authors’ response : Corrected

Line 277: ‘around day 80‘; looking at the Figure it’s more like around day 100.

 Authors’ response: Referee is right. Corrected!

Line 360: ‘with altitude that OI‘; should it read ‘than OI‘?

Authors’ response:  Corrected

Line 408:  ‘emission are‘ -> ‘emissions are‘ or ‘emission is‘

 Authors’ response: Corrected

Caption Fig. 1, line 3: ‘at 135 nm is the 1S-1D oxygen doublet‘; are these really the correct electronic states?

 Authors’ response : No, this is completely wrong. This is corrected.

Caption Fig. 2, line 4: ‘green‘ -> ‘red‘

Authors’ response : Corrected

Reviewer 4 Report

Review of "Daily variability in the terrestrial UV airglow" by Immel et al.

I thank the authors for their response to my earlier comments and the changes they have Ade to the paper. The correlation analysis extends the analysis of the differences in the behavior of the OI and N2 airglow.

Major Point
1) Variability of signals.
While the authors have extended the discussion of the 200-day data set, I had trouble identifying where the variability of the OI is greater than that of the N2 airglow. Do the authors quantify and compare the variability in the two signals? In Figure 4 it appears in the second row that the OI emission is more variable than the LBH emission, but I did not see where this was clearly quantified in the paper.

Minor Points
Line 247: The authors note that the brightness of the emissions likely varies with the F10.7 index. Can the authors cite collations between the signals and the F10.7 and Ap indices?
Line 249: The authors say that they are plotting trends in the signals in Figure 4. This is not the case, they are plotting the signals.
Line 263: The sentence "Included .... " seems redundant.
Line 271: "There is no evidence of an effect in the SH OI 135.6 counts, or either of the LBH counts" might better read "There is no evidence of an effect in the SH OI 135.6 or the LBH counts".

Author Response

Authors’ response: Thank you for the further detailed and insightful commentary.

Major Point
1) Variability of signals.
While the authors have extended the discussion of the 200-day data set, I had trouble identifying where the variability of the OI is greater than that of the N2 airglow. Do the authors quantify and compare the variability in the two signals? In Figure 4 it appears in the second row that the OI emission is more variable than the LBH emission, but I did not see where this was clearly quantified in the paper.

Authors’ response: The variations of each emission relative to the 7-day mean are similar, with RMS/Mean ranging from 2.5-3.5%, now reported.

Minor Points
Line 247: The authors note that the brightness of the emissions likely varies with the F10.7 index. Can the authors cite collations between the signals and the F10.7 and Ap indices?

Authors’ response: It is certainly possible to do, but requires an additional detrending of the data over the period of interest. The instrument response to secular/seasonal trends in brightness over the period are not present in either index, and therefore dominate the correlation calculation without some correction. Given that the correct treatment of the seasonal variations requires information from a model or other source with its own nuances, we feel that reporting correlations with these indices does not benefit the analysis we have undertaken here.

Line 249: The authors say that they are plotting trends in the signals in Figure 4. This is not the case, they are plotting the signals.

Authors’ response : Point taken, the text is corrected where data are reported as “trends” or “variations”

Line 263: The sentence "Included .... " seems redundant.

Author’s response: Section is rewritten

Line 271: "There is no evidence of an effect in the SH OI 135.6 counts, or either of the LBH counts" might better read "There is no evidence of an effect in the SH OI 135.6 or the LBH counts".

Author’s response: This description is rewritten to include more detail on the behaviors observed.