Ionospheric Variability over the Brazilian Equatorial Region during the Minima Solar Cycles 1996 and 2009: Comparison between Observational Data and the IRI Model

Round 1

Reviewer 1 Report

 

The authors present a revision of foF2 and hmF2 parameters over São Luis, Brazil, during solar minimum 23 (1996) and solar minimum 24 (2009). Also, they explore the influence of low solar flux over the approximations of the IRI model, which is very relevant to understand possible errors in the ionosphere models. The study performed is well presented and described; however, some suggestions are made to highlight the results.

 

Mayor comments

 

• Line 166. "Figure 2 summarizes the geomagnetic condition of the data distribution based on Kp index…." Please, explain in more detail the construction of these plots.

 

• Figure 2. Thinking about the nature of the variability of Kp, the geomagnetic disturbances can be caused by coronal mass ejections or corotating interaction regions. Thus, in your histogram of Figure 2a, the contribution of both large-scale events is mixed. However, it seems you want to correlate the low solar flux in 2009 with the low Kp activity (?), which appears incorrect since active regions contribute more to solar flux. I need help understanding this analysis's idea or something that needs to be added.

   

• Discussion. It is missing a connection with the geophysical conditions shown in Figures 1 and 2. These figures are only mentioned in Section 2; however, the discussion should include the interpretation of these figures. For instance, “The results presented here clearly revealed the impacts of the decrease in the level of solar extreme ultraviolet radiation in 2009…” could be “Based on the low level of solar extreme ultraviolet radiation in 2009 showed in Figure 1, we found an important impact of …”

   

• Discussion. The comparison between 1996 and 2009 is complicated because the number of days analyzed is significantly different. Could you discussed how it could be affect your results.

• The approximations of IRI look very similar in both years (from Figures 3 and 4), which could mean that IRI is using a basic calibration based on old data. What were your initial parameters in the IRI model to estimate hoF2 and foF2? It is crucial to describe possible differences with other enter parameters.

• Discussion, line 422. “In turn, Santos et al. (2022a) have shown that even a small variation in geomagnetic activity (represented by Kp index) can impact the parameters of height and frequency of intermediate layers over the Brazilian low latitude region (Cachoeria Paulista, 22.42° S; 45°W) during the solar minimum period of 2009. These results indicate that, although this period has been extremely calm, it is still sensitive to geomagnetic disturbances.” Here is missing a connection with the geophysical conditions shown in Figure 2. In this way, this figure takes place in your analysis. So, you can highlight this figure with something phrases in the discussion. For example, “… During this period, according to Figure 2, geomagnetic variations were present…”

• It is difficult to read the discussion. As was made in the results section, you can divide the discussion into three subsections related to the topics explored. For example: 1) Influence of solar flux over ionosphere; 2) Approximation of the IRI model during anomalous solar minimums; 3) Atmospheric tides activity.

• Figure 6. Why is there much difference between the polinomial fitting and the data? Explain more about the methods to obtain theses approximations, please.

 

 

Minor comments

 

• Figure 2. The notation in the text(1⁺) and in theplots (1°) are differents.

• Line 250. “the model critical frequency of the F2 layer …” could be better “the F2 model of critical frequency”

• Results, line 214. “Figure 3 also reveals that: i) The F layer rise at sunset due to the PRE in the equinoxes of 1996 was ~ 40 km higher than in 2009…” Please, indicate in your plot and/or text what hour corresponds to sunset.

 

Author Response

We are grateful to the referee #1 for the helpful and comments on the original version of our manuscript. Please, see attached our responses.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments

to manuscript “Ionospheric variability over Brazilian equatorial region during  the minima solar cycles 1996 and 2009: comparison between observational data and the IRI model” by A. M. Santos et al.

 The behavior of the Brazilian equatorial ionosphere during the solar minimum periods, 1996 and 2009 that cover the solar cycles 22/23 and 23/24 is investigated in the manuscript. The authors analyzed the variations in the critical frequency f₀F₂ of the F₂ layer and the height of its maximum h_mF₂ based on the digisond data. These variations were compared with the IRI model. Atmospheric tides were also analyzed.

In general, any observational data can contribute to the improvement of existing models. This is also true for the data discussed in this manuscript. On this basis, I can recommend the article for acceptance after a minor  revision.

1. Authors must correct grammatical mistakes. They can easily find them by carefully reading the manuscript.

2. In Figure 2, it is not clear which order polynomial was used for the polynomial approximation of the results. This can be indicated in the text.

3. The seasonal mean values of h_mF2 and f_oF₂ showed significant deviations between the corresponding values for 2009 and 1996. Maybe this is due, among other things, to the fact that the amount of data in 1996 is 2-4 times less than in 2009. What do the authors think about this?

