the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Jan 2026
Ionospheric responses to the 14 October 2023 annular solar eclipse over Brazil: A case study of fixed-frequency isoline variations
Abstract. This study investigates the ionospheric response to the 14 October 2023 annular solar eclipse using data from stations deployed in the Brazilian equatorial and low latitudes. While most research focuses on total electron content (TEC) and ionization reduction, this work uniquely examines the temporal evolution of fixed-frequency isolines (3–8 MHz) to evaluate variations in the vertical motion and structural dynamics of the ionosphere. Observations were conducted at three stations with varying degrees of obscuration i.e., Araguatins (89 %), Jataí (55 %), and São José dos Campos (38 %). Results demonstrate a significant reduction in the altitude of iso-frequency lines at all stations following the onset of the partial eclipse, with the most pronounced effects occurring at Araguatins due to its proximity to the eclipse path. A time lag of approximately 1.5 hours was observed between maximum obscuration and the minimum altitude of the isolines. Interestingly, a slight initial increase in altitude was detected at all sites immediately after the start of the partial eclipse, likely due to plasma convergence toward the eclipse path driven by thermospheric cooling. The recovery phase was found to be latitudinally dependent as well, Araguatins (equatorial) recovered faster than the low-latitude stations, a phenomenon attributed to the interplay between the equatorial fountain effect and eclipse-induced atmospheric pressure gradients. These findings highlight that even an annular eclipse with partial obscuration significantly disturbs the ionospheric dynamics over 1,500 km away from the central shadow path, emphasizing the high sensitivity of the Earth's ionosphere to transient solar occultation.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Annales Geophysicae.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-6570', Anonymous Referee #1, 08 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2025-6570/egusphere-2025-6570-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2025-6570-RC1 -
AC1: 'Reply on RC1', Igo Paulino, 12 May 2026
Referee: This manuscript presents a case study of ionospheric altitude variations during the October 2023 annular solar eclipse using observations from three Brazilian stations—Araguatins, Jataí, and São José dos Campos—experiencing different levels of solar obscuration (89%, 55%, and 38%, respectively). The focus on the temporal evolution of fixed-frequency isolines (3–8 MHz) provides a novel and useful approach for examining vertical motion and structural changes in the ionosphere. The analysis reveals altitude reductions, time lags, and latitude-dependent recovery processes. The study is timely and well-motivated, particularly in the context of eclipse impacts on the equatorial and low-latitude ionosphere. The data analysis is generally sound, and the figures illustrating isoline dynamics (Figs. 2–5) are clear and informative. However, minor revisions are required to improve quantitative rigor, resolve ambiguities in physical interpretation, enhance figure clarity, and better contextualize the findings within the existing literature on this eclipse event. The following minor comments are offered to improve clarity, accuracy, and quantitative rigor.
Authors: The authors would like to thank the Referee #1 for their encouraging remarks and positive feedback. We appreciate the effort spent on this review and the guidance provided to improve the manuscript. All suggestions will be implemented and all concerns will be fully addressed in this revised version.
Referee: The paper would benefit from a brief paragraph at the end of the Introduction outlining the manuscript structure. For example: “This paper is organized as follows: Section 2, 3, 4 describe..."
Authors: We are grateful for this suggestion and agree with the ponto of view of the Referee #1. As suggested, this sentence will be added to the Introduction in the revised version of our paper.
Referee: Lines 97–98 state that the observed altitude reduction “can be either due to a decrease in ionization at higher altitudes or downward motion,” but the manuscript does not subsequently disambiguate between these mechanisms. The authors should clarify which interpretation is favored by the observations (or under what conditions each applies), supported by quantitative or physical arguments.
Authors: We fully agree with Reviewer #1 and sincerely appreciate this key observation. This is indeed a crucial point regarding the interpretation of our findings. Based on the data used, it is currently not possible to distinguish which specific mechanism is dominant or whether there is a competition between the two processes. In order to improve this point, we are currently analyzing the further data in an attempt to estimate the vertical plasma drift and identify potential differences compared to the control days. Additionally, we intend to conduct a more precise review of the existing literature to better support our discussion and improve this section in the final version of the manuscript.
