General comment:

This study discusses the differences in PMSE (Polar Mesosphere Summer Echoes) between the Northern and Southern Hemispheres, based on long-term radar observations in both the Antarctic and Arctic regions. Specifically, the study examines seasonal, altitude-dependent, and local time-dependent variations. The paper carefully discusses how the minimum detectable volume reflectivity changes with the specifications and observation modes of the radar. By focusing only on cases where the volume reflectivity exceeds a certain threshold, the analysis minimizes the impact of variations introduced by different radar systems and observation modes. While many of the findings have been previously suggested, their validation using long-term, well-calibrated observation data adds significant value to this study.

Based on the above, I believe that this paper, with minor revisions, would be suitable for acceptance in Annales Geophysicae.

Specific comments:

• Upon thorough review, I noticed that there seems to be no mention of range resolution. Including this information somewhere in the paper would be beneficial.
• ll. 161-169: In addition to providing specific dates for the earliest and latest onset of the PMSE season, it might be beneficial to include the respective years as well. Doing so would offer readers a valuable reference when investigating inter-annual variations around the mesopause.
• ll. 287-295: Tidal effects are mentioned as one of the factors contributing to local time dependency of PMSE. On the other hand, previous studies, including Murphy et al. (2004), have pointed out seasonal variations in the amplitude and phase of tides in the Antarctic region. It would be valuable if the paper could discuss whether the local time dependency shown in lower panels of Figure 3 changes with the season, or at least remains qualitatively consistent. Such discussion could include how this relates to the research on the seasonality of tides.

Technical comments:

• An abbreviation SH (Southern Hemisphere) is not used in the manuscript though it is defined in ll. 24.
• ll. 202: "Andøa" should be "Andøya".
• ll. 267:  "PMSE particles" might be "NLC (or PMC) particles".

References:

• Murphy, D. J., et al. (2006), A climatology of tides in the Antarctic mesosphere and lower thermosphere, J. Geophys. Res., 111, D23104, doi:10.1029/2005JD006803.

This paper presents a long dataset of carefully calibrated VHF radar measurements in the mesopause region, and uses them to catalogue the properties of PMSEs, and to compare the properties of these echoes in the Northern and Southern Hemisphere. The paper is clearly written, with good diagrams, and draws appropriate conclusions. The results largely confirm previous work, but with much longer data records the variability of the PMSEs can be more thoroughly investigated. I would have liked to see a more substantial discussion of the importance of this work, and the questions that remain unanswered (if any), which the authors may care to address. Overall though the paper is a throrugh and rigorous piece of observational science that is publishable in its present form.

The manuscript describes a long-term climatological comparison for PMSE at 69 deg latitudes in the Arctic and Antarctica. It uses calibrated PMSE observations from 23 boreal summers at Andoya (in the Arctic) and 15 austral summers (in Antarctica). The manuscript focuses on the similarities and differences at these two locations related to echo strengths, height dependency, and diurnal and seasonal variation in terms of occurrence rates. While most of the results reconfirm the previous studies conducted at these two locations, the comparison of the echo strength at these two locations indicates that they are indeed comparable, which is opposite to earlier studies.  The authors point out that this occurred due to discrepancies in the calibration process at the Antarctica site. 

While the manuscript provides a good summary for the analysis of 20 plus years of PMSE observations, the authors could have also focused more in understating the science behind the observed features and this is missing in the manuscript.

Comments: 

While the new results rectify the previous comparisons, especially in the case of PMSE echo strengths, the other results remain the same and reconfirm previous results. Considering these facts, incorporating other parameters (that are considered links with PMSE occurrence) is important to take the PMSE research to the next level. In the Discussions section, although authors tried for this using assumptions and the references (some of them are from old studies), it could have been very useful if they used measurements of temperature, winds, water vapor, etc. from either ground or space observations (for at least for a few years) to investigate their links to PMSE.

It is interesting to note that PMSE diurnal occurrence at Andoya goes to a minimum at approximately 21 hrs local time (Fig 5). As noted above, utilizing observations of other parameters may provide reasons for this to occur consistently for more than 20 years.

GENERAL COMMENTS

This manuscript compares long term observations of Polar Mesosphere Summer Echoes (PMSEs) made by one VHF radar in the Arctic and by a second one in the Antarctic. It relies on the fact that the radars have been calibrated in order to allow direct comparisons of signal strengths. Although this manuscript effectively extends work carried out in a few earlier publications (which have been referenced), it benefits strongly from the much longer datasets available. This makes the results much more robust and leads to some changes in the conclusions. The manuscript should be be of interest to other researchers focussing on the polar summer mesopause regions.  

The manuscript is generally clearly written. However, there are a number of places where clarification of the analysis would be of benefit to the reader. I have listed these issues in the SPECIFIC COMMENTS section below. I regard the manuscript to be ready for publication after only minor revisions have been made.

SPECIFIC COMMENTS

1) It would be useful to include the coordinates of the Andøya radars and of the Davis radar where they are first mentioned in the main body of the text, i.e. on lines 14 and 35, respectively. Although these details are given in the abstract, that is separate from the main text.

