Influences of sudden stratospheric warmings on the ionosphere above Okinawa

Hocke, Klemens; Wang, Wenyue; Ma, Guanyi

doi:https://doi.org/10.5194/egusphere-2024-485

Preprints

https://doi.org/10.5194/egusphere-2024-485

Preprints

21 Feb 2024

| 21 Feb 2024

Influences of sudden stratospheric warmings on the ionosphere above Okinawa

Klemens Hocke, Wenyue Wang, and Guanyi Ma

Abstract. We analyzed the ionosonde observations at Okinawa (26.7° N, 128.1° E) for the years from 1972 to 2023. Okinawa is at the border between the low latitude and the mid-latitude ionosphere. The inﬂuences of sudden stratospheric warmings (SSWs) on the ionosphere are strong at low latitudes but less obvious at middle and high latitudes. We divided the dataset into winters with major SSWs in the northern hemisphere (SSW years) and into winters without major SSWs (no SSW years). During the SSW years, the daily cycle of the F2 region electron density maximum (NmF2) was stronger than in the no SSW years. The relative NmF2 amplitudes of solar and lunar tidal components (S₂, O₁, M₂, MK₃) are stronger by 3 to 8 % compared to no SSW years. The semidiurnal amplitude, averaged for 29 SSW events, has a signiﬁcant peak at the central date of the SSW (epoch time 0 of the composite analysis). The SSW inﬂuence is not strong: the semidiurnal amplitude is about 38.2 % in SSW years and about 32.9 % in no SSW years (relative to NmF2 of the background ionosphere). However, there is a sharp decrease of the amplitude by about 10 % after the SSW peak was reached. The amplitude of the diurnal component does not show a single peak at the central date of the SSW. We present the maximal semidiurnal amplitudes of the SSWs since 1972. The SSW of 31 December 1984 has the strongest amplitude (162 %) in the ionosphere above Okinawa. The most surprising ﬁnding of the study are the strong lunar tides with relative amplitudes of about 10 % and the discovery of a terdiurnal lunar tide (5 %) in NmF2 during SSW years. The periods of the ionospheric lunar tides agree with the periods of ocean tides.

Received: 19 Feb 2024 – Discussion started: 21 Feb 2024

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Klemens Hocke, Wenyue Wang, and Guanyi Ma

Status: closed

RC1:
'Comment on egusphere-2024-485', Anonymous Referee #1, 14 Mar 2024

Review of “Influences of sudden stratospheric warmings on the ionosphere above Okinawa” by Hocke et al.
The manuscript presents results of the impact of sudden stratospheric warmings (SSWs) on the ionosphere based on a long-term observational record. The observations are based on the ionosonde at Okinawa, and cover the time period from 1972-2023, encompassing 29 SSW events. The ionosonde observations are used to determine the influence of SSWs on the solar semidiurnal tide. The authors additionally find enhancements in the diurnal, semidiurnal, and terdiurnal lunar tides. Although a number of previous studies have investigated the influence of SSWs on the ionosphere, they have primarily focused on case studies. The present study is based on 29 SSW events, and thus provides a statistical view of how SSWs impact the ionosphere. The results are therefore new and would be suitable for publication. However, prior to publication I believe that several aspects of the manuscript need to be clarified. Specific detailed comments are provided below.
1. In order to understand variability that is not related to SSWs, the authors perform a composite analysis of the variability in years without SSWs. Though I agree with the approach that the authors have taken, I believe that they should provide additional details about the no SSW events. The no SSW years are referenced to a central date (e.g., Figures 3 and 4); however, in the absence of a SSW it is not clear how the central date is determined. The authors should provide additional details on how the central date is defined for the no SSW cases. Additionally, for scientific reproducibility, I suggest that the authors include a listing of the years that are used for the no SSW cases.
2. In lines 123-124, the authors state that “we start with foF2 since the composite shows a clearer effect than for NmF2”. Since foF2 and NmF2 are directly related to each other, one would expect that the results for foF2 and NmF2 are also similar, though perhaps scaled differently since NmF2 ~ foF2^2. Furthermore, it is unclear to the reviewer why having foF2 being the original measurement (lines 124-125) would impact the results since the conversion from foF2 to NmF2 does not introduce additional errors. The authors should provide additional explanation as to why the conversion from foF2 to NmF2 influences the results.
3. The authors state that “the SSW effect [in NmF2] is not so obvious as for foF2” (line 142). However, Figure 3 shows the absolute values of foF2 and Figure 4 shows the relative values for NmF2. It is therefore difficult to directly compare the results. Would the enhancements be similar if both are shown in either absolute or percentage variations?
4. In determining the results for Figure 5, do the authors consider the effects of geomagnetic activity? Although such effects may be small in the composite analysis, for any individual event it is possible that geomagnetic effects could large. It is recommended that the authors consider only using geomagnetically quiet days when determining the maximum amplitude in any given year.

