SAO, AO, QBO, and Long-term trend of the peak OH airglow emission

Gu, Sheng-Yang; Wang, Dong; Tang, Liang; Wei, Yafei

doi:https://doi.org/10.5194/egusphere-2023-910

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https://doi.org/10.5194/egusphere-2023-910

Preprints

11 May 2023

| 11 May 2023

SAO, AO, QBO, and Long-term trend of the peak OH airglow emission

Sheng-Yang Gu, Dong Wang, Liang Tang, and Yafei Wei

Abstract. Based on the volume emission rate of the OH airglow observed by TIMED/SABER, we fitted the peak emission rate and the peak height of the OH airglow and analyzed the seasonal and interannual variations of both. The results show similar latitudinal variations in the semiannual oscillation (SAO) and annual oscillation (AO) of peak emission rate and peak height: the amplitude of SAO is greatest in equatorial regions and AO is greatest in mid-latitudes. For interannual variations, we find that OH airglow emission in equatorial regions is modulated by the quasi-biennial oscillation (QBO), while the QBO signal at other latitudes is much weaker than in equatorial regions and can be ignored. The QBO in OH airglow is consistent with the phase variation of the QBO in the tropical lower stratosphere (30 km), which is also consistent with the phase variation of the QBO in the migrating diurnal tide. As an important kinetic process affecting OH airglow emission, we suggest that the tides play an important role in the modulation of the OH airglow by the QBO. In addition, we have analyzed the relationship between peak OH airglow emission and solar activity. The results show a good correlation between peak emission rate and solar activity, with a correlation coefficient of 0.89, while peak height shows no significant solar cycle variation, with a correlation coefficient of −0.66. The modulation of peak emission rate by solar activity has significant latitudinal variation. The modulation effect is weakest in the equatorial region and greatest at mid-latitudes in both hemispheres.

Received: 05 May 2023 – Discussion started: 11 May 2023

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Sheng-Yang Gu, Dong Wang, Liang Tang, and Yafei Wei

Status: closed

RC1:
'Comment on egusphere-2023-910', Anonymous Referee #1, 02 Jun 2023

Summary
This paper presents an analysis of the peak altitude and peak emission rate of the hydroxyl (OH) emission rates observed for the past 20 years by the SABER instrument on the NASA TIMED satellite. SABER measures OH emission in two distinct spectral intervals, one centered at 2.0 um and the other at 1.6 um. The paper analyzes and presents results for the 1.6 um channel. The 20 years of SABER data enable comprehensive analysis of various temporal features that appear in the peak height and peak altitude of the OH emission. The analysis presented in the paper shows clear evidence for semi-annual and annual oscillations in the peak features and evidence for influence of the stratospheric quasi-biennial oscillation on the peak features. The results presented often confirm the results of previous papers regarding the presence of temporal features in the OH emission. The paper is clearly written.
The results in the paper are presented as ‘engineering’ analysis of the temporal variations in the OH emission. There is very little, if any, quantitative physics or chemistry given to explain the observed behavior. As an example, the discussion of solar variability is primarily of correlations between the OH variability and the F10.7 solar radio flux index. Solar variability affects temperature, composition, and dynamics. But the paper does not attempt to quantify which of these effects is dominant. The paper does not explain why there is strong latitude dependence in many of the analyses. However, the SABER dataset includes temperature, atomic oxygen, atomic hydrogen. These datasets could be explored along with model simulations (the WACCM model would be ideal for this) to put the results in context. In addition, although a secondary concern, the paper does not state why the peak altitude and peak emission rates are important physically. They are clearly markers for atmospheric variability. But is the variability important and why? One could imagine that since the reaction of H + O3 is the largest source of heating in the mesopause region, the variation in intensity and location of the OH emission means that the energetics of the mesopause are being altered significantly. Discussions such as this are necessary to place the results in a physical context as the reported variability largely reproduces prior works.
Recommendation
The recommendation is to reject the paper and invite submission of a new paper that contains much more detailed physical explanation of the observed OH behavior. This should include comparisons with the WACCM model and evaluations with other SABER data products. The paper should provide a quantitative explanation of features such as the latitudinal dependence of the semiannual and annual oscillations and should go into detail about the relative roles of temperature, chemistry, and dynamics in producing the observed variability.
A second recommendation is to evaluate the variability of the OH emissions on pressure surfaces and not on altitude. Studying variations at fixed altitudes mix variations in emission as the pressure surfaces rise and fall around the altitudes as the atmosphere warms and cools over the year and over the solar cycle. Pressure is the natural vertical coordinate of the SABER data.
Specific Comments
Title – the title contains the word ‘trend’ in relation to the peak OH emission. The word ‘trend’ typically implies looking at the long-term change of a parameter due to some forcing that is fundamentally changing the atmosphere such as increasing carbon dioxide concentrations. The original time series is analyzed in a way to remove variability (such as the AO, SAO, QBO, and solar cycle) and the linear trend of the residual is computed to derive a change (typically parameter per decade units). The paper does not appear to contain a trend analysis of this type. Please correct the title and the few places in the text where the word ‘trend’ occurs
Data use – An error was discovered in the SABER data for dates after December 16, 2019. A new version, v2.08 is available for data after that date. Please visit the SABER data web site to review and please discard all v2.0 data after Dec 16 2019.
Line 23-24. This sentence is an example of the lack of quantitative understanding of the airglow that comes across in the paper. The airglow intensity is not directly related to density and only indirectly related to temperature through the temperature dependence of the rate coefficient for the reaction of H and O3. The entire Introduction is full of generalities that makes one question whether the paper truly understands the physics of airglow generation including how and why it varies. A revised Introduction should directly address the physics/chemistry of OH formation and how it may vary, thus setting up the results and analysis with a model such as WACCM later in the paper.
Line 116-117 – please provide a reference citation to the vibrational states that contribute to each of the SABER OH channels.
Line 125 – the instantaneous field of view of the SABER instrument is 2 km. SABER samples the atmosphere at a much higher cadence than every 2 km and so it may appear that the vertical resolution is much higher. The paper needs to discuss the effects of the finite field of view on the ability to determine and analyze variations of the peak height and emission.
Line 136 – please provide a reference citation to the OMNI database and spell out the acronym.
Line 161, Section 3.1. The authors are encouraged to examine the data with Fourier techniques to see if other periodic features are evident.
Lines 181 to 200 – Any new manuscript should include explanations of the origins of the AO, SAO, and QBO, and how these influence the OH emissions. In particular, the AO should be primarily driven by the annual variation of earth-sun distance. So there is an annual cycle of solar radiation along with varying solar radiation on an 11 year cycle. Does any of the 11 year cycle ‘alias’ into the annual cycle? Could the Fourier techniques mentioned above help sort out different cyclic variations?
Line 240-250 – please explain physically how the QBO in the stratosphere modulates the OH emission in the mesosphere.
Line 259-270. SABER has temperature, atomic oxygen, atomic hydrogen, and ozone data. These could be analyzed in concert with the OH data and WACCM model results to produce a complete picture of the relative importance of temperature, chemistry, and dynamics in producing the observed variations in the OH emission.
Line 300- Instead of using the F10.7 proxy, it is suggested to use the actual solar irradiance measured by the SORCE and SEE instruments over the past 20-plus years. Focus on the wavelength regions that drive most of the heating in the mesopause region. This may provide a much better result than using F10.7.

