Summary

This manuscript presents a statistical study of O2Atm(0-0) auroral emissions at 762 nm as observed by the MATS satellite during three months of operations from February to April 2023. The Authors identified auroral signatures in the limb images taken along the track of the satellite by developing a detection algorithm isolating them from the O2 airglow signatures. They visually inspected all the auroral events and determined the geomagnetic coordinates (geomagnetic latitude and magnetic local time, MLT) of the corresponding tangent point as well as the altitude of the emission peak for the 378 of them which were retained for the statistical analysis. They then analysed the distributions of the auroral events as a function of geomagnetic latitude, MLT, and geomagnetic activity as measured with the Kp index. They also examined the dependence of the emission peak altitude with MLT and Kp index, as well as the distribution of spectral intensity as a function of peak altitude. The main findings are that the auroral events occur at lower geomagnetic latitude with increasing Kp, that the peak altitude is around 103 km for most conditions, and that the spectral intensity tends to be lower when the peak is found at a higher altitude.

The manuscript is well written, the methodology is carefully detailed, and the topic of the study is very suitable for ANGEO Communicates. However, the statistical analysis is relatively superficial, and it is hence difficult to identify significant new results provided by the study. I believe that the manuscript could become suitable for publication if the following comments and suggestions are given consideration.

Major comment

As stated above, currently the main results presented in the manuscript do not really bring much novel scientific insight in O2 auroral emissions. The fact that the auroral oval expands with increasing geomagnetic activity is very well-known and documented; the 762 nm aurora peak altitude and brightness are found to be similar to those derived in past studies. However, with the unique new dataset provided by the MATS mission, and given the scarcity of the literature on O2 auroral emissions, there is a lot of room for making new findings. I may suggest a few avenues that the Authors could consider to get there, some of which would require expanding slightly the statistical treatment of the data, while others could in fact mostly rely on surveying the existing literature:

* Could the altitude thickness of the auroral emissions be evaluated (using e.g. the full width at half maximum as the metric) and be a parameter studied as a function of MLT, geomagnetic latitude, Kp index, spectral intensity, etc.?

* Could another geomagnetic index than Kp be considered – for instance an auroral electrojet such as AE or SME? These might be more insightful than Kp if looking at the properties of the aurora such as peak altitude and brightness, as they respond specifically to auroral/substorm activity.

* Rather than a mere qualitative assessment that the O2 762 nm auroral emission geomagnetic latitude decreases with increasing Kp index, could a parametrisation of this latitude as a function of Kp (or other index) be derived? It could then be discussed with respect to the latitudinal dependence of other auroral emission lines on geomagnetic activity, enabling one for instance to determine whether O2 762 nm occurs in a specific part of the auroral oval and is associated with a certain type of particle precipitation.

* Can any interhemispheric asymmetries be noted when the events do not take place close to the equinox?

* How do the results – in terms of O2 762 nm auroral emission altitude, MLT and geomagnetic latitude distribution, brightness – compare to what is known about other auroral emission lines?

* Would it be insightful to look into the cross-oval extent of the detected auroral events (again as a function of MLT, geomagnetic latitude, geomagnetic activity)? Could this be estimated by evaluating the width of the parabola associated with the signature of auroral emissions in keograms?

* Is it possible to get additional information by analysing the full images rather than keograms? In Fig. 1b–c, there seems to be some structuring of the auroral emissions visible along the horizontal direction of the images. Making use of the 2D nature of the MATS data could lead to findings that were not possible from rocket measurements, which are intrinsically 1D.

To clarify: I do not request that the Authors look into all of those questions, of course. But I think if at least one of those ideas – or another one not from this list – could be addressed, it might bring in novelty that would radically enhance the impact of the paper and make it worthy of prompt publication.

Minor comments

– l. 4–5 (abstract), “This emission (...) plays a big role in the study of atmospheric airglow and aurora”: This statement is quite vague; it could be worth giving a concrete example of what role this emission plays.

– l. 13: If possible, please provide a reference.

– l. 16: See also Kirillov & Belakhovsky (2021) for a recent work on this topic.

– l. 44–45, “and the monthly dependence of the auroral altitude”: I did not find which part of the results section addresses this point.

– l. 49: Generally, the term “lower thermosphere” is used instead of “low thermosphere” in the literature; please consider adopting it.

– l. 77: Would it be possible to explain why a third-order polynomial was chosen for the regression in the removal of the airglow contribution? Especially in the example selected for Fig. 1, it seems that the fitted background in panel f) is very flat; it this typical? What is the reasoning behind considering a third-order polynomial in the general case?

– l. 85: Please indicate (here or later in the section) the value of the retained threshold, to ensure reproducibility of the results.