Citation: https://doi.org/10.5194/egusphere-2024-485-RC1
- AC2: 'Reply on RC1', Klemens Hocke, 22 Mar 2024
  
  Thank you for the positive comments and your improvements!
  Point 1: We agree and we will provide tables of the central dates for the selected 29 SSW events and the 29 no SSW events in the revised manuscript. The central dates of the no SSW events are selected arbitrarily between December and March. Often we took month and day of a true SSW event and shifted it to a no SSW year. We tried to keep the distribution of the no SSW events similar to those of the SSW events. Thanks to your comment, we found a small error in the no SSW list since we used one date two times. As a consequence , we removed this shortcoming and replotted the figures new. The changes are small in the new version. We will add one figure with the composite of the O1 amplitude in the new version.
  Point 2: The different shapes of the composite curves of foF2 and NmF2 have been a puzzle for us, and we controlled our programs with a lot of effort. In so far, we are sure that this result is robust and true. In the revised manuscript, we explain the different composite curves by the following argument. "The difference is possibly due to the fact that the average of a series of numbers $(foF2)_i$ ($i$ is number of SSW event) has a different behaviour than the average of the corresponding series of quadratic numbers $(foF2^2)_i$ in the composite of $NmF2$. Thus, the shapes of the composite curves are different for $foF2$ and $NmF2$.
  Point 3: Please find attached the composite of the absolute NmF2 values. The peak is not so strong as in the composite of foF2.
  Point 4: We agree. In the revised manuscript we will indicate the SSW events with high geomagnetic activity (Kp > 4). There is only one true SSW event with high Kp value (31.12.1984). Interestingly, this event showed the maximal amplification of the semidiurnal amplitude of NmF2. There are three events in the no SSW list but these cases showed low or moderate amplification of the semidiurnal amplitude of NmF2.
  
  Citation: https://doi.org/10.5194/egusphere-2024-485-AC2
RC2:
'Comment on egusphere-2024-485', Anonymous Referee #2, 17 Mar 2024

The manuscript titled 'Influences of sudden stratospheric warmings on the ionosphere above Okinawa' by Hocke et al. provides a fresh take on the impact of SSWs on the ionosphere. The past 15 years of research on this topic has mostly focused on the SSW effects at low-latitudes, with few studies reporting on the effects at mid- and high-latitudes. This observational work is noteworthy in the sense that it is analyzing multiple SSW events and provides statistical results of the SSW effects at Okinawa through composite analysis of NmF2 and foF2. The authors have made a dexterous use of the long-term NmF2 and foF2 observations at Okinawa in this study and that should be commended. Of particular interest, is the analysis of the lunar tides in NmF2 and the new discovery of terdiurnal lunar tidal enhancement during SSWs is particularly a novel finding. This manuscript provides a lot of new results that will be of great interest to the aeronomy community. I recommend that the paper should be accepted after a minor revision mainly due to its novel findings.
Minor comments:
Line 18: Please be more specific with regards to the reversal of the zonal mean zonal wind of the stratospheric polar vortex.
Line 43: ‘latitudes’
Line 56: ‘whether’
Line 82: bracket is missing in the citation of McInturff (1978)
Line 141: The authors mention that the effect of SSW is not so obvious in NmF2 as for fof2. It will be better if the authors could elaborate on the reason behind this statement as it is not discussed anywhere later in the manuscript.