Citation: https://doi.org/10.5194/egusphere-2023-910-RC1
- AC1: 'Reply on RC1', Dong Wang, 03 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-910/egusphere-2023-910-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-910-AC1
RC2:
'Comment on egusphere-2023-910', Anonymous Referee #2, 07 Jun 2023

General comments:
This manuscript deals with the variability in OH emission altitude and OH peak emission rate over a period of more than 20 years based on limb emission measurements with the SABER instrument on TIMED. Different effects and impacts are investigated, e.g. seasonal variations, the effects of the QBO and solar cycle variations. Such a study is in principle of relevance for the mesosphere/mesopause community, because many groups employ ground-based measurements of OH emissions to retrieve mesopause temperature and knowledge about the variability of the OH emission height is important. I have several major and many minor concerns regarding this manuscript, however, and believe that at least a major revision is required before the paper should be accepted in ACP.
I briefly mention my major concerns first, followed by specific comments. It is not really clear, what the new aspects of this study are. It seems to me that all of the shown effects have already been reported in earlier studies. If not, please highlight the new results explicitly. The discussions and arguments often quite weak and it is not clear, what is based on speculation and what on – perhaps – earlier studies, particularly in terms of the role of tides. In addition, the authors chose to analyse variations in peak emission rate and peak altitude. One can certainly do that, but there are other quantities that would be more valuable for the ground-based OH community. I think you should not only analyze peak emission rate, but also the vertically integrated emission rate. The latter is much more important for the many ground-based observers. And this can be done quite easily. You can certainly keep the results on peak emission rate, but add results on the vertically integrated emission rate. Also, the centroid altitude, i.e. altitude weighted by the emission rate profile, would also be a good quantity and is in my opinion more useful than the peak altitude. Finally, there are many linguistical issues, incomplete sentences or sentences, whose meaning is unclear.