Referee: The manuscript repeatedly states that an increase in the altitude of iso-frequencies occurred immediately after the start of the partial eclipse (Lines 9–10, 142–144, 194–196, 225–226). However, this claim is not uniformly supported by the figures. Inspection of Figures 2 (Araguatins), 3 (Jatai) and 4 (São José dos Campos) indicates that: - A discernible initial altitude increase is mainly evident in lower-frequency isolines (3–4 MHz, corresponding to lower ionospheric altitudes) at the onset of the partial eclipse; - Higher-frequency isolines (7–8 MHz, corresponding to higher altitudes), show little to no uplift and, in some cases (particularly at São José dos Campos), begin decreasing immediately at the onset of the partial eclipse. Revise the Abstract, Results, Discussion, and Conclusions to clearly state that the initial uplift is most pronounced at lower frequencies/lower altitudes, while higher-frequency isolines show limited or no increase and, in some cases, an immediate decrease. Quantify the altitude changes (in km) for the lower-frequency isolines.
Authors: Thank you Referee #1 for the important comments. We will ensure that all the suggested corrections will be addressed in our revised submission.
Referee: The manuscript states that a slight increase in the altitude of the 6 MHz isoline was observed at all stations immediately after the onset of the partial eclipse. However, Figure 5 reveals station- dependent behavior: -Araguatins: No clear initial increase; instead, a small dip followed by a gradual diurnal rise; - Jataí: All three daily curves show an upward inflection after eclipse onset. - São José dos Campos: A clear decrease immediately after eclipse onset, continuing beyond eclipse end. This discrepancy should be explicitly addressed in the text, and the statement generalized or revised accordingly.
Authors: Again, we agree with the Referee #1 and we would like to express our gratitude for detailed review. We will proceed to revise the manuscript accordingly, incorporating the suggested changes into the upcoming text.
Referee: Improve the color scheme of the isoline plots: the 3–8 MHz isolines should be shown using discrete, dark, and clearly contrasting colors to enhance readability and prevent ambiguity between adjacent isolines.
Authors: While we agree with Referee #1 regarding the color scheme used in these figures, it is important to note that these images are direct outputs from the software UDIDA, which imposes certain limitations for substantial modifications. Nevertheless, we are currently working on an enhanced version of these Figures that we believe will improve the visual quality.
Referee: - Line 8: Clarify location → “a time lag of approximately 1.5 hours at Araguatins”; - Lines 14–15: Rephrase to avoid confusion: “.. with obscuration as low as Sau Jose dos Campos (38%)”; - Line 28: Typo -> “enhacement” -> enhancement; -Line 50: Correct obscuration value for Jataí -> 55%, consistent with Abstract and Figure 6; Line 93: Briefly explain PRE mechanism; - Line 115: Date error -> 2023, not 2025; - Line 122: The sentence “A very slight progressing of the frequencies though the altitudes..” is unclear. Please rephrase for clarity.
Authors: Thank you for the suggestions. All the minor points were fixed.
Referee: - Line 154: Quantify illumination: at 89% obscuration, ~11% of the solar disk remains visible; - Lines 135–140: The following factual inconsistencies were noted between the text and Figure 5; - Line 135: The text states the response “occurred immediately following the maximum eclipse.” However, Figure 5 shows that the 6 MHz descent at Jataí begins roughly 30 minutes before maximum eclipse; - Line 136: The text states the altitude “remained lower until approximately 23:00 UTC.” Figure 5 indicates the Jataí 6 MHz isoline recovers much earlier, near 21:40 UTC. - Line 139: The text states the São José dos Campos behavior “resembles the Jatai case.” This is misleading; Figure 5 shows a distinctly different response, with a clearer and more immediate descent at São José dos Campos. - Line 140: The text states the altitude remained lower “until 22:30.” Figure 5 shows recovery occurring much earlier, near 21:20 UTC. Please verify these timings against Figure 5 and correct the text accordingly. - Lines 182–185: Address why responses at Jataí (55%) and São José (38%) are described as “similar” despite different obscuration levels.
Authors: Thank you very much for the suggestions. We have taken them all into account and are happy to implement them for the revised version.
Referee: The "Main Findings" listed in the Conclusion (Lines 220–226) contain a contradictory sequence. They state:1. "A reduction in the altitude ... was observed at all stations after the onset..."; 2. "An increase in the altitude ... was observed at all stations immediately after the commencement..." This phrasing incorrectly implies both a decrease and an increase occurred at the same time ("after the onset"). The observed sequence was: a brief initial increase (predominantly in lower-frequency isolines) at the onset, followed later by a more pronounced reduction at the peak of the eclipse. Please revise the listed findings to clarify this correct chronological order.
Authors: Yes, we will revise this part of the manuscript in accordance with the suggestions provided and we express our gratitude for the careful review of our work provided by the Referee #1.