Citation: https://doi.org/10.5194/egusphere-2024-485-RC2
- AC1: 'Reply on RC2', Klemens Hocke, 22 Mar 2024
  
  Thank you for your positive review and the minor corrections which we will include all in the revised manuscript! The different shapes of the composite curves of foF2 and NmF2 have been a puzzle for us, and we controlled our programs with a lot of effort. In so far, we are sure that this result is robust and true. In the revised manuscript, we explain the different composite curves by the following argument. "The difference is possibly due to the fact that the average of a series of numbers $(foF2)_i$ ($i$ is number of SSW event) has a different behaviour than the average of the corresponding series of quadratic numbers $(foF2^2$)$_i$ in the composite of $NmF2$. Thus, the shapes of the composite curves are different for $foF2$ and $NmF2$.
  
  Citation: https://doi.org/10.5194/egusphere-2024-485-AC1

Status: closed

RC1:
'Comment on egusphere-2024-485', Anonymous Referee #1, 14 Mar 2024

Review of “Influences of sudden stratospheric warmings on the ionosphere above Okinawa” by Hocke et al.
The manuscript presents results of the impact of sudden stratospheric warmings (SSWs) on the ionosphere based on a long-term observational record. The observations are based on the ionosonde at Okinawa, and cover the time period from 1972-2023, encompassing 29 SSW events. The ionosonde observations are used to determine the influence of SSWs on the solar semidiurnal tide. The authors additionally find enhancements in the diurnal, semidiurnal, and terdiurnal lunar tides. Although a number of previous studies have investigated the influence of SSWs on the ionosphere, they have primarily focused on case studies. The present study is based on 29 SSW events, and thus provides a statistical view of how SSWs impact the ionosphere. The results are therefore new and would be suitable for publication. However, prior to publication I believe that several aspects of the manuscript need to be clarified. Specific detailed comments are provided below.
1. In order to understand variability that is not related to SSWs, the authors perform a composite analysis of the variability in years without SSWs. Though I agree with the approach that the authors have taken, I believe that they should provide additional details about the no SSW events. The no SSW years are referenced to a central date (e.g., Figures 3 and 4); however, in the absence of a SSW it is not clear how the central date is determined. The authors should provide additional details on how the central date is defined for the no SSW cases. Additionally, for scientific reproducibility, I suggest that the authors include a listing of the years that are used for the no SSW cases.
2. In lines 123-124, the authors state that “we start with foF2 since the composite shows a clearer effect than for NmF2”. Since foF2 and NmF2 are directly related to each other, one would expect that the results for foF2 and NmF2 are also similar, though perhaps scaled differently since NmF2 ~ foF2^2. Furthermore, it is unclear to the reviewer why having foF2 being the original measurement (lines 124-125) would impact the results since the conversion from foF2 to NmF2 does not introduce additional errors. The authors should provide additional explanation as to why the conversion from foF2 to NmF2 influences the results.
3. The authors state that “the SSW effect [in NmF2] is not so obvious as for foF2” (line 142). However, Figure 3 shows the absolute values of foF2 and Figure 4 shows the relative values for NmF2. It is therefore difficult to directly compare the results. Would the enhancements be similar if both are shown in either absolute or percentage variations?
4. In determining the results for Figure 5, do the authors consider the effects of geomagnetic activity? Although such effects may be small in the composite analysis, for any individual event it is possible that geomagnetic effects could large. It is recommended that the authors consider only using geomagnetically quiet days when determining the maximum amplitude in any given year.

Citation: https://doi.org/10.5194/egusphere-2024-485-RC1
- AC2: 'Reply on RC1', Klemens Hocke, 22 Mar 2024
  