Specific comments:
Title: “and long-term trend”
Long-term trends are not discussed at all in this paper, as far as I can tell. Please remove this from the title.
Line 10: “The results show similar latitudinal variations in the semiannual oscillation (SAO) and annual oscillation (AO) of peak emission rate and peak height: the amplitude of SAO is greatest in equatorial regions and AO is greatest in mid-latitudes.“
I don’t think this statement is correct and it probably does not convey the intended meaning, e.g. the latitudinal variation of peak height and peak emission rate are anti-correlated.
Line 14: “The QBO in OH airglow is consistent with the phase variation of the QBO in the tropical lower stratosphere (30km), which is also consistent with the phase variation of the QBO in the migrating diurnal tide.“
This is quite a vague statement and I’m not sure it is correct. What specifically is consistent between the variations? Is the QBO-effect on the migrating diurnal tide really understood? Statements of this kind appear throughout the manuscript, but it is unclear, whether this is well established (are there earlier studies? If yes, they should be cited) or speculation? The current manuscript does certainly not provide any evidence that this is the mechanism driving the QBO signature in OH emissions.
Line 16: “As an important kinetic process“
I would call not tides a kinetic process. They are a “dynamical process“
Line 16: ”we suggest that the tides play an important role in the modulation of the OH airglow by the QBO“
Again, the paper provides no evidence that this is the main mechanism.
Line 23: ”Airglow is the product of photochemical processes in the middle and upper atmosphere, and its radiation intensity is related to atmospheric temperature and atmospheric density.“
This is very general and vague. I suggest deleting the second part of the statement or mentioning specific processes.
Line 35: ”observed strong semiannual“ -> ”observed a strong semiannual“
Line 38: “with a maximum at the equinoxes and a minimum at the solstices“ -> ”with maxima at the equinoxes and minima at the solstices“
Line 39: ”URAS satellite” -> “UARS satellite”
Line 47: “mesopause thermometer (MTM)“ -> ”Mesospheric Temperature Mapper (MTM)“
Line 48: “variations of O(1S)“ -> “variations of O(1S) green line“ (there is also a UV line originating from the 1S state of O.
Line 54: “With increasing latitude, annual oscillation dominates at higher latitudes.“
Please rephrase, sentence logic suboptimal.
Line 57: “has made great progress.“ -> “great progress has been made“
Line 60: “Batista et al. (1994) found a positive correlation between …“
Please mention how long their time series was.
Line 65: ”Pertsev and Perminov (2008) analyzed the response of hydroxyl airglow to solar activity …“
Please mention the OH bands they observed.
Line 72 on SCIAMACHY measurements: the following reference is also relevant for your study and provides a more in-depth analysis of the variability in SCIAMACHY OH(3-1) and OH(6-2) nightglow measurements:
Teiser, G, and C. von Savigny, Variability of OH(3-1) and OH(6-2) emission altitude and volume emission rate from 2003 to 2011, J. Atmos. Sol.-Terr. Phys., 161, 28 - 42, 2017.
Line 73: ”Gao et al. (2016) used the airglow data observed by SABER to analyze the response of NO“
I don’t think this is an NO airglow emission?
Line 79: “Yee et al. (1997) analyze three-day observations“
What do you mean by “three-day observations”?
Line 94: ”In addition to OH airglow intensity, peak emission rate and peak height are important parameters that can be used to describe the airglow emission rate.“
I think that the vertically integrated emission rate is more valuable than the peak emission rate. Ground-based instruments observe the first, not the latter.
Line 99: “low thermosphere” -> ”lower thermosphere”
Line 108: “An overview is given in Section 4.“
Not really clear what the overview is about? Perhaps this sentence can be deleted?
Line 114: “but also the NO OH emissions in the excited state“
? What do you mean by “NO OH emissions”? And “emissions in the excited” state does not make much sense.
Line 115: “radiation to come from“ -> „coming from“ or just „from“
Line 118: “observed by bandpass filters with a central wavelength of 1.6 μm“
This was mentioned in the previous sentence already.
Line 119: “SABER can observe the latitude range from 53° in the winter hemisphere to 83° in the summer hemisphere.“
Not sure this is correct? There is a two month yaw cycle, right?
Equation (1): Parentheses missing around argument of the sine function.
Line 127: „in the later paper“ -> “in the following“ ?
Figure 1 and related 1: I think it would be better to determine centroid altitudes, i.e. altitude weighted by the emission rate profile.
Line 135: Please use another section title. The reader does not know yet what LRO means. I addition “low resolution OMNI“ is not a suitable title.
Line 136: What does OMNI stand for?
Line 146: „The vertical resolution of the data is changed from 0.667°“
Something is wrong here. The vertical resolution cannot be in degrees.
Line 147: „Their wind field data“
Unclear, what „their“ refers to.
Line 149: „(0.625°N,103.125°S)“
Line 152: „were first averaged daily and latitudinally“
? Do you mean zonal means, i.e. averaging over longitude?
Same line: „The averaged results were averaged by latitude range“
What does this mean?
Same sentence: “and the result obtained by averaging was used as the value of the grid centroid.“
It is unclear to me what “grid centroid“ means here.
Line 154: “cover the global 24h place time“
?? Meaning unclear.
Line 167: “The maximum value is at the equinox and the minimum value is at the solstice, and the extreme value at the September equinox is larger than that at the March equinox,“
I disagree, your plot shows the opposite behaviour.
Line 173: “The same phenomenon has been identified in previous studies and is thought to be the role of tides in this“
Sentence incomplete or wrong.
Line 187: “The phase of SAO is delayed from near day 90“
Unclear, what this means or what the intended meaning is.
Line 188: “The peak emission rate has its largest amplitude during the equinox, which is consistent with diurnal tides (Burrage et al., 1995).“
I don’t think it is that easy. We don't know what local solar time the measurements are made at the equinoxes! Also, the statement is not really precise. In what way do you see consistency here. What specific mechanism do you have in mind?
Line 189: “The peak emission rate is associated with diurnal tides and ist seasonality is likely to be caused by the seasonal variation of diurnal tides.“
