Assessment of Polar Ionospheric Observations by VIPIR/Dynasonde at Jang Bogo Station, Antarctica: Part 1—Ionospheric Densities

Round 1

Reviewer 1 Report

The paper compares bottomside TEC and NmF2 measurements from a dynasonde operated at Jang Bogo Station (JBS), Antarctica with vertical GPS TEC measurements from a c-located GPS receiver. The authors describe limitations and sources of uncertainties in correlations. While the error sources put forward are not surprising or novel, the comparisons themselves provide value and are a good resource for understanding the behavior of the ionosphere in a dynamic polar region. The paper is well-written and insightful; however a number of clarifications are required to aid the narrative.

1. The quantification of the bottomside TEC (bTEC) from the dynasonde needs to be expanded since this is a key metric central to all the calculations. Specifically, it would be useful if the authors noted if this measurement is simply an integration of the ionospheric densities from a fit chapman profile around the peak reflection altitude? If so, one would assume that the correlation uncertainty would depend substantially on whether the hmF2 is close to the IPP chosen or not. The two follow-up questions related to this are:
   • What is the IPP chosen for the TEC analysis? Is it fixed at 350km or varied?
   • What are the statistics of the hmF2 or the peak reflection altitude versus the IPP chosen and do the differences correlate with the disagreements in correlation ?
2. Is GPS the only constellation used for TEC analysis ? How many constellations can the receiver record data from ? Presumably including GLONASS, Beidou and Galileo can increase the data volume? A sentence can be inserted in 2.2 about this.
3. Figure 4: Do the presented data correspond only to quiet days ? If so, how is the median filtering done to obtain the yellow line which appears continuous? If there are data gaps for the active days how is that accounted for in the median profile? Is the median taken only for a day? The authors are encouraged to insert a brief description about this
4. Figure 5: The yellow font for GPS TEC in the figure legend is very difficult to read. A different color is suggested. (Minor suggestion)
5. Have the authors considered analyzing the correlations only including data when the aurora boundary (maybe from DMSP data) does not expand to the IPP latitudes? This is admittedly an arduous exercise beyond the scope of the paper, but some comments about the validity of this technique in relation to future analysis can suffice for this paper. (Minor)
6. The authors discuss E-region events or aurora events as sources of uncertainties in the correlation (or lack thereof). The G-condition is another possible observation in ionosonde data, when the peak density is with the F1 peak and not the F2 peak. Lobzin and Pavlov (2001) published a preliminary analysis on the occurrence probability and it was indicated that the probability of occurrence maybe higher in higher latitudes or solar minima. What is the role of this error in the statistical analysis presented in the paper?

Author Response

Please see the attached file.

Author Response File: Author Response.docx

Reviewer 2 Report

1. In the introduction, the historical comparison between GPS and ground-based ionospheric sounding should be introduced.What is the novelty of this manuscript different with other similar studies ?
2. It is well known the observation of ionosonde has a good correlation with GNSS measurements, especially in quiet geomagnetic days. Therefore, it is not significant if the assessment only focuses on quiet geomagnetic days. I think the assessment during severe space events is valuable to conduct.
3. Weddell sea ionospheric anomaly is an important dynamic feature in Antarctic region. Both the ionospheric observations of GNSS and VIPIR should be used to validate this phenomenon.

Author Response

Please see the attached file.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper presents the analysis and observation of the polar ionosphere with Dynasonde analysis software at Jang Bogo Station (JBS), Antarctica. In the present study, an initial assessment of the JVD ionospheric densities is attempted by the comparison with GPS TEC measurements which are simultaneously obtained from the GPS receiver at JBS during the solar minimum period of 2017 – 2019. The results presented in the paper are of great interest for the study of polar regions, polar ionosphere and impact of different solar and geomagnetic factors on polar ionosphere. I would especially like to mention chapter 4 of the article, where the problems of determining ionosonde sounding are described in detail and exhaustive explanations of determining ionospheric parameters are given.

I recommend the manuscript for publication in the journal “Remote sensing” after minor change

1. Line 50-57. It is not clear to me why data on incoherent scattering radars in the northern hemisphere are given. If this is general data about ground-based observatories in the polar region of the northern hemisphere, then the data is insufficient. Perhaps it is better to focus on measuring equipment data in Antarctica and the high latitudes of the southern hemisphere. If it is necessary to give a general assessment of ground-based observatories in polar latitudes, then this issue should be considered in more detail, not only incoherent scattering radars, which are not mentioned anywhere else in the work.

