the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 May 2026
Nocturnal variability of total electron content (TEC) at Koudougou (Burkina Faso) during three geomagnetic storms of solar cycle 24: implications for equatorial ionospheric irregularities
Abstract. This study analyses the nocturnal variability of GPS-derived total electron content (TEC) at Koudougou, Burkina Faso (12°15’N, 2°20’W), during three geomagnetic storms with Sudden Storm Commencement (SSC) that occurred during solar cycle 24 (22 June 2015, 14 July 2012 and 8 October 2013). The objective is to identify the geophysical conditions favourable to the generation or suppression of equatorial ionospheric irregularities. In this work, nocturnal variability is quantified by the hourly nocturnal standard deviation σ_TEC (20:00–05:00 LT) and by the relative perturbation δTEC (%). The reference TEC is constructed from the median of five geomagnetically quiet days (Kp < 2) of the month of each storm. A Superposed Epoch Analysis (SEA) is applied to the three events aligned at T = 0, corresponding to the SSC time. The storms of 22 June 2015 (SYM-H_min = −198.8 nT) and 14 July 2012 (SYM-H_min = −114.2 nT) are dominated by the negative phase (δTEC reaching −91.4 % and −99.5 % respectively), with σ_TEC classified as moderate to reduce during the main phase (3.38 and 3.79 TECU), characteristic of the disturbance Dynamo. In contrast, the storm of 8 October 2013 (SYM-H_min = −64 nT) exhibits a prolonged positive phase (δTEC_max = +77.6 %, 16 hours of positive phase) and an intense σ_TEC over the three analysed nights (5.0 to 7.9 TECU), the signature of a dominant Prompt Penetration Electric Field (PPEF). SEA of the three events reveals a transition from a brief median positive phase (T = −21 h, +29.7 %) to a prolonged median negative phase (T = +1 h to +34 h, minimum −80.3 % at T = +32 h). These results quantitatively highlight, over the West African sector, the generation/suppression duality of nocturnal ionospheric irregularities during disturbed periods, illustrating the antagonistic role of PPEF and the disturbance Dynamo.
- Preprint
(1756 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 03 Jul 2026)
-
RC1: 'Comment on egusphere-2026-2200', Anonymous Referee #1, 01 Jun 2026
reply
-
AC1: 'Reply on RC1', Kiswendsida Théophile Guissou, 11 Jun 2026
reply
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-2200/egusphere-2026-2200-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Kiswendsida Théophile Guissou, 11 Jun 2026
reply
-
RC2: 'Comment on egusphere-2026-2200', Anonymous Referee #2, 29 Jun 2026
reply
Reviewer Comments
Manuscript Title
Nocturnal variability of total electron content (TEC) at Koudougou (Burkina Faso) during three geomagnetic storms of solar cycle 24: Implications for equatorial ionospheric irregularities
General Comments
In other words, the title can be interpreted as follows:
How does the ionospheric total electron content (TEC) above Koudougou evolve during the night under the influence of three geomagnetic storms, and what do these variations reveal about the development of equatorial ionospheric irregularities that may disrupt GNSS systems and radio communications?
According to the authors, the main objectives of this work are to:
-
characterize the nighttime TEC response at Koudougou during geomagnetic storms;
-
identify the dominant electrodynamic mechanism (PPEF or disturbance dynamo) during SSC geomagnetic storms;
-
explain the physical factors responsible for the different behaviors observed during the three storm events.
What makes this study potentially original?
This work focuses on a poorly documented region, namely West Africa, and more specifically Koudougou (Burkina Faso). This region remains significantly less studied than Brazil or India with respect to the ionospheric response to geomagnetic storms. Consequently, high-quality GNSS observations from Koudougou represent an important regional contribution.
Most previous studies have investigated the overall TEC response during geomagnetic storms. In contrast, the present study specifically examines the nighttime evolution of TEC, particularly during the post-sunset period (20:00–05:00 LT), when equatorial plasma bubbles are most likely to develop. This addresses a more focused and scientifically relevant problem.
The principal scientific contribution of this work is that nighttime δTEC and σTEC together provide a practical diagnostic for distinguishing PPEF-dominated and disturbance dynamo-dominated electrodynamic regimes over the West African equatorial ionosphere, thereby providing indirect evidence of conditions that are either favorable or unfavorable for the development of equatorial ionospheric irregularities.
However, the manuscript requires substantial revision before it can be considered for publication.
Although the proposed interpretation is physically plausible, the manuscript never demonstrates that ionospheric irregularities actually occurred. Rather, the study identifies signatures that are consistent with the development or suppression of irregularities without directly observing them. This distinction should be clearly stated throughout the manuscript, particularly in the discussion and conclusion.
Furthermore, the title refers to "implications for equatorial ionospheric irregularities," yet the manuscript does not explain why σTEC can be considered a reliable indicator or proxy for ionospheric irregularities. The physical relationship between σTEC and plasma irregularities should be explicitly justified and supported by appropriate references.