Are there any references to back this up? Also, “the peak emission rate is associated with diurnal tides“ doesn’t really make sense.
Line 196: “is greatest on day 183 of the first year“
?? I don’t really understand this. The AO has a period of 1 year, i.e. one maximum every year on the same day of the year??
Line 197: “The amplitude of the 50°S reaches its maximum on day 140 of the second year and the 50°N reaches its maximum on day 337 of the first year“
See last point.
Line 199: “and annual oscillation is more frequent at higher latitudes“
"more frequent" is not a good choice of words in this context
Line 200: “We note the semiannual and annual variations in OH airglow intensity provided by Reid et al. (2014), who analyzed filter photometer measurements at Buckland Park“
If you would determine vertically integrated emission rates, you could do an apples-to-apples comparison with Reid et al.
Line 204: “and the trend of the phase and amplitude change of the peak height“
What does trend mean here? Please specify or replace? Trend is usually/often the “linear trend“
Line 211: “reaches its maximum amplitude on day 2 of the second year and the 50°N reaches its maximum on day 178 of the second year.“
Again, the AO has a period of 1 year; why do you have to specify year 1/year 2?
Line 222: “The atomic oxygen distribution derived by Russell and Lowe (2003) using the hydroxyl emission and oxygen green line emission observed by WINDII exploits precisely the relationship between OH emission and atomic oxygen.“
This is only partly correct. The [O] retrievals from the OI green line do not exploit the relationship between the OH emission and O.
Line 227: “One of the mesospheric quasi-biennial oscillation“
?? Meaning unclear.
Line 227: “One of the mesospheric quasi-biennial oscillation (MQBO) shows a similar spatial distribution structure to the mesospheric semiannual oscillation“
What spatial distribution (dimensions?) do you mean here?
Line 229: “In addition, the Christmas Island MF radar (2°N, 130°W) also detects an MQBO with the same phase and the same peak height,“
The same phase and peak height compared to what?
Line 258: “Xu et al. (2009) analyzed the quasi biennial oscillation of the migrating diurnal tide based on data from TIMED observations.“
Which observations, i.e. which atmospheric parameter(s)? And from which instrument? Altitude range?
Line 259: “After comparison, we find that the distribution of OH airglow emission at low latitudes is similar to that of the migrating diurnal tide in temperature.“
Unclear, how you came to this conclusion? This is a complicated matter and you should describe in detail what you mean.
Line 258 – 270: The reasoning in this paragraph is not really stringent, the arguments not precise and the underlying mechanisms are not addressed. For example in line 269: “The stratospheric QBO phase variation is consistent with the tides in the MLT region and the airglow emission phase variation.“ In which way is there a consistency between the two phenomena? This is completely unclear.
Line 275: “Figure 6 shows the correlation with latitude,“
No, this figure does not show a correlation with latitude.
Line 276: “and stratospheric QBO“
One cannot correlate data with the “stratospheric QBO“. You mean the zonal wind at 10/30 hPa above Singapore, right?
Line 296: “then OH airglow emission is likely to vary with solar activity“
Replace “then“ with "the" ... or just "OH airglow emissions are likely to vary.."
Line 308: “with an advance in the response of OH airglow emission in 2008,“
What does “advance“ mean here specifically? I suggest rephrasing this sentence.
Line 314: “Von Savigny (2015) found no clear long-term trends or 11-year solar cycle features in the OH emission height time series.“
The above mentioned paper by Teiser & von Savigny (2017) would also be relevant here.
Line 320: “although showing an opposite trend to solar activity“
I suggest not using “trend“ in this context.
Line 330: “Analysis of the solar cycle dependence on the peak emission rate and its solar response.“
Sentence incomplete.
Same line: “A global annual mean series scatter plot“
Wording not precise.
Line 334: “To investigate the solar activity dependence of the OH airglow.“
Sentence incomplete.
Lines 330 – 335: these sentences are partly redundant and not in very good shape overall.
Line 337: “A is a constant and B is the coefficient“
B is also a constant.
Line 362: “-0.13 km/sfu“ -> “-0.13 km/(100 sfu)“
Line 378: “For QBO, we find that the phase variation of OH airglow emission in the equatorial region remains consistent with the stratospheric QBO phase variation.“
What exactly does consistency mean here? Is this just: "there is a general connection" or "we understand the underlying physical mechanism"?
Line 380: “mixing rate“ -> “mixing ratio“ (and concentration is more relevant here than the mixing ratio)
Line 380: “As the stratospheric zonal wind field moves eastward,“
Wording not precise (wind field moves eastward): Also, what altitudes/latitudes does this refer to?
Line 382: “We, therefore, speculate that tides play an important role in the modulation of OH airglow emission by QBO.“
Earlier in the paper this was not phrased that carefully, but presented rather as a fact. The arguments in this paragraph are not very convincing.
Figure 9: This figure should be removed in my opinion. The underlying mechanisms are not well explained. If there are earlier papers, please cite them. And: O^3 (!) + H -> OH^* + O^2 (!!) is incorrect.
Line 395: “As a result, the latitudinal variations of peak emission rate and height were similar“
?? Why similar? They were anticorrelated, right?
Line 401: “appears on day 183 of the first year at 25°N, delayed towards the poles. The maximum amplitude occurs at 50°S on day 140 of the second year and at 50°N on day 337 of the first year“
Same point as above. Why do you need to specify the year here?
Line 404: “The phase of the SAO at peak height“
??
Line 405: “the AO is delayed from 20°S on day 186 of the first year to day 2 of the second year at 50°S and day 178 of the second year at 50°N.“
See point above.
Line 411: “so that the phase variation remains consistent with the stratospheric zonal winds“
Again, this is not precise; consistent in which respect?
Same line: “When the wind field is to the east,“
??
Lines 414 – 419: Are there earlier studies backing this up? Last sentence: How do you know this? No evidence for this has been presented.
Line 422: “while peak OH airglow emission heights show no significant solar cycle variations.“
?? Why not, the correlations coefficients (i.e. their absolute values) are quite large.
Next sentence: Please delete it – this is only speculation.

