Unexpected Regional Zonal Structures in Low Latitude Ionosphere Call for a High Longitudinal Resolution of the Global Ionospheric Maps

Round 1

Reviewer 1 Report

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Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Manuscript ID: remotesensing-1688661

Title: Unexpected Zonal Differences in Low Latitude Ionosphere Call for a Higher Longitudinal Resolution of the Global Ionospheric Maps

Authors: Libo Liu, Yuyan Yang, Huijun Le, Yiding Chen, Ruilong Zhang, Hui Zhang, Wenjie Sun, and Guozhu Li

In this study, authors report unexpected strong longitudinal structures in the low latitude ionosphere detected by the total electron content (TEC) observations of a network of Global Navigation Satellite System (GNSS) receivers in Asia.

In this form the manuscript remotesensing-1688661 cannot be accepted for publication on Remote Sensing, I therefore suggest some changes as follows;

As general remark I suggest authors to follow the structure of the manuscript as indicated in the instruction for authors of journal Remote Sensing, in particular a paper should be organised in the following sections:

•  Introduction.
• Materials and Methods.
• Results.
• Discussion.
• Conclusions.
• Patents.

It is, for example, very important for the readers to be able to distinguish between Results and Discussion.

As particular remarks;

• In the abstract, the sentence between lines 24-26 is not clear and must be reorganised.

• At line 92, I suggest authors to add a section 2. Materials and Methods and to renumber the following paragraph 2.1 Data Sources and TEC Estimation.

• At line 110 the acronym used should be renamed VTEC (vertical TEC) in place of TEC.

• At line 127, authors should explain/circumstantiate better the usage of a cut-off angle of 30° adding references.

• At line 146, authors must add a section 3. Results and renumber the following section 3.1 Zonal differences in TEC.

• At line 147, authors must renumber the section 14-16 December (DOY 348-350) 2019 as 3.1.1

• At line 202, in the Figure 3, authors should add the scale of distances (km) and the orientation bar (North, South, East, West).

• At line 213, authors must renumber the following section as: 3.1.2.

• At line 246, Figure 6 (as indicated for Figure 3) authors should add the scale of distances (km) and the orientation bar (North, South, East, West).

• At line 257, authors must renumber the following paragraph as 3.1.3.

• In the following section Discussion authors have indicated also the results, adding some in paragraphs.

• At line 426, the sentence “in this report” must be changed with “In this work” or something of similar, in fact this manuscript is structured as an article and not as a report.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

General Comments

In this work the authors reported cases of strong zonal gradients in TEC at low latitudes, over a narrow longitude and latitude range in the northern hemisphere of the Asian sector. They observed these gradients by analyzing data from the five geostationary satellites (GEO) of the Chinese BeiDou Navigation Satellite System. The authors investigated the possible contributors of these gradients, such as, solar and geophysical conditions, local time effect, equatorial electroject, and equatorial electric field. All the possible contributors were discarded. However, in terms of ionospheric response the authors observed lower hmF2 values associated to the time intervals of strong longitudinal TEC gradients. These longitudinal gradients were not clearly observed by the authors when analyzing the pictures of the Global Ionospheric Maps (GIMs). Thus, the authors mention that the smaller scale longitudinal structures with strong differences challenge the presentation of the current GIMs, and they suggest a finer spatial resolution being adopted in future products of GIMs. In the reviewer’s opinion, the paper has the potential to be published in Remote Sensing (RS), but there are still questions that need to be answered and other points to be fixed. For example, it is hard not to feel by finishing reading this paper a frustrating feeling that the authors have not presented the possible physical(s) drive(s) of these longitudinal gradients, even in a speculative manner. I really think the authors can do that in this case, since they have provided good results of the discarded contributors. They can discuss this in depth, pointing out the paths to be followed. For example, in the conclusions the authors mentioned “We look forward to observations of the thermospheric compositions and winds and electric fields with finer zonal resolutions providing clues on the physical drivers.”, but they have not provided those in the discussions. I believe the authors do not have simultaneous in-situ measurements to analyze these possibilities, however, if they have, so they should provide and analyze that in this work, and not in a future study. There is no reason to slice this study and does not make much sense to finish this work as main conclusion that GIM maps must improve longitudinal resolution.

Specific Comments

(1) Abstract

Page 1, line 17: The sentence in the first line of the abstract will sound better if the reason(s) for the growing interests is added. Otherwise, it can be displaced to other part of the abstract when the reader can easily associate such reason(s) for the “growing interests”.

