the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 May 2023
High-time resolution analysis of meridional tides in the upper mesosphere and lower thermosphere at mid-latitudes measured by the Falkland Islands SuperDARN radar
Andrew J. Kavanagh
Neil Cobbett
Paul Breen
Tim Barnes
Abstract. Solar tides play a major role in the dynamics of the upper mesosphere and lower thermosphere (MLT). Hence, a comprehensive understanding of these tides is important for successful modelling of the MLT region. Most ground-based observations of tidal variations in the MLT have been from meteor radar measurements with a temporal resolution of 1 hr. Here, we take a different perspective on these tidal variations using high-resolution 1-min neutral wind measurements from the Falkland Islands SuperDARN radar. This analysis shows that these higher-resolution data can be used to identify higher frequency tidal components than are typically observed by meteor radars (up to a heptadiurnal component). It also shows evidence of significant power in these higher frequency components, particularly in the quaddiurnal component, which may be particularly suitable for a global analysis using high-resolution SuperDARN neutral wind measurements. The high-resolution analysis also shows evidence of fluctuations with a frequency of 1.5 cycles/day, as well as higher frequency fluctuations, accompanying a quasi-two-day wave. We discuss the limitations of this high-resolution analysis method, and the new opportunities that it may provide. We conclude that higher-resolution SuperDARN neutral wind measurements need to be better exploited in the future, as they provide a complementary way of studying tides and waves in the MLT.
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Gareth Chisham et al.
Status: final response (author comments only)
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RC1: 'Comment on angeo-2023-15', Anonymous Referee #1, 17 Jul 2023
Reviewer’s report on the manuscript by Gareth Chisham et al. titled “High-time resolution analysis of meridional tides in the upper mesosphere and lower thermosphere at mid-latitudes measured by the Falkland Islands SuperDARN radar” (manuscript #: angeo-2023-15)
This manuscript proposes a new analysis algorithm of the meridional tides at MLT altitude based on the mid-latitude SuperDARN radar observation of meteor echoes with relatively high temporal resolution. As a result, they found higher-order tidal waves that could not be detected using the previous studies based on 1-hour averaging.
The results sound interesting because they identified higher components of the tidal waves, which were not apparent in previous studies. On the other hand, I have a few concerns, such as the validity of identifying meteor echoes and citations of previous works, as shown below. The manuscript needs (mostly minor) revisions before being accepted for publication in the Annales Geophysicae journal.
1. Insufficient evaluation of the algorithms of identifying meteor echoes.
The authors identified meteor echoes as those having altitude ranges of about 100 km and low spectral width, based on the results by Chisham and Freeman (2013). I have a concern about the possibility of the contamination of non-meteor echoes, such as E-region echoes, sporadic E echoes, and PMSEs. They have not fully discussed these possibilities, especially because they deal with echoes with Doppler velocities larger than 100 m/s.
In addition to the echoes I already mentioned, there are other echoes such as HAIR (high aspect angle irregularity region, Milan et al., 2003) and FAIR (far aspect angle irregularity region, St.-Maurice and Nishitani, 2020), both located in the lower E-region ionosphere around 100 km altitude. They are not meteor echoes and do not always move with the ambient neutral wind. Some of them have similar characteristics as meter echoes (e.g., located around 100 km altitude and having low spectral width). Therefore, possible contamination of these echoes (E-region, sporadic E-region, PMSE, HAIR, and FAIR echoes) should be discussed in the text.
2. Citation of previous studies.
The most promising way to solve the problem of distinguishing meteor echoes is to obtain the raw time series data, as reported by Yukimatu and Tsutsumi (2002) and Tsutsumi et al. (2009). Meter echoes should appear in the raw time series data as the echoes with a sudden increase of the echo power, followed by its exponential decay. Chisham and Freeman (2013) argue that the standard SuperDARN radars do not record raw time series data. However, the function of recording raw time series data has been implemented in the standard SuperDARN radar operation software, and several SuperDARN radars actually record raw time series data. I do not require the authors to analyze the raw time series data in the current study. However, it is not the right manner to ignore previous studies completely. Works related to raw time series data analysis should be introduced in the discussion.
Minor comments:
Line 173 “persistant” should be “persistent”
References:
Milan, S. E., Lester, M., Yeoman, T. K., Robinson, T. R., Uspensky, M. V., and Villain, J.-P.: HF radar observations of high-aspect angle backscatter from the E-region, Ann. Geophys., 22, 829–847, https://doi.org/10.5194/angeo-22-829-2004, 2004.