Citation: https://doi.org/10.5194/egusphere-2023-910-RC2
- AC2: 'Reply on RC2', Dong Wang, 03 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-910/egusphere-2023-910-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-910-AC2
- AC3: 'Reply on RC2', Dong Wang, 03 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-910/egusphere-2023-910-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-910-AC3

Status: closed

RC1:
'Comment on egusphere-2023-910', Anonymous Referee #1, 02 Jun 2023

Summary
This paper presents an analysis of the peak altitude and peak emission rate of the hydroxyl (OH) emission rates observed for the past 20 years by the SABER instrument on the NASA TIMED satellite. SABER measures OH emission in two distinct spectral intervals, one centered at 2.0 um and the other at 1.6 um. The paper analyzes and presents results for the 1.6 um channel. The 20 years of SABER data enable comprehensive analysis of various temporal features that appear in the peak height and peak altitude of the OH emission. The analysis presented in the paper shows clear evidence for semi-annual and annual oscillations in the peak features and evidence for influence of the stratospheric quasi-biennial oscillation on the peak features. The results presented often confirm the results of previous papers regarding the presence of temporal features in the OH emission. The paper is clearly written.
The results in the paper are presented as ‘engineering’ analysis of the temporal variations in the OH emission. There is very little, if any, quantitative physics or chemistry given to explain the observed behavior. As an example, the discussion of solar variability is primarily of correlations between the OH variability and the F10.7 solar radio flux index. Solar variability affects temperature, composition, and dynamics. But the paper does not attempt to quantify which of these effects is dominant. The paper does not explain why there is strong latitude dependence in many of the analyses. However, the SABER dataset includes temperature, atomic oxygen, atomic hydrogen. These datasets could be explored along with model simulations (the WACCM model would be ideal for this) to put the results in context. In addition, although a secondary concern, the paper does not state why the peak altitude and peak emission rates are important physically. They are clearly markers for atmospheric variability. But is the variability important and why? One could imagine that since the reaction of H + O3 is the largest source of heating in the mesopause region, the variation in intensity and location of the OH emission means that the energetics of the mesopause are being altered significantly. Discussions such as this are necessary to place the results in a physical context as the reported variability largely reproduces prior works.
Recommendation
The recommendation is to reject the paper and invite submission of a new paper that contains much more detailed physical explanation of the observed OH behavior. This should include comparisons with the WACCM model and evaluations with other SABER data products. The paper should provide a quantitative explanation of features such as the latitudinal dependence of the semiannual and annual oscillations and should go into detail about the relative roles of temperature, chemistry, and dynamics in producing the observed variability.
A second recommendation is to evaluate the variability of the OH emissions on pressure surfaces and not on altitude. Studying variations at fixed altitudes mix variations in emission as the pressure surfaces rise and fall around the altitudes as the atmosphere warms and cools over the year and over the solar cycle. Pressure is the natural vertical coordinate of the SABER data.
Specific Comments
Title – the title contains the word ‘trend’ in relation to the peak OH emission. The word ‘trend’ typically implies looking at the long-term change of a parameter due to some forcing that is fundamentally changing the atmosphere such as increasing carbon dioxide concentrations. The original time series is analyzed in a way to remove variability (such as the AO, SAO, QBO, and solar cycle) and the linear trend of the residual is computed to derive a change (typically parameter per decade units). The paper does not appear to contain a trend analysis of this type. Please correct the title and the few places in the text where the word ‘trend’ occurs
Data use – An error was discovered in the SABER data for dates after December 16, 2019. A new version, v2.08 is available for data after that date. Please visit the SABER data web site to review and please discard all v2.0 data after Dec 16 2019.
Line 23-24. This sentence is an example of the lack of quantitative understanding of the airglow that comes across in the paper. The airglow intensity is not directly related to density and only indirectly related to temperature through the temperature dependence of the rate coefficient for the reaction of H and O3. The entire Introduction is full of generalities that makes one question whether the paper truly understands the physics of airglow generation including how and why it varies. A revised Introduction should directly address the physics/chemistry of OH formation and how it may vary, thus setting up the results and analysis with a model such as WACCM later in the paper.
Line 116-117 – please provide a reference citation to the vibrational states that contribute to each of the SABER OH channels.
Line 125 – the instantaneous field of view of the SABER instrument is 2 km. SABER samples the atmosphere at a much higher cadence than every 2 km and so it may appear that the vertical resolution is much higher. The paper needs to discuss the effects of the finite field of view on the ability to determine and analyze variations of the peak height and emission.
Line 136 – please provide a reference citation to the OMNI database and spell out the acronym.
Line 161, Section 3.1. The authors are encouraged to examine the data with Fourier techniques to see if other periodic features are evident.
Lines 181 to 200 – Any new manuscript should include explanations of the origins of the AO, SAO, and QBO, and how these influence the OH emissions. In particular, the AO should be primarily driven by the annual variation of earth-sun distance. So there is an annual cycle of solar radiation along with varying solar radiation on an 11 year cycle. Does any of the 11 year cycle ‘alias’ into the annual cycle? Could the Fourier techniques mentioned above help sort out different cyclic variations?
Line 240-250 – please explain physically how the QBO in the stratosphere modulates the OH emission in the mesosphere.
Line 259-270. SABER has temperature, atomic oxygen, atomic hydrogen, and ozone data. These could be analyzed in concert with the OH data and WACCM model results to produce a complete picture of the relative importance of temperature, chemistry, and dynamics in producing the observed variations in the OH emission.
Line 300- Instead of using the F10.7 proxy, it is suggested to use the actual solar irradiance measured by the SORCE and SEE instruments over the past 20-plus years. Focus on the wavelength regions that drive most of the heating in the mesopause region. This may provide a much better result than using F10.7.