The discussion devotes considerable attention to the respective roles of the PPEF and the disturbance dynamo. However, it does not sufficiently address the central scientific question raised by the title:
What have we learned about nighttime equatorial ionospheric irregularities over West Africa?
The discussion should place greater emphasis on the scientific implications of the observed nighttime TEC signatures for the occurrence or suppression of ionospheric irregularities rather than primarily describing the electrodynamic mechanisms.
In addition, the study is explicitly devoted to nighttime TEC variability, yet several analyses and figures extend over the entire 24-hour storm interval. Since equatorial plasma bubbles predominantly develop after sunset, the authors should justify why a full-day analysis is necessary or, preferably, focus their detailed interpretation on the nighttime period relevant to the stated objectives.
Finally, the conclusion is too brief. It should more clearly highlight the scientific contributions of the study, explain how the results advance our understanding of the nighttime equatorial ionosphere over West Africa, and explicitly answer the scientific question posed in the title.
Specific Comments
Typographical and Presentation Issues
The appropriate label in terme of LT is HH:MM not NNH as the authors did it in most of the figures
The x-axis labels are missing in Figures 3(a–e).
The figure captions should be revised and written in clear, standard scientific English.
The same comments apply to Figures 4 and 5, particularly Figure 5(a).
## Scientific Comments
### Lines 60–62
> "The vertical total electron content (VTEC) data come from the GPS receiver at Koudougou (dip: +8.24°), installed in December 2008 within the framework of the International Heliophysical Year (IHY) with the support of ENST-Bretagne. This station is part of the equatorial SCINDA network."
The authors state that the receiver used in this study was provided by ENST-Bretagne and that it is part of the SCINDA (Scintillation Network Decision Aid) network.
This statement is not entirely accurate. The SCINDA network was fully funded by the U.S. Air Force Research Laboratory (AFRL) through its partnership with Boston College. ENST-Bretagne contributed to the installation of some GPS receivers but was not responsible for the establishment or funding of the SCINDA network itself.
---
Magnetic Latitude and EIA
The manuscript states:
"Koudougou is in West Africa at geographic coordinates 12°15′N, 2°20′W. Its magnetic latitude, calculated with the IGRF-13 model, is approximately 2°N, placing it in the immediate vicinity of the foot of the northern crest of the equatorial ionization anomaly (EIA). The station is under the direct influence of the African equatorial electrojet, conferring on this position a high sensitivity to variations in the equatorial vertical E×B drift."
How can a station located at approximately 2°N magnetic latitude be considered to be in the vicinity of the northern crest of the Equatorial Ionization Anomaly (EIA) ? The northern EIA crest is typically located around ±15° magnetic latitude, which, in the African sector, corresponds approximately to the latitude of Tamanrasset (southern Algeria). This statement should therefore be revised and scientifically justified.
Suggestions for Improving the English
## Abstract
**Lines 15–18**
Current:
> "...with σTEC classified as moderate to reduce during the main phase..."
Suggested:
> "...with σTEC classified as moderate to low during the main phase."
---
**Lines 22–23**
Current:
> "...generation/suppression duality of nocturnal ionospheric irregularities..."
Suggested:
> "...dual role in the generation and suppression of nocturnal ionospheric irregularities."
---
**Lines 23–24**
Current:
> "...the antagonistic role of PPEF and the disturbance Dynamo."
Suggested:
> "...the antagonistic roles of PPEF and the disturbance dynamo."
---
## Introduction
**Line 43**
Current:
> "...PPEF and disturbance Dynamo signatures."
Suggested:
> "...PPEF and disturbance dynamo signatures."
---
**Lines 49–50**
Current:
> "...offers a geophysically particularly suitable position..."
Suggested:
> "...offers a particularly suitable geophysical location..."
---
**Lines 55–57**
Current:
> "...identify the dominant signature mechanism (PPEF and disturbance Dynamo)..."
Suggested:
> "...identify the dominant electrodynamic mechanism (PPEF or disturbance dynamo)..."
---
## Data and Methodology
**Lines 72–74**
Current:
> "...analyse the electrodynamic mechanisms responsible for ionospheric disturbances..."
Suggested:
> "...analyze the electrodynamic mechanisms responsible for ionospheric disturbances..."
---
**Lines 100–102**
Current:
> "...electron enhancement, typically associated with the PPEF..."
Suggested:
> "...an electron enhancement, typically associated with PPEF..."
---
## Results
**Lines 132–134**
Current:
> "...indicating that the Koudougou station presents strong nocturnal irregularity activity..."
Suggested:
> "...indicating that the Koudougou station exhibits strong nocturnal irregularity activity."
---
**Lines 139–140**
Current:
> "...These curves serve as quiet reference..."
Suggested:
> "...These curves serve as a quiet-time reference."
---
**Lines 151–152**
Current:
> "...consistent with suppression of ionospheric irregularities..."
Suggested:
> "...consistent with the suppression of ionospheric irregularities."