2. Line 137. Change “1016” to 10 to the power of 16.
3. Line Abbreviation IPPs it is explained only in line 186. Please add decryption at the first mention in the text
4. Line By what formula was the correlation calculated? Need the formula for calculating the correlation.
5. 3. Solar activity during the study period is not presented and is not discussed in the text. Add F10.7 index for the days under study.
6. Line 267. Fig. 4. It is not clear which days of what year are shown in the figure. In the caption to the picture it is written that “during the period of 2017-2019”, however, in the figure itself, the DOY are from 1 to 365. Explain this point in the caption to the picture or in the text.
7. Lines 306, 314, 326, 333. Unlike the Kp planetary index, the F10.7 index is not dimensionless. F10.7 index units is s.f.u. (solar flux units). Add s.f.u. in the text.

Author Response

Please see the attached file.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments for author File: Comments.docx

Author Response

Dear Reviewer,

We already provided our answers for your first review comments.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have addressed all of my concerns. I suggest consideration for publication.

Author Response

Dear Reviewer,

Thank you for your valuable comments and suggestions and finally accepted our revised manuscript to be published on this journal.

Reviewer 2 Report

The quality of the revision improves significantly, the manuscript can be accepted in the present form.

Author Response

Dear Reviewer,

Thank you for your valuable comments and suggestions and finally accepted our revised manuscript to be published on this journal.

Reviewer 4 Report

Most of the responses provided by the authors are not satisfactory.
For example, in major comment 1, it was asked to mention how many quiet days per year are used and how frequently ELDI  noted during the quiet time condition.
The reviewers replied, "We used all data for low Kp conditions during 2017-2019, which should be the majority of the data during the period. It is hard to identify the ELDI profiles (NmE > NmF2) from Dynasonde measurements but can only be considered though the lower peak heights. However, since we used only for low Kp conditions and the ELDI mostly occurs during disturbed times, it should probably be excluded in the analysis most of the time. This was mentioned in the section 2.1 in the manuscript".
Since the title reads as an 'assessment' the detailed data information is essential, and the authors should make sure how the assessment has carried, and how much quantity of the samples was used?
I have raised a question about the ELDI because the authors often used this as one of the error factors in the discussion. So, naturally, they also should explain how frequently or how often they have seen this feature?

Similarly, they have a misunderstanding about the correlation analysis. To get a good or better correlation coefficient magnitude (in the present case density) the two data sets need not be the same, but their temporal variation should be similar.
For example, in this study, the correlation coefficient of NmF2 (density in one particular altitude) and VTEC (volume integration) is high. It does not mean their magnitudes are similar. It is just because of their temporal variations.
Comment 7, In my opinion, the section 4 is really not necessary for this study since the focus is only during the quiet condition, provide more details about the scientific and technical controlling factors of the TEC in Dynasonde and GPS-TEC in the polar region. In particular:
Reply by the authors: "The main objective of this study is the assessment of the observed data from the JVD and therefore it is meaningful to show the limitations of the JVD measurements".
Comment on the reply: Yes, I agree this is an assessment, but in the beginning the authors mentioned they focus on the quiet time, so it is more important to provide more details about the data quality and technical biases, scientific reasoning for the particular condition. If not, you should elaborate on both the condition (quiet and disturbed) and discuss the limitations of these situations. Since you have mentioned quiet time is the focus. Given that situation, how or what role does the disturbed condition have to explain the results?
7i) Reply by the authors, "In the polar winter ionosphere, the solar production is nearly absent, which reduces the F-region and topside ionospheric densities, but the energetic particle precipitations may increase the E-region density even during quiet times. These characteristics of the polar ionosphere may result in relatively small differences between VTEC and bTEC. Please see Line 261-268. However, there may also be some quality issues in measurements from both GPS receiver and JVD, particularly in winter."
Comment: this is the reason in comment 1 I asked about the ELDI occurrence during quiet time. But, it is not something difficult to quantify. If the E region density is enhanced, does not it contribute to the VTEC? That is an interesting point that should be looked at in detail.
7ii) I think the authors can look into the details of how many days the EPP (effect during quiet) was noted during the reported duration.

Author Response

Dear Reviewer,

We would like to apologize for not being satisfactory to respond to your earlier comments. we tried to answer to all your comments. Please see the attached file.

Author Response File: Author Response.docx