Citation: https://doi.org/10.5194/egusphere-2023-910-RC1
- AC1: 'Reply on RC1', Dong Wang, 03 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-910/egusphere-2023-910-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-910-AC1
RC2:
'Comment on egusphere-2023-910', Anonymous Referee #2, 07 Jun 2023

General comments:
This manuscript deals with the variability in OH emission altitude and OH peak emission rate over a period of more than 20 years based on limb emission measurements with the SABER instrument on TIMED. Different effects and impacts are investigated, e.g. seasonal variations, the effects of the QBO and solar cycle variations. Such a study is in principle of relevance for the mesosphere/mesopause community, because many groups employ ground-based measurements of OH emissions to retrieve mesopause temperature and knowledge about the variability of the OH emission height is important. I have several major and many minor concerns regarding this manuscript, however, and believe that at least a major revision is required before the paper should be accepted in ACP.
I briefly mention my major concerns first, followed by specific comments. It is not really clear, what the new aspects of this study are. It seems to me that all of the shown effects have already been reported in earlier studies. If not, please highlight the new results explicitly. The discussions and arguments often quite weak and it is not clear, what is based on speculation and what on – perhaps – earlier studies, particularly in terms of the role of tides. In addition, the authors chose to analyse variations in peak emission rate and peak altitude. One can certainly do that, but there are other quantities that would be more valuable for the ground-based OH community. I think you should not only analyze peak emission rate, but also the vertically integrated emission rate. The latter is much more important for the many ground-based observers. And this can be done quite easily. You can certainly keep the results on peak emission rate, but add results on the vertically integrated emission rate. Also, the centroid altitude, i.e. altitude weighted by the emission rate profile, would also be a good quantity and is in my opinion more useful than the peak altitude. Finally, there are many linguistical issues, incomplete sentences or sentences, whose meaning is unclear.