Referee: Please provide quantitative estimates for the reported altitude changes to clarify the magnitude of the ionospheric response. Include approximate values (in km or as a percentage) for:1. The initial increase in altitude at onset (for lower-frequency isolines, e.g., 3–5 MHz) at each station; 2. The subsequent altitude decrease near maximum obscuration (for 3–8 MHz isolines) at each station. Example: “At Araguatins, the 3–4 MHz isolines increased by ~X km at onset, while the 3–8 MHz isolines decreased by ~Y km near maximum obscuration.” - Line 229: Maintain consistency -> use 38%, not 39%.
Authors: In the revised version of the manuscript, we will include estimated values for both altitude and percentages, as suggested. We agree with Referee #1 that incorporating these estimates significantly enhances the clarity and quality of the text. Thank you for the valuable feedback.
Referee: Line 232: “Data can be requested from…” is not compliant with open-science policies. Required actions: - Archive data in a public repository (e.g., CEDAR, Zenodo, Figshare, Pangaea); - Provide a DOI, or state that the data will be archived upon acceptance; Clarify availability of the UDIDA software (open-source repository link or access conditions).
Authors: Thank you for your suggestions. We have deposited the research data in the Zenodo repository and the corresponding access link will be provided in the revised version of this manuscript. Furthermore, additional details regarding the UDIDA software, including licensing terms and installation instructions, will be incorporated into the revised version of the manuscript.Citation: https://doi.org/10.5194/egusphere-2025-6570-AC1
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AC1: 'Reply on RC1', Igo Paulino, 12 May 2026
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RC2: 'Comment on egusphere-2025-6570', Anonymous Referee #2, 09 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2025-6570/egusphere-2025-6570-RC2-supplement.pdf
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AC3: 'Reply on RC2', Igo Paulino, 14 May 2026
Referee: This manuscript presents a case study of the ionospheric response to the 14 October 2023 annular solar eclipse using ionosonde observations from three locations in the Brazilian sector: Araguatins, Jataí, and São José dos Campos. This manuscript reports altitude variations in the iso-frequency observations from the three ionosonde stations during and after the eclipse. This study uses ionograms with 5-minute temporal resolution and fixed-frequency observations at 1.67 minutes, which is a very useful experimental setup for investigating eclipse-induced variations (height/density) in the ionosphere. The results are quite interesting and insightful, and have the potential to help the community better understand eclipse-induced ionospheric variations. However, in the current form, the manuscript contains some unclear interpretations of the results. Before publication, the reviewer wants the authors to revise the manuscript to improve clarity and the physical interpretation of the results shown in the figures.
Authors: The authors thank Referee #2 for the time dedicated to evaluating this study. The constructive comments and suggestions received are highly appreciated and will certainly be useful to refine the quality of the article.
Referee: Line 6: `Results demonstrate reduction in altitude…' In the line, the author reported are duction in altitude of the iso-frequency observation at all stations following the onset of the partial eclipse. For instance, what can I see from the figure 2, the lower frequencies shows increase in altuitude in the growth of the eclipse and in the recovery phase of the eclipse there is a decrease in altitude, but for the higher frequencies (7-8 MHz) there is a dip after the eclipse onset and then slight increase and a delayed dip in the recovery phase. So make it clear in your abstract and the discussion about these variations, as these variations are the key observation of the manuscript. And what is the plausible mechanism causing these variations?? For example, Barad and Sripathi (2023) reported both iso-height and iso-frequency variations during the 15 January 2010 annular solar eclipse, with a temporal resolution of 2 minutes, similar to this study.You can also find various literature on height/density variations during the eclipse. Kindly review these investigations and prepare a more thorough discussion.
Authors: We would like to thank you for your important comments. Since Reviewer #1 raised a similar concern, we are committed to reviewing the full text to ensure that all ambiguities and inconsistencies are removed. Furthermore, we appreciate the recommended literature. We will examine the suggested paper to use it as a support to incorporate improvements in the Discussion.
Referee: As the eclipse occurs in the local afternoon and the end is toward dusk, it would be very helpful for readers if you could add the sunset terminator to the plots as a reference for evening-time ionospheric variations.
Authors: Thank you for the suggestion, we will provide it in the revised version.
Referee: Could you please calculate the height variations during the eclipse period using the quiet-time base values, or the percentage change in height?