St.-Maurice, J.-P., & Nishitani, N. (2020). On the origin of far-aspect angle irregularity regions seen by HF radars at 100-km altitude. Journal of Geophysical Research: Space Physics, 125, e2019JA027473. https://doi.org/10.1029/2019JA027473
Tsutsumi, A.S. Yukimatu, D.A. Holdsworth, M. Lester, Advanced SuperDARN meteor wind observations based on raw time series analysis technique, Radio Science, 44 (2009), p. RS2006, 10.1029/2008RS003994
A.S. Yukimatu, M. Tsutsumi, A new SuperDARN meteor wind measurement: raw time series analysis method and its application to mesopause region dynamics, Geophysical Research Letters, 29 (2002), p. 1981, 10.1029/2002GL015210
Citation: https://doi.org/10.5194/angeo-2023-15-RC1 -
AC1: 'Reply on RC1', Gareth Chisham, 17 Aug 2023
The authors identified meteor echoes as those having altitude ranges of about 100 km and low spectral width, based on the results by Chisham and Freeman (2013). I have a concern about the possibility of the contamination of non-meteor echoes, such as E-region echoes, sporadic E echoes, and PMSEs. They have not fully discussed these possibilities, especially because they deal with echoes with Doppler velocities larger than 100 m/s. In addition to the echoes I already mentioned, there are other echoes such as HAIR (high aspect angle irregularity region, Milan et al., 2003) and FAIR (far aspect angle irregularity region, St.-Maurice and Nishitani, 2020), both located in the lower E-region ionosphere around 100 km altitude. They are not meteor echoes and do not always move with the ambient neutral wind. Some of them have similar characteristics as meter echoes (e.g., located around 100 km altitude and having low spectral width). Therefore, possible contamination of these echoes (E-region, sporadic E-region, PMSE, HAIR, and FAIR echoes) should be discussed in the text.
The following comment is a response to both reviewers:
The large majority of echoes measured by SuperDARN at near ranges (<400 km) are a result of scattering from meteor trails, which drift at the neutral wind speed. However, it is true that there is occasional contamination of these neutral wind measurements by E-region echoes, although these are only a significant problem in and near the auroral zone where field-aligned (and other) ionospheric irregularities are ubiquitous. Chisham and Freeman (2013) removed a large proportion of this E-region contamination from echoes measured by the Saskatoon SuperDARN radar, within the auroral zone, by understanding the differences in the probability distributions of the velocity error and spectral width error of the different echo types (as explained in detail in section 2.4 of that paper). Although not removing this E-region contamination completely, the filtering proposed in that paper reduced it significantly.
The echoes measured by the mid-latitude FIR SuperDARN radar and presented in this paper are very different to those observed at auroral latitudes. The near ranges are overwhelmingly dominated by meteor echoes. FIR is located at mid-latitudes (~55 degrees south geographic latitude; ~40 degrees south geomagnetic latitude), a significant distance from the auroral region, which only appears at far ranges in the FIR field-of-view during disturbed times. Hence, E-region echoes are very rare in the FIR data set, especially during the solar minimum interval studied here. At the near-ranges where meteor echoes are observed they are almost non-existent. Hence, contamination of meteor echoes by E-region echoes is not a problem with the FIR observations.
Visual inspection of daily plots of scatter throughout the analysis interval indicates that the only potential contamination of the meteor echoes within the FIR near-range data is from PMSE/MSE and sea scatter (following E-region reflection). This contamination is rarely a problem at the nearest ranges and occurs predominantly around noon/early afternoon in the summer months. This can be shown by a statistical investigation of echo characteristics at near ranges, which shows the domination of meteor echo characteristics at the nearest ranges. The potential for contamination of the data set from non-meteor echoes will now be discussed in more detail in the revised version of the paper, with a figure that demonstrates the changes in echo characteristics due to different scattering targets at near ranges.
The most promising way to solve the problem of distinguishing meteor echoes is to obtain the raw time series data, as reported by Yukimatu and Tsutsumi (2002) and Tsutsumi et al. (2009). Meter echoes should appear in the raw time series data as the echoes with a sudden increase of the echo power, followed by its exponential decay. Chisham and Freeman (2013) argue that the standard SuperDARN radars do not record raw time series data. However, the function of recording raw time series data has been implemented in the standard SuperDARN radar operation software, and several SuperDARN radars actually record raw time series data. I do not require the authors to analyze the raw time series data in the current study. However, it is not the right manner to ignore previous studies completely. Works related to raw time series data analysis should be introduced in the discussion.
The reviewer is right that the paper should discuss the previous methods that have been used to measure SuperDARN meteor echoes at high-time resolution. The method presented by Yukimatu and Tsutsumi (2002) is the only method that can presently be used to study individual SuperDARN meteor echoes in detail. We now intend to discuss this method in the introduction of the revised paper. However, our view is that this method is impractical for studying very long data sets to study phenomena such as atmospheric tides. Hence, we feel that the approach that we have taken with the SuperDARN data analysis is the most suitable for the phenomena that we wish to analyse.