Specific comments:
Title: “and long-term trend”
Long-term trends are not discussed at all in this paper, as far as I can tell. Please remove this from the title.
Line 10: “The results show similar latitudinal variations in the semiannual oscillation (SAO) and annual oscillation (AO) of peak emission rate and peak height: the amplitude of SAO is greatest in equatorial regions and AO is greatest in mid-latitudes.“
I don’t think this statement is correct and it probably does not convey the intended meaning, e.g. the latitudinal variation of peak height and peak emission rate are anti-correlated.
Line 14: “The QBO in OH airglow is consistent with the phase variation of the QBO in the tropical lower stratosphere (30km), which is also consistent with the phase variation of the QBO in the migrating diurnal tide.“
This is quite a vague statement and I’m not sure it is correct. What specifically is consistent between the variations? Is the QBO-effect on the migrating diurnal tide really understood? Statements of this kind appear throughout the manuscript, but it is unclear, whether this is well established (are there earlier studies? If yes, they should be cited) or speculation? The current manuscript does certainly not provide any evidence that this is the mechanism driving the QBO signature in OH emissions.
Line 16: “As an important kinetic process“
I would call not tides a kinetic process. They are a “dynamical process“
Line 16: ”we suggest that the tides play an important role in the modulation of the OH airglow by the QBO“
Again, the paper provides no evidence that this is the main mechanism.
Line 23: ”Airglow is the product of photochemical processes in the middle and upper atmosphere, and its radiation intensity is related to atmospheric temperature and atmospheric density.“
This is very general and vague. I suggest deleting the second part of the statement or mentioning specific processes.
Line 35: ”observed strong semiannual“ -> ”observed a strong semiannual“
Line 38: “with a maximum at the equinoxes and a minimum at the solstices“ -> ”with maxima at the equinoxes and minima at the solstices“
Line 39: ”URAS satellite” -> “UARS satellite”
Line 47: “mesopause thermometer (MTM)“ -> ”Mesospheric Temperature Mapper (MTM)“
Line 48: “variations of O(1S)“ -> “variations of O(1S) green line“ (there is also a UV line originating from the 1S state of O.
Line 54: “With increasing latitude, annual oscillation dominates at higher latitudes.“
Please rephrase, sentence logic suboptimal.
Line 57: “has made great progress.“ -> “great progress has been made“
Line 60: “Batista et al. (1994) found a positive correlation between …“
Please mention how long their time series was.
Line 65: ”Pertsev and Perminov (2008) analyzed the response of hydroxyl airglow to solar activity …“
Please mention the OH bands they observed.
Line 72 on SCIAMACHY measurements: the following reference is also relevant for your study and provides a more in-depth analysis of the variability in SCIAMACHY OH(3-1) and OH(6-2) nightglow measurements:
Teiser, G, and C. von Savigny, Variability of OH(3-1) and OH(6-2) emission altitude and volume emission rate from 2003 to 2011, J. Atmos. Sol.-Terr. Phys., 161, 28 - 42, 2017.
Line 73: ”Gao et al. (2016) used the airglow data observed by SABER to analyze the response of NO“
I don’t think this is an NO airglow emission?
Line 79: “Yee et al. (1997) analyze three-day observations“
What do you mean by “three-day observations”?
Line 94: ”In addition to OH airglow intensity, peak emission rate and peak height are important parameters that can be used to describe the airglow emission rate.“
I think that the vertically integrated emission rate is more valuable than the peak emission rate. Ground-based instruments observe the first, not the latter.
Line 99: “low thermosphere” -> ”lower thermosphere”
Line 108: “An overview is given in Section 4.“
Not really clear what the overview is about? Perhaps this sentence can be deleted?
Line 114: “but also the NO OH emissions in the excited state“
? What do you mean by “NO OH emissions”? And “emissions in the excited” state does not make much sense.
Line 115: “radiation to come from“ -> „coming from“ or just „from“
Line 118: “observed by bandpass filters with a central wavelength of 1.6 μm“
This was mentioned in the previous sentence already.
Line 119: “SABER can observe the latitude range from 53° in the winter hemisphere to 83° in the summer hemisphere.“
Not sure this is correct? There is a two month yaw cycle, right?
Equation (1): Parentheses missing around argument of the sine function.
Line 127: „in the later paper“ -> “in the following“ ?
Figure 1 and related 1: I think it would be better to determine centroid altitudes, i.e. altitude weighted by the emission rate profile.
Line 135: Please use another section title. The reader does not know yet what LRO means. I addition “low resolution OMNI“ is not a suitable title.
Line 136: What does OMNI stand for?
Line 146: „The vertical resolution of the data is changed from 0.667°“
Something is wrong here. The vertical resolution cannot be in degrees.
Line 147: „Their wind field data“
Unclear, what „their“ refers to.
Line 149: „(0.625°N,103.125°S)“
Line 152: „were first averaged daily and latitudinally“
? Do you mean zonal means, i.e. averaging over longitude?
Same line: „The averaged results were averaged by latitude range“
What does this mean?
Same sentence: “and the result obtained by averaging was used as the value of the grid centroid.“
It is unclear to me what “grid centroid“ means here.
Line 154: “cover the global 24h place time“
?? Meaning unclear.
Line 167: “The maximum value is at the equinox and the minimum value is at the solstice, and the extreme value at the September equinox is larger than that at the March equinox,“
I disagree, your plot shows the opposite behaviour.
Line 173: “The same phenomenon has been identified in previous studies and is thought to be the role of tides in this“
Sentence incomplete or wrong.
Line 187: “The phase of SAO is delayed from near day 90“
Unclear, what this means or what the intended meaning is.
Line 188: “The peak emission rate has its largest amplitude during the equinox, which is consistent with diurnal tides (Burrage et al., 1995).“
I don’t think it is that easy. We don't know what local solar time the measurements are made at the equinoxes! Also, the statement is not really precise. In what way do you see consistency here. What specific mechanism do you have in mind?
Line 189: “The peak emission rate is associated with diurnal tides and ist seasonality is likely to be caused by the seasonal variation of diurnal tides.“
Are there any references to back this up? Also, “the peak emission rate is associated with diurnal tides“ doesn’t really make sense.
Line 196: “is greatest on day 183 of the first year“
?? I don’t really understand this. The AO has a period of 1 year, i.e. one maximum every year on the same day of the year??
Line 197: “The amplitude of the 50°S reaches its maximum on day 140 of the second year and the 50°N reaches its maximum on day 337 of the first year“
See last point.
Line 199: “and annual oscillation is more frequent at higher latitudes“
