| 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.
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.