Line 173 “persistant” should be “persistent”
This will be corrected in the revised paper.
Citation: https://doi.org/10.5194/angeo-2023-15-AC1
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AC1: 'Reply on RC1', Gareth Chisham, 17 Aug 2023
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RC2: 'Comment on angeo-2023-15', Anonymous Referee #2, 18 Jul 2023
In this paper, the authors presented high-resolution analysis of meridional winds obtained from meteor echoes observed by the superdarn radar. High-resolution (1 min) continuous wind observations are very important for addressing the influence of the entire spectrum of waves on the MLT region. However, the data procedure is vaguely described. How many underdense meteors can be detected per minute or per hour? Need to include a plot showing time variation of meteor counts. The authors need to show clearly how meteor echos and E-region echoes are differentiated, with a few examples.
Citation: https://doi.org/10.5194/angeo-2023-15-RC2 -
AC2: 'Reply on RC2', Gareth Chisham, 17 Aug 2023
In this paper, the authors presented high-resolution analysis of meridional winds obtained from meteor echoes observed by the superdarn radar. High-resolution (1 min) continuous wind observations are very important for addressing the influence of the entire spectrum of waves on the MLT region. However, the data procedure is vaguely described.
We don’t agree that the data procedure is vaguely described. The data procedure is standard as for many previous SuperDARN meteor scatter observations. Our paper includes a concise explanation of the meteor echo analysis and is well referenced regarding the measurements of meteor echoes with SuperDARN, and too much more detail in this paper would involve the repetition of text and information which is easily found in the referenced articles.
How many underdense meteors can be detected per minute or per hour? Need to include a plot showing time variation of meteor counts.
We already state in our paper `Figure 3 of Hibbins et al. (2011) presented the seasonal and diurnal variation of the mean number of meteor echoes observed during the first year of FIR operations.’ However, we can see that it would be instructive to show the number of meteor echoes typically observed per hour in the data set, so in the revised version of the paper we will add a figure showing this information. As the SuperDARN integration time is one minute, it is only possible to observe a single meteor echo during each one-minute interval. This is standard for SuperDARN meteor echo observations.
The authors need to show clearly how meteor echos and E-region echoes are differentiated, with a few examples.
The following comment is a response to both reviewers:
The large majority of echoes measured by SuperDARN at near ranges (<400 km) are a result of scattering from meteor trails, which drift at the neutral wind speed. However, it is true that there is occasional contamination of these neutral wind measurements by E-region echoes, although these are only a significant problem in and near the auroral zone where field-aligned (and other) ionospheric irregularities are ubiquitous. Chisham and Freeman (2013) removed a large proportion of this E-region contamination from echoes measured by the Saskatoon SuperDARN radar, within the auroral zone, by understanding the differences in the probability distributions of the velocity error and spectral width error of the different echo types (as explained in detail in section 2.4 of that paper). Although not removing this E-region contamination completely, the filtering proposed in that paper reduced it significantly.
The echoes measured by the mid-latitude FIR SuperDARN radar and presented in this paper are very different to those observed at auroral latitudes. The near ranges are overwhelmingly dominated by meteor echoes. FIR is located at mid-latitudes (~55 degrees south geographic latitude; ~40 degrees south geomagnetic latitude), a significant distance from the auroral region, which only appears at far ranges in the FIR field-of-view during disturbed times. Hence, E-region echoes are very rare in the FIR data set, especially during the solar minimum interval studied here. At the near-ranges where meteor echoes are observed they are almost non-existent. Hence, contamination of meteor echoes by E-region echoes is not a problem with the FIR observations.
Visual inspection of daily plots of scatter throughout the analysis interval indicates that the only potential contamination of the meteor echoes within the FIR near-range data is from PMSE/MSE and sea scatter (following E-region reflection). This contamination is rarely a problem at the nearest ranges and occurs predominantly around noon/early afternoon in the summer months. This can be shown by a statistical investigation of echo characteristics at near ranges, which shows the domination of meteor echo characteristics at the nearest ranges. The potential for contamination of the data set from non-meteor echoes will now be discussed in more detail in the revised version of the paper, with a figure that demonstrates the changes in echo characteristics due to different scattering targets at near ranges.
Citation: https://doi.org/10.5194/angeo-2023-15-AC2
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AC2: 'Reply on RC2', Gareth Chisham, 17 Aug 2023
Gareth Chisham et al.
Gareth Chisham et al.
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