"more frequent" is not a good choice of words in this context
Line 200: “We note the semiannual and annual variations in OH airglow intensity provided by Reid et al. (2014), who analyzed filter photometer measurements at Buckland Park“
If you would determine vertically integrated emission rates, you could do an apples-to-apples comparison with Reid et al.
Line 204: “and the trend of the phase and amplitude change of the peak height“
What does trend mean here? Please specify or replace? Trend is usually/often the “linear trend“
Line 211: “reaches its maximum amplitude on day 2 of the second year and the 50°N reaches its maximum on day 178 of the second year.“
Again, the AO has a period of 1 year; why do you have to specify year 1/year 2?
Line 222: “The atomic oxygen distribution derived by Russell and Lowe (2003) using the hydroxyl emission and oxygen green line emission observed by WINDII exploits precisely the relationship between OH emission and atomic oxygen.“
This is only partly correct. The [O] retrievals from the OI green line do not exploit the relationship between the OH emission and O.
Line 227: “One of the mesospheric quasi-biennial oscillation“
?? Meaning unclear.
Line 227: “One of the mesospheric quasi-biennial oscillation (MQBO) shows a similar spatial distribution structure to the mesospheric semiannual oscillation“
What spatial distribution (dimensions?) do you mean here?
Line 229: “In addition, the Christmas Island MF radar (2°N, 130°W) also detects an MQBO with the same phase and the same peak height,“
The same phase and peak height compared to what?
Line 258: “Xu et al. (2009) analyzed the quasi biennial oscillation of the migrating diurnal tide based on data from TIMED observations.“
Which observations, i.e. which atmospheric parameter(s)? And from which instrument? Altitude range?
Line 259: “After comparison, we find that the distribution of OH airglow emission at low latitudes is similar to that of the migrating diurnal tide in temperature.“
Unclear, how you came to this conclusion? This is a complicated matter and you should describe in detail what you mean.
Line 258 – 270: The reasoning in this paragraph is not really stringent, the arguments not precise and the underlying mechanisms are not addressed. For example in line 269: “The stratospheric QBO phase variation is consistent with the tides in the MLT region and the airglow emission phase variation.“ In which way is there a consistency between the two phenomena? This is completely unclear.
Line 275: “Figure 6 shows the correlation with latitude,“
No, this figure does not show a correlation with latitude.
Line 276: “and stratospheric QBO“
One cannot correlate data with the “stratospheric QBO“. You mean the zonal wind at 10/30 hPa above Singapore, right?
Line 296: “then OH airglow emission is likely to vary with solar activity“
Replace “then“ with "the" ... or just "OH airglow emissions are likely to vary.."
Line 308: “with an advance in the response of OH airglow emission in 2008,“
What does “advance“ mean here specifically? I suggest rephrasing this sentence.
Line 314: “Von Savigny (2015) found no clear long-term trends or 11-year solar cycle features in the OH emission height time series.“
The above mentioned paper by Teiser & von Savigny (2017) would also be relevant here.
Line 320: “although showing an opposite trend to solar activity“
I suggest not using “trend“ in this context.
Line 330: “Analysis of the solar cycle dependence on the peak emission rate and its solar response.“
Sentence incomplete.
Same line: “A global annual mean series scatter plot“
Wording not precise.
Line 334: “To investigate the solar activity dependence of the OH airglow.“
Sentence incomplete.
Lines 330 – 335: these sentences are partly redundant and not in very good shape overall.
Line 337: “A is a constant and B is the coefficient“
B is also a constant.
Line 362: “-0.13 km/sfu“ -> “-0.13 km/(100 sfu)“
Line 378: “For QBO, we find that the phase variation of OH airglow emission in the equatorial region remains consistent with the stratospheric QBO phase variation.“
What exactly does consistency mean here? Is this just: "there is a general connection" or "we understand the underlying physical mechanism"?
Line 380: “mixing rate“ -> “mixing ratio“ (and concentration is more relevant here than the mixing ratio)
Line 380: “As the stratospheric zonal wind field moves eastward,“
Wording not precise (wind field moves eastward): Also, what altitudes/latitudes does this refer to?
Line 382: “We, therefore, speculate that tides play an important role in the modulation of OH airglow emission by QBO.“
Earlier in the paper this was not phrased that carefully, but presented rather as a fact. The arguments in this paragraph are not very convincing.
Figure 9: This figure should be removed in my opinion. The underlying mechanisms are not well explained. If there are earlier papers, please cite them. And: O^3 (!) + H -> OH^* + O^2 (!!) is incorrect.
Line 395: “As a result, the latitudinal variations of peak emission rate and height were similar“
?? Why similar? They were anticorrelated, right?
Line 401: “appears on day 183 of the first year at 25°N, delayed towards the poles. The maximum amplitude occurs at 50°S on day 140 of the second year and at 50°N on day 337 of the first year“
Same point as above. Why do you need to specify the year here?
Line 404: “The phase of the SAO at peak height“
??
Line 405: “the AO is delayed from 20°S on day 186 of the first year to day 2 of the second year at 50°S and day 178 of the second year at 50°N.“
See point above.
Line 411: “so that the phase variation remains consistent with the stratospheric zonal winds“
Again, this is not precise; consistent in which respect?
Same line: “When the wind field is to the east,“
??
Lines 414 – 419: Are there earlier studies backing this up? Last sentence: How do you know this? No evidence for this has been presented.
Line 422: “while peak OH airglow emission heights show no significant solar cycle variations.“
?? Why not, the correlations coefficients (i.e. their absolute values) are quite large.
Next sentence: Please delete it – this is only speculation.

Citation: https://doi.org/10.5194/egusphere-2023-910-RC2
- AC2: 'Reply on RC2', Dong Wang, 03 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-910/egusphere-2023-910-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-910-AC2
- AC3: 'Reply on RC2', Dong Wang, 03 Aug 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-910/egusphere-2023-910-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-910-AC3

Sheng-Yang Gu, Dong Wang, Liang Tang, and Yafei Wei

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Short summary

Based on SABER observations, the seasonal and interannual variability of the peak emission rate and height of OH airglow is analyzed. The results show that the latitudinal variations of the semi-annual oscillation (SAO) and annual oscillation (AO) of peak emission rates and heights are similar. In addition, we find that the OH airglow emission is modulated by the quasi-biennial oscillation (QBO) in the equatorial region and solar activity.


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