| 13 Nov 2025
Tracking Ionospheric Changes during Solar Eclipses: Concepción Historical Data
Abstract. Solar eclipses offer a unique natural experiment to probe ionospheric responses to sudden reductions in solar radiation. This study examines the Concepción (36.79° S, 73.03° W/Chillán (36.64° S, 71.99° W) ionosphere during 16 selected eclipses, out of 21 identified events between 1958 and 2024, using a historical ionogram dataset spanning several decades. Critical frequencies (foE, foF1, foF2) and virtual heights (h’E, h’F1, h’F2/F) were extracted from digitized and scaled ionograms to quantify eclipse-induced perturbations. Diurnal variations show typical dips in the E- and F1-layer critical frequencies, while F2-layer responses are more complex and variable. Regression analysis was performed exclusively on critical frequencies, revealing a nearly linear decrease of foE with increasing solar obscuration, whereas virtual heights exhibited inconsistent behavior due to neutral winds, plasma transport, and other dynamical processes. High-cadence observations, available for select events, provided further insight into short-term ionospheric variability. Only the 2 July 2019 and 14 December 2020 eclipse responses were previously published. The study highlights the rescue and standardization of historical ionograms, many originally on fragile or hazardous 35 mm film, emphasizing the scientific value of long-term datasets. Predictions for the 06 February 2027 eclipse indicate an expected foE decrease of ~0.7 MHz at Chillán, offering a timely opportunity to validate the regression models and assess predictive skill across solar cycle conditions.
Competing interests: Manuel Bravo is the guest editor of the special issue.
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.
I consider that this is a valuable work. On one side, it gathers many historical data, and this is something that in my opinion is extremely important since it implies data preservation, control and also their digitalization, which is a complex task considering that some of the records had flaws and damages. So, this is a key contribution of this work.
I also find it valuable the identification of the eclipses together with the ionosonde data for the corresponding periods. Even if most of them are not adequate for analyzing the effect of the eclipses on the ionosphere parameters as the authors intended to.
Another valuable aspect, is the analysis focusing on the effect on the critical frequencies (foE, foF1 and foF2) of the maximum darkness of each eclipse, instead of the effect of the different degrees of obscuration within a given eclipse. This is novel to me.
Based on all this, I consider that this work should be accepted for publication but it needs a major revision. Following are my comments and suggestions:
Major comments:
(1) In the Abstract it is written "Regression analysis was performed exclusively on critical frequencies, revealing a nearly linear decrease of foE with increasing solar obscuration"
This looks like you analyze the dependence with the obscuration along an eclipse. And this is not what is done in this work. Or at least what I understood. You analyze the dependence with the maximum obscuration of an eclipse.
I think this should be stated clearly in the abstract. In fact, this is the novelty of the work, in addition to the valuable work of collecting the data. Even though the results obtained, in my opinion, lack statistical significance, and a more clear explanation of the method and exclusion of events and data are needed.
(2) In the Introduction, the authors mention that the "purpose of this work is to characterize the response of the Concepción (36.79°S, 73.03°W) ionosphere under solar eclipse conditions over several solar cycles, so that its response can be associated with parameters such as the amount of darkness, season, time of day, etc., in order to predict the response for future eclipses." but you do not do all this because the limitations in the data.
In fact, the effect you analyze is of the maximum obscuration per eclipse over ionospheric parameters.
I do not see either the analysis of this effect in terms of time or solar cycle phase.
In Figure 4 I see all the four events selected without any discrimination.
I think that either you can mention that this was the initial purpose, before gathering the data, and explain then that the work is more like a compilation of data during eclipse events (which is very valuable, even if the data are not well suited for the analysis you do), or state from the beginning what is made in this work. And also mention clearly that you analyze not the effect of the obscuration degree within a given eclipse but of the effect of the maximum obscuration per eclipse.
I understand that this is not an easy task, since precisely, you have to gather data of many events, which are not so frequent.
(3) From 21 periods, only 4 are analyzed. Statistically this means something. First you discard those eclipses without data. But then you select 4 within those with data, which show an effect.
Those not included means something to your statistics. If you have for example 10 eclipses and only in 4 you detect the effect, the phenomenon detected has also a percentage of occurrence. Which in the case of being 4 to 10, it will be 40 %. With which the significance of your regression coefficients are even lower.
(4) I think that you choose one single point of foE, foF1 and foF2 for each of the four selected eclipses. Do you think that the time resolution of the data may have some influence? Since it is not the same if you choose an foE data, for example, from a time series with 5 minute resolution, than if you have a 15 minute resolution. Unless the variability of the percentage of darkness within an eclipse around the maximum darkness, varies very slowly. Please explain this.
(5) About Figure 4. What are the statistical significance of these regressions? You have very few points; only four.
(6) In the Conclusion: "This work analyzed the response of the Concepción/Chillán ionosphere to 16 selected solar eclipses ..." You did not analyze all them. You have gathered the data, and presented them. But you have analyzed only 4 events. Unless you discuss also the events which you do not include in the regression analysis.
I think that you should clearly mention which are your valid results, and also explain the difficulties on carrying a study like this.
For example, that even if you have the data, they have many times gaps which prevent the study, or due to the analysis has to be based on high temporal resolution data, the disturbances are more notorious and blur the eclipse effect, for example. I assume you may have more arguments.
Or may be I am wrong. So please, elaborate on this.
(7) A data availability section is missing, which I think is very important. You should say where are the data available, or at least how anyone can have access to them.
Minor comments:
(1) In the abstract: "Concepción (36.79°S, 73.03°W/Chillán (36.64°S, 71.99°W)" should be" Concepción (36.79°S, 73.03°W)/Chillán (36.64°S, 71.99°W)"
(2) h’F2/F ? Shouldn't it be just h'F2 ?
(3) Line 47: Smith and King, 1981; Bremer, 1992; Ortiz de Adler et al., 1997; Jarvis et al., 1998; Foppiano et al., 1999 ... The years should go between parenthesis.
(4) Line 91: "(1957–1994," should be "(1957–1994),"
(5) Line 101: "Software de Corrección de Ionogramas (SoCIo)". Which is it source ? or reference ?
(6) Figure 1: In the panel showing Eclipse on 13-11-2012 the dashed line indicating maximum darkness is missing, since it coincides with the End time. What does this mean? Why isn't there a period after maximum darkness of recovery?
(7) 18-3-87 is also missing the dashed line indicating maximum darkness. In this case I can see it coincides with the beginning. Why this happens?
(8) Figure 4: I think the the last panels correspond to delta F2. Please check.
(9) Line 181: Which are the four selected events? You should mention them clearly
(10) Line 206: "This degradation reflects the additional variability introduced by diurnal, seasonal, and solar cycle variations in baseline foE values" In the case of percentage values you can assume that you are kind of independent of hour or solar cycle phase, since you are dividing the difference in each parameter by the median value. So I would expect an effect when you consider the absolute difference, not the relative. Please explain.