4. In my opinion, the differences in drift speed of about 0.5-1.0 m/s on the equinoxes in 1996 and 2009 are too small to have an impact.

I think that taking into account these comments can improve the manuscript.

Author Response

We would like to acknowledge the comments given by the referee. We have carried out a revision of the manuscript taking into account all the referee’s comments and suggestions.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors investigated the hmF2 and foF2 parameters over the Brazilian equatorial region during the solar minima years of cycles 23 and 24 and also compared the results with the empirical IRI-2016 model to test the performance of the model. The parameters show the obviously lowered values during 2009 than the corresponding year 1996 with visible discrepancies between the observed and model outputs. The significant results of critical height (hmF2) and seasonal difference in PRE effects are also discussed in this work. From my observation, the work done in this manuscript is reasonable with the explanations presented appropriately. The discussion and conclusion sections are well presented. I have a few comments that the authors need to incorporate to qualify for acceptance in the revised version.

Comments:

1.      Line 58 The long‑term impact of solar activity on the ionosphere has been discussed by Mosses et al., (2022) [https://doi.org/10.1007/s12145-022-00863-y] over an African equatorial GNSS station which I found a more relevant work and should be cited after referring the results demonstrated in the article.

2.      Line 83 A full stop is expected before the sentence “Coley et al. (2010) discussed such contraction by comparing the observational data with the IRI model.”

3.      Line 138 I understand the authors have used the IRI-2016 model. However, there is a recent version of the model (IRI-2020) now available since February 2022. The authors should put a note why did not they refer the latest version in this study and refer the article mentioning the new additions in IRI-2020 (https://doi.org/10.1029/2022RG000792).

4.      In Figure 2, I do not understand the Kp index values in the x and Y axes labels. Please clarify. Also, the symbol Kp in the text should match that in the figure (KP)

 

5.      Figures 3 and 4 The authors mentioned local time variation of the parameters. I suggest adding a local time axis for better interpretation.

Author Response

We would like to thank the referee #3 for the comments and suggestions to revise our manuscript. 

Author Response File: Author Response.docx

Reviewer 4 Report

This paper describe the low latitude ionosphere in Brazil's response to the solar focring, geomagnetic forcing and the lower atmospheric forcing. They not only use the local observations such as digisonde, but also the IRI 2007 model. They found that the hmf2 and nmf2 in 2009 is lower than in 1996, except for summer. Also the PRE occurrence in 2009 is several minutes earlier than in 1996. It is anticipated that there is is differences between observation and models because the model is an empirical model. The paper is in good shape and well organized. It can be published after several minor aspects are solved.

 

For introduction, the author give almost all previous studies. However, they shall point out the motivation, what do these previous studies lack, and why the authors want to carry out this study, what advantages do they have over previous studies??

 

Line 419-426 here the author mention that some studies found the low latitude ionosphere is sentive to geomagnetic activity. They shall also acknowledge a recent study by Cai et al. 2021, who provide both thermosphere and ionosphere observations and simulations to found they can still be impacted by minor geomagnetic disturbances (Kp=1.7) during solar minimum

Cai, X., Burns, A. G., Wang, W., Qian, L., Pedatella, N., Coster, A., et al. (2021). Variations in thermosphere composition and ionosphere total electron content under “geomagnetically quiet” conditions at solar-minimum. Geophysical Research Letters, 48, e2021GL093300. https://doi.org/10.1029/2021GL093300

Author Response

We thank the reviewer # 4 for your comments. 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

 

2nd REVIEW

Manuscript ID: atmosphere-2043523

Title: Ionospheric variability over Brazilian equatorial region during the minima solar cycles 1996 and 2009: comparison between observational data and the IRI model

Author: ANGELA SANTOS et al.

 

The authors present a good improvement concerning the original manuscript. They have revised the original work, answering all the comments and requests. The study performed is well presented and described. However, I have some suggestions more.

 

Major comment

 

Figures 5 and 6. Use the same scale in all plots. It will help to simplify the comparison between FoF2 and hmF2 parameters at different hours.

 

Line 497. “Our results, for example, showed that the impacts of the tides on height and frequency parameters over the equatorial region were very expressive, especially, the peak height, which was strongly higher in 1996 than in 2009 for almost all the days analyzed.” Please, try to formalize the disscusion, avoid phrases such as “very expressive”. Also, your description requires more quantities or percentages to quantify the comparison between these parameters. This applies to the whole section.

 

Minor comments

 

Line 400. “… ∆foF3” Please, check this.

 

 

 

Author Response

We are grateful to referee #1 for all the comments and suggestions. Please, see attached our responses.

Best Regards,

Ângela Santos

Author Response File: Author Response.docx