– Figure 1: Please add axis labels in panels e) and f), as well as a colour bar for the data shown in panels a–e). Please define also ‘TPlat’ explicitly (for instance in the caption). You may also consider adding in panel e) a y-axis indicating the altitude of the tangent point corresponding to the pixel row numbers.

– Fig. 1 caption: One of the ‘(ii.)’s should be ‘(iii.)’.

– l. 101: Would it be possible to comment on the fact that 378 events were retained for the study, while a back-of-the-envelope calculation suggests that, during the ~81 days of MATS data used in this study, there have been approximately 4860 auroral oval crossings? Is it so that O2 762 nm auroral emission is not always present in the auroral oval? Are there limitations in the instrument’s operations related to e.g. lighting conditions in the atmosphere? Are the auroral signatures very often too complex for the events to be retained by the algorithm? It would be interesting to see the temporal distribution (i.e. as a function of the date in early 2023) of the obtained auroral events, for each hemisphere.

– l. 113: How representative are the statistics in the cases where only three events are in a given data bin? Would it remove many data points if selecting a higher threshold for calculating the mean and standard error of the mean?

– l. 117–118, “A notable feature of this plot is the lower altitude of events between 5 and 7 MLT, especially in the southern hemisphere”: It seems to me that it is in fact only the case in the southern hemisphere, as the few northern-hemisphere data points are at altitudes very closed to the average values. Please correct the statement.

– Figure 2: The chosen colour map is not adequate, as it is not accessible to people with colour vision deficiencies. Please refer to the ANGEO guidelines to revise the figure (https://www.annales-geophysicae.net/submission.html#figurestables), and consider using a suggested tool such as Coblis.

– Figure 3: Would it be possible to provide a measure of the uncertainty on the average altitude values in panel a)? Besides, panel b) is missing its y-axis label (name of the plotted parameter).

– Fig. 3 caption: “One-hour average” suggests a temporal average (e.g. from a time series), but here I think you are referring to the average of events occurring within the same 1-hour MLT bin. Please consider rephrasing to avoid ambiguity. In addition, please indicate whether the boundaries of the Kp bins are included or excluded (i.e., is the first bin from Kp = 0 to Kp = 3– or to Kp = 3? If the latter, then I presume that the second bin starts at Kp = 3+).

– l. 124: If using the phrase “clear correlation”, please calculate a relevant correlation coefficient as part of the data analysis.

– l. 127–128: The statement about the auroral intensities expressed in kR is difficult to verify by looking at the figures. Would it make sense to present the spectral intensities shown in Fig. 3b directly as limb brightness using the conversion described in l. 121–122, for instance?

– l. 131: Earlier (l. 114), it read ‘104 km’; please harmonise.

– l. 134–135, “For MLTs of 5 to 7, the average altitude of the peak tends to be below average”: As mentioned above, this seems to only be the case for the southern hemisphere; please rephrase.

– l. 135–136: More precisely than energetic electrons, in the morning sector, the role of pulsating aurora has been emphasised in producing lower auroral peak altitudes compared to the evening and midnight sectors (see e.g. Partamies et al., 2022, on the 557.7 nm and 427.8 nm aurora).

– l. 137: ‘left panel’ --> ‘right panel’

– l. 137–138: The reported general trend between peak altitude and brightness is not at all obvious from Fig. 3b. Further statistical processing and a revised figure would be necessary to make an assessment on this matter.

– l. 138–139: The reference to Cattell et al. (2006) does not seem optimal, as the statement it is meant to support is not at all the focus of the cited paper. In fact, the statement does not necessarily hold – see for instance Fig. 5 of Tesfaw et al. (2022), where it is clear that energy flux and characteristic electron energy are not always following the same trend. Please revise this sentence.

– l. 142–143: Would it be possible to provide an estimate of how often events with a complex spatial distribution of the aurora may have been missed? It would be interesting to know for instance if the O2 762 nm aurora generally consists of a single arc or if multiple structures can be seen during a single oval crossing. If only single arcs have been retained for the study due to the event selection algorithm, this may induce a bias in the results, which would be worth evaluating and discussing in more detail.

– Although it considered a different emission line of O2 (1.27 µm), it would be worth referring to the recent study by Gao et al. (2020) using 18 years of SABER data, since their methods are adjacent to yours, and it would prove insightful to discuss how your results compare to theirs. So little has been published on O2 auroral emissions that it would be worth mentioning the more recent literature addressing it.

Cited references

– Gao et al. (2020), https://doi.org/10.1029/2020JA028302

– Kirillov & Belakhovsky (2021), https://doi.org/10.1029/2020JD033177

– Partamies et al. (2022), https://doi.org/10.5194/angeo-40-605-2022

– Tesfaw et al. (2022), https://doi.org/10.1029/2021JA029880