  Thank you for the positive comments and your improvements!
  Point 1: We agree and we will provide tables of the central dates for the selected 29 SSW events and the 29 no SSW events in the revised manuscript. The central dates of the no SSW events are selected arbitrarily between December and March. Often we took month and day of a true SSW event and shifted it to a no SSW year. We tried to keep the distribution of the no SSW events similar to those of the SSW events. Thanks to your comment, we found a small error in the no SSW list since we used one date two times. As a consequence , we removed this shortcoming and replotted the figures new. The changes are small in the new version. We will add one figure with the composite of the O1 amplitude in the new version.
  Point 2: The different shapes of the composite curves of foF2 and NmF2 have been a puzzle for us, and we controlled our programs with a lot of effort. In so far, we are sure that this result is robust and true. In the revised manuscript, we explain the different composite curves by the following argument. "The difference is possibly due to the fact that the average of a series of numbers $(foF2)_i$ ($i$ is number of SSW event) has a different behaviour than the average of the corresponding series of quadratic numbers $(foF2^2)_i$ in the composite of $NmF2$. Thus, the shapes of the composite curves are different for $foF2$ and $NmF2$.
  Point 3: Please find attached the composite of the absolute NmF2 values. The peak is not so strong as in the composite of foF2.
  Point 4: We agree. In the revised manuscript we will indicate the SSW events with high geomagnetic activity (Kp > 4). There is only one true SSW event with high Kp value (31.12.1984). Interestingly, this event showed the maximal amplification of the semidiurnal amplitude of NmF2. There are three events in the no SSW list but these cases showed low or moderate amplification of the semidiurnal amplitude of NmF2.
  
  Citation: https://doi.org/10.5194/egusphere-2024-485-AC2
RC2:
'Comment on egusphere-2024-485', Anonymous Referee #2, 17 Mar 2024

The manuscript titled 'Influences of sudden stratospheric warmings on the ionosphere above Okinawa' by Hocke et al. provides a fresh take on the impact of SSWs on the ionosphere. The past 15 years of research on this topic has mostly focused on the SSW effects at low-latitudes, with few studies reporting on the effects at mid- and high-latitudes. This observational work is noteworthy in the sense that it is analyzing multiple SSW events and provides statistical results of the SSW effects at Okinawa through composite analysis of NmF2 and foF2. The authors have made a dexterous use of the long-term NmF2 and foF2 observations at Okinawa in this study and that should be commended. Of particular interest, is the analysis of the lunar tides in NmF2 and the new discovery of terdiurnal lunar tidal enhancement during SSWs is particularly a novel finding. This manuscript provides a lot of new results that will be of great interest to the aeronomy community. I recommend that the paper should be accepted after a minor revision mainly due to its novel findings.
Minor comments:
Line 18: Please be more specific with regards to the reversal of the zonal mean zonal wind of the stratospheric polar vortex.
Line 43: ‘latitudes’
Line 56: ‘whether’
Line 82: bracket is missing in the citation of McInturff (1978)
Line 141: The authors mention that the effect of SSW is not so obvious in NmF2 as for fof2. It will be better if the authors could elaborate on the reason behind this statement as it is not discussed anywhere later in the manuscript.

Citation: https://doi.org/10.5194/egusphere-2024-485-RC2
- AC1: 'Reply on RC2', Klemens Hocke, 22 Mar 2024
  
  Thank you for your positive review and the minor corrections which we will include all in the revised manuscript! The different shapes of the composite curves of foF2 and NmF2 have been a puzzle for us, and we controlled our programs with a lot of effort. In so far, we are sure that this result is robust and true. In the revised manuscript, we explain the different composite curves by the following argument. "The difference is possibly due to the fact that the average of a series of numbers $(foF2)_i$ ($i$ is number of SSW event) has a different behaviour than the average of the corresponding series of quadratic numbers $(foF2^2$)$_i$ in the composite of $NmF2$. Thus, the shapes of the composite curves are different for $foF2$ and $NmF2$.
  
  Citation: https://doi.org/10.5194/egusphere-2024-485-AC1

Klemens Hocke, Wenyue Wang, and Guanyi Ma

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Short summary

We find a clear SSW effect in the foF2 series at Okinawa. In the composite of 29 SSW events, the amplitude of the semidiurnal cycle of foF2 shows a peak at the SSW onset time in SSW years. For the first time, we find a lunar terdiurnal component with a relative amplitude of about 5 % in SSW years. The lunar diurnal and semidiurnal components have relative amplitudes of about 10 % in SSW years. The periods of the lunar ionospheric tidal variations are at the periods of ocean tides.


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