Authors: That is a significant suggestion, thank you. We are currently working on performing these calculations and intend to examine the results by comparing them with control days. If the findings regarding ionospheric altitudes are helpful for the interpretations, we may include them in the manuscript. However, we will certainly present these results in our formal response to the referees within the revised version of the paper.
Referee: If possible, could you show the EEJ variations to clearly interpret the role of electric fields during the eclipse and their impact on the EXB drift, causing these height variations during the eclipse passage?
Authors: We appreciate the pertinent suggestion from the Referee #2 and we will present the EEJ variation on the day of the eclipse, comparing it with quiet days. However, evaluating the possible effects of the EEJ on the complex dynamics of the F-region may not be so straightforward.Especially considering that the stations are all outside the influence of the EEJ. Nevertheless, we will do our best to introduce this discussion into the manuscript.
Referee: Can you quantify, for different stations, the actual time lag between the maximum obscuration and the maximum decrease in altitude of the iso-frequency observations to better understand the physical processes?
Authors: Yes, that is certainly possible. We will incorporate those changes and include them in the revised version as suggested. Thank you for the suggestion!
Referee: - Line 28 ‘and enhacement in’… correct i;. - Line 51 ‘approximately 70%’, …provide the correct obscuration; - In line 207 ‘? demonstrated’… correct the typo; - Line 229 ‘only 39%’…Give the actual value.
Authors: Thank you for correcting those minor points. We will fix all of them for the upcoming version of the manuscript.Citation: https://doi.org/10.5194/egusphere-2025-6570-AC3
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AC3: 'Reply on RC2', Igo Paulino, 14 May 2026
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RC3: 'Comment on egusphere-2025-6570', Anonymous Referee #3, 09 Feb 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2025-6570/egusphere-2025-6570-RC3-supplement.pdf
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AC2: 'Reply on RC3', Igo Paulino, 12 May 2026
Referee: This manuscript addresses a pressing issue: the study of the ionosphere's response to an annular solar eclipse. The study contains interesting observational results.
Authors: We are grateful to Referee #3 for the detailed feedback and constructive criticism provided. We are committed to preparing an improved version of the manuscript, ensuring that each concern of the referee is carefully addressed.
Referee: The figures should show the times of sunrise and sunset on Earth and in the ionosphere.
Authors: Thank you, we will incorporate this suggestion for the revised version of the manuscrit.
Referee: It is unclear whether the authors are using true or virtual heights.
Authors: Our apologies for that, we will provide this information for the upcoming version.
Referee: The work lacks summary tables for altitude increases/decreases, delay times of key processes, etc.
Authors: Thank you for this suggestion. Since Referee 1 also raised a point regarding the quantization of altitudes and percentages, we will incorporate a table into the manuscript as suggested to address both concerns.
Referee: What does the decrease in altitude after the eclipse reflect compared to control days? It might be more appropriate to use reference days instead of control days.
Authors: In the revised version, we will expand our discussion, which will certainly help address this important question for the readers. We agree with your suggestion and are happy to change the terminology as suggested. Thank you!
Referee: It is necessary to separate the effects associated with a decrease in electron density and variations in vertical drift velocity.
Authors: Thank you for your observation. We recognize that this is the main point to be addressed in the manuscript, as it was highlighted by all three referees. At the same time, it is essential to clarify the inherent limitations of the data regarding our ability to distinguish between these specific effects. We are currently exploring the use of foF2 data and vertical drift measurements in an effort to identify potential variations relative to the reference days. If we find any evidence that aids in the interpretation of the results, we will certainly incorporate those findings into the revised version of the paper.
Referee: There are no formulas describing the main effects. It would be helpful for the authors to study and consider some of the next papers in their review, as well as papers by other authors.
Authors: Thank you for these suggestions. We are currently evaluating the most effective way to incorporate mathematical formulas that can assist in the interpretation of our results, taking into account the specific characteristics of our dataset. We would like to thank you for the suggested references. We will review the papers and will certainly include some of them in our bibliography.
Referee: There is a contradiction in the conclusions 1 and 4. Altitude reduces at onset and increases at commencement. How is this possible?
Authors: Yes, thank you for this observation. We will fix it.
Referee: Please consider whether the terms “phase progression” and “recovery phase” are appropriate here, or if alternative terminology might be more precise.
Authors: Sure, thank you. We will definitely refine these terms. However, because this particular approach is not yet widely featured in the literature, maybe we can not find more usual terms.Citation: https://doi.org/10.5194/egusphere-2025-6570-AC2
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AC2: 'Reply on RC3', Igo Paulino, 12 May 2026
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