2) The first reference to "mesopause jumps" on line 37 would benefit from slightly more detail. Given that the mesosphere is subject to interhemispheric coupling, it is not immediately clear whether the breakdown of the polar vortex being referred to is in the northern or southern hemisphere. I note that a more detailed explanation is given beginning on line 314.  

3) The meaning of abbreviation LMT, which first appears on line 51, should be spelled out. I'm assuming that it refers to Local Mean Time. Moreover, it would be useful to briefly describe what this term means so that it is not confused with the time within the local time zone.

4) The authors use the terms "signal strength" and "volume reflectivity" interchangeably. Although this is somewhat justified, it can be confusing in instances such as on line 132: "1% of the echoes detected by the Davis radar in 2005–2012 period (Q_0.01 ) have a signal strength of η <= 1.6 10^−16 m^−1 . . . ". There are several more similar instances throughout the manuscript.

5) I found some of the explanations, at the start of section 3.2.1, of what is being shown in Figure 3 a bit confusing. The following sentence, starting on line 152, was particularly confusing: "The top panels show the mean frequencies of all echo detections normalized to the seasonal maximum value over time and altitude with respect to the daily measurement period." The same point is made much more clearly in the caption for Figure 3, which simply states "The seasonal height distribution of PMSE in the top plots are normalized to its maximum". Conversely, the description from the caption of Figure 3 for what is shown in the middle panels makes it sound as though only a single range gate is being considered. The corresponding text starting on line 155 is clearer. However, I would change the wording slightly to something like "Here, an occurrence is triggered if the volume reflectivity exceeds the minimum threshold at any altitude and at any time during the day".

6) In the description of first and final PMSE dates in the paragraph beginning on line 161, and in Table 3, it would be of interest if the years during which the earliest/latest occurrences were seen were included as well as the days.

7) Given that PMSE occurrence is much lower in the first and final months of the season than during the middle two months, in relation to Fig. 5 the authors should state which dates were considered for calculating the mean diurnal variations. For clarification, if PMSEs are deemed to have occurred over 4 km of the 8 km altitude regions considered, does that represent an occurrence rate of 50%? In the context of the middle panels of Fig. 3, this would constitute an occurrence rate of 100%.

8) Also in relation to Fig. 5, it would be useful if the authors could distinguish between single year means (i.e. those represented by the individual black lines) and multi year means (i.e. those represented by the blue and red lines). In some cases in Section 3.2.2, when a reference is made to "mean values", it not immediately clear which type of mean is being referred to.

9) The variable name "η_min" is used in Table 2 to indicate the minimum values in the distributions of volume reflectivities. The same variable name is subsequently used on lines 144 and 248 to indicate the minimum threshold value of volume reflectivity. It would be better to use a name such as "η_thresh" in order to avoid confusion.

TECHNICAL CORRECTIONS

1) For completeness, the meaning of the abbreviation "VHF" should be given in full on line 14.

2) The name "Andøya" has been misspelled as "Andoya" on line 71, as "Andøa" on line 202, as "Andøoya" on line 252, and as "Andoøya" in the caption for Figure 3.

3) The words "relative to the solstice" on line 171 are not required since the abbreviation "rts" has already been used in this sentence.

4) The units of power on line 365 should be shown as "watts" with a lower case "w" rather than as "Watt".

5) The urls for a few of the references incorrectly begin with the "https://doi.org" part repeated - see Becker et al. (2015) on line 402, Bremer et al. (2009) on line 408, Latteck and Bremer (2017) on line 431, Latteck et al. (2021) on line 446, Lübken et al. (2015) on line 455, and Lübken et al. (2017) on line 458.

6) The paper by Latteck et al. (2008) now has a (presumably retrospective) doi (https://doi.org/10.5194/angeo-26-2795-2008), which has not been included in the reference on line 440.

7) The references for Latteck et al. (2010) on line 441 and for Latteck et al. (2012) on line 443 have omitted the names of the journals - "Advances in Radio Science" and "Radio Science", respectively.

8) The word "by" in the following sentence on line 194 - "The distance of the local minima to each other is even larger BY about 2.7 h." - gives the wrong meaning. It should be replaced by a word such as "at". Similarly the word "of" should be replaced by a word such as "by" in the sentence on line 304 - " the vertical extent of Polar Mesospheric Clouds (PMC) could be attributed to colder temperatures OF 4–7 K".

9) In relation to the right hand panel of Fig. 6 (see line 198), it is not easy to distinguish between the "dots" and "stars". However, the intended feature is clear.

10) On line 234, do the authors mean "in terms of all values larger than 99% (see Q_0.99 in Tab. 2)" as they have written or "in terms of all values BELOW/UP TO the 99% quartile"?

11) Use of the term "vice versa" is not appropriate in the sentence on line 291, although I agree with what the authors are trying to say, i.e. that meridional winds can transport cold air from higher to lower latitudes and warm air from lower to higher latitudes.