Page 1, lines 24-26: The sentence starting as “The cases on December…” is understandable, but the English writing must be improved.

(2) Introduction

Page 2, lines 63-64: It is not clear what does mean “first several days”. Should it be something like “first days of the storm time period”? Does it include only the main phase of a geomagnetic storm? In the same line, the sentence starting as “During the following days,…”, in this case the “following days” here refers only to the days of the recovery phase of a geomagnetic storm? Rewrite the sentences to make it more understandable.

Page 2, lines 78-79: This part of the sentence “…when the wave-like longitudinal variations in the low latitude ionosphere act much stronger over the normal state.” is confusing in its present form. Rewrite sentence for clarity.

Page 2, line 89: Correct sentence as follows - “The cases studied in this work suggest that… capture such horizontal structures.”.

(3) Data Sources and TEC Estimation

Page 3, line 103: Rewrite sentence, such as, “The distribution and location of the GNSS receivers can be found in the works of [27, 30].

Page 3, line 109: It sounds better as follows -  “For simplicity, here after we call the vertical TEC as TEC only.”

Page 3, lines 113-114: Rewrite sentence as follows – “The receiver and satellite differential code biases (DCBs) should be…”

Page 3, lines 116-129: I also recommend reading the Section “TEC and DCB estimation” from the work of Prol et al. (2018), GPS Solutions, at https://doi.org/10.1007/s10291-018-0701-6.

Page 3, line 139: Correct the verb to “investigating” instead of “investigated”.

Page 3, line 141: Complete sentence as suggested - “The CODE TEC GIMs and MIT TEC maps can be downloaded, respectively, from the FTP site…”.

(4) Zonal Differences in TEC

Page 4: Add a paragraph introducing Section 3 with a brief description of the main results to be presented. 

(5) 14-16 December (DOY 348-350) 2019

Page 4, Fig. 1: The date of 15-Dec is repeated twice in the top of two different panels. Missing 16-Dec in the plot.

Page 5, line 172: Rewrite sentence as follows – “…from non-GEO TEC observations under the situations with large plasma density gradients.”

Page 4, Fig. 1: It would be better if the values of the GEO IPPs and elevations for the GEO PRNs are summarized in a table instead of labeled inside the panels.

Page 4, line 161: Change “fantastic feature” for “striking feature”.

Page 4, lines 161-163: Improve the English writing in the entire sentence – “…on 15 December (DOY 161 349)…,especially those of C05.”

Page 6, Fig. 3: The smallest triangles are not visible in the map. Change the scale size of the triangle for a better representation of the TEC differences, even at those regions of smaller longitudinal differences in TEC. Also, the legend of the figure does not describe properly what is shown in the map.

(6) 14-16 March (DOY 74-76) 2020

Page 7, Fig. 4: The date of 15-Mar is repeated twice in the top of two different panels. Missing 16-Mar in the plot.

(7) Discussion / Patterns of Longitudinal Differences

Page 10, line 292: Correct the word “soma” to “some”.

Page 10, lines 295-296: Improve sentence as follows - “…occurred in the days of enhanced TEC values.”.

(8) Possible Contributors of Longitudinal Differences

Page 12, lines 332-334: The sentence starting as “TEC is expected to…” seems not to agree with that shown before between lines 293-296.

(9) Conclusions

The conclusions of the work have also to include some remarks related to the discarded contributors mentioned in Section 4.2.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear Authors of manuscript remotesensing-1688661,

This work completed successfully the revision process, and is now ready for publication on Remote Sensing.

Author Response

Thanks for your approval. We have checked and modified the English language and style. All changes are tracked based on the former version of the manuscript.

Best regards

Libo Liu

Reviewer 3 Report

General Comments

The authors corrected the manuscript as pointed out in the previous report. Figures and text were improved. They provided the rationale for not using in-situ measurement data, and in the present version the conclusions are well supported by the results. After reading the corrected version of this manuscript, I suggest accepting it after the minor adjustments as described below.

Page 3, line 114: Correct to Differential Code Biases instead of "Different" Code Biases.

Page 7, line 207: Write the initial of the word "Triangle" in lower case.

Page 19, line 571: The citation to Rideout and Coster (2006) is not numbered in the list of references.

Author Response

Thanks for providing those further valuable comments.

We accept all comments and have made necessary modifications. To improve the English presentation, we have invited a Pakistan to have a complete modification. All changes are tracked based on the former version of the manuscript.

Best regards

Libo Liu