---
**Lines 162–164**
Current:
> "...during J+1 recovery..."
Suggested:
> "...during the recovery day (J+1)."
---
**Lines 174–176**
Current:
> "...reflecting persistent intense ionospheric irregularity activity, the signature of a dominant PPEF."
Suggested:
> "...reflecting persistent intense ionospheric irregularity activity, consistent with the influence of a dominant PPEF."
---
## Discussion
**Lines 202–203**
Current:
> "...this disturbing electric field..."
Suggested:
> "...this disturbance electric field..."
---
**Lines 220–222**
Current:
> "...without explicitly distinguishing PPEF and disturbance Dynamo regimes."
Suggested:
> "...without explicitly distinguishing PPEF and disturbance dynamo regimes."
---
## Conclusion
**Lines 227–229**
Current:
> "...dominated by the disturbance Dynamo..."
Suggested:
> "...dominated by the disturbance dynamo."
---
**Lines 232–233**
Current:
> "...illustrating the temporal succession of PPEF and disturbance Dynamo."
Suggested:
> "...illustrating the temporal succession of PPEF and the disturbance dynamo."
-
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 121 | 34 | 14 | 169 | 18 | 16 |
- HTML: 121
- PDF: 34
- XML: 14
- Total: 169
- BibTeX: 18
- EndNote: 16
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The author analyzes the TEC behavior measured at the equatorial station Koudougou, Burkina Faso, during three geomagnetic storms, with the objective to identify the geophysical conditions favorable to the generation or suppression of equatorial ionospheric irregularities. They use an index called nocturnal variability, which is quantified by the hourly TEC nocturnal standard deviation σ_TEC (20:00–05:00 LT). They also use a δTEC index, which was defined as the TEC perturbation in relation to the median of the five quietest days of the month. The subject is important and the lack of studies over the region justifies the authors’ effort to conduct the investigations. Nevertheless, there are several points in the manuscript that need to be better justified and clarified. It can not be accepted for publication in the present form.
The first point to be verified is the affirmation (lines 100-104) that a positive phase in TEC (electron enhancement) is typically associated with the PPEF, while a negative phase (depletion) is associated with the disturbance Dynamo or atmospheric composition changes [O/N₂]. Unfortunately, the interpretation is not that simple. The analysis of a possible influence of PPEF shall be made by carefully examining the time variation of the IMF Bz component. In a simplified view, a rapid southward turning of IMF Bz generates an undershielding electric field which penetrates to the low latitudes with the same polarity as the quiet time electric field (westward during the night). On the other hand, an abrupt northward turning of IMF Bz, after a prolonged southward orientation, may cause an overshielding electric field which can penetrate to the equator with polarity opposite to the quiet time electric field (eastward during the night). Besides the overshielding electric field, there is the disturbance dynamo electric field, which also has polarity opposite to the quiet time electric fields and last longer than the overshielding perturbations. Those disturbed electric fields will alter the electrodynamics at the equatorial region and the plasma distribution. The manuscript does not explain how those disturbances can alter the TEC and be responsible for the observed positive and/or negative phases.
Lines 160-164: The explanation given by the authors to the increase of σ_TEC to 5.86 TECU during J+1 recovery, suggesting a progressive return of the ascending E×B drift after the weakening of the disturbance can not be confirmed by looking at the data. The IMF Bz component does not display indication of PPEF or of an ascending EXB drift. In fact, during the long and steady MS recovery phase, IMF Bz remains always negative. No abrupt variation of IMF Bz is seen.
Another point worth noting is the use of the superposed epoch analysis (SEA) for this type of data. It seems to this reviewer that this technique is not appropriate for the analysis. The effect of the disturbed electrics field depends on factors such as the local time at which the storm started, its duration, if it was an isolated storm or if substorms had occurred, etc. I suggest suppressing this part of the manuscript, but part (b) of Figure 6 could be retained because it gives an integrated view of σ_TEC during the three storms.
The reference to the work of Fejer et al. (2008) on lines 200-202 is not appropriate to this context. Fejer et al. (2008) analyses only the “Quiet time equatorial F region vertical plasma drift model derived from ROCSAT-1 observations”. Furthermore, at night a disturbance Dynamo generates an eastward-directed electric field, not “a westward-directed nocturnal electric field” as stated in the manuscript.
The reference Tulasi Ram et al. (2014) on line 204 is not in the reference list. The paper listed was published in 2009 (wrongly listed as 2014); it deals with a distinct subject. I have found a paper from that group (Tulasi Ram et al., 2014) which also does not deal with plasma irregularities. These two examples of inappropriate references in the same paragraph raise a red flag regarding the accuracy of the references used in the manuscript.
Other points that need attention:
It is necessary to improve the resolution of the figures. It is impossible to read some of the text inside them. Moreover, the text on the figures should be in English.
Please check the consistency between the values of dip and magnetic latitude listed on lines 60 and 64 respectively.