the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jun 2025
Analysis of the Mother’s Day Storm Effects on Equatorial Power Grids Near the Equatorial Electrojet Region
Abstract. Power transmission lines are susceptible to geomagnetic risks known as geomagnetically induced currents (GICs). These currents arise from rapid variations in the geomagnetic field at Earth’s surface, which induce geoelectric fields in the ground and drive GICs into the grounded neutral points of conductive infrastructure such as power transmission networks. In this study, we analyzed the behavior of two equatorial transmission lines —where the Equatorial Electrojet (EEJ) exerts a strong influence on electromagnetic variations —using measured current and voltage records from these lines, along with magnetic data from the geomagnetic superstorm of 10–11 May 2024, one of the most intense events of the past two decades. Magnetic observatories at Tatuoca (TTB), Kourou (KOU), and São Luís (SLZ) were selected to characterize regional field variations via the time derivative of the horizontal geomagnetic component (dH/dt). We then computed Pearson correlation coefficients between two distinct storm phases and the electrical parameters of the lines. The dH/dt proxy for GIC activity exceeded ±36 nT/min at all sites and peaked above 65 nT/min at TTB and SLZ. Strong to very strong correlations emerged during the storm’s initial and main phases (first period analized), while correlations weakened to moderate levels during recovery. These findings provide a solid foundation for future studies and inform the development of preventive measures by power-grid operators under intense geomagnetic activity.
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Status: open (until 04 Oct 2025)
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RC1: 'Comment on egusphere-2025-2381', Anonymous Referee #1, 26 Jun 2025
reply
General characteristics of the manuscript and general comments:
The manuscript assesses the impact of the most intense magnetic storm of recent years, which occurred on 10-11 May 2024, on the Brazilian electrical transmission system located in an area influenced by the equatorial electrojet. As the locations where the geomagnetically induced currents (GICs) threaten power grids, we generally consider regions on the Earth's surface at high geomagnetic latitudes. However, the authors stress in the study that during very intense storms, another current system may also cause large and rapid changes in the geomagnetic field at the Earth's surface, and they point to the equatorial electrojet.
Geomagnetic records from three Brazilian observatories – TTB, SLZ, and KOU – are studied. Two of the observatories (namely TTB and SLZ) are under a certain influence of the equatorial electrojet, and the third one (KOU) is a little more distant from this region. The authors rely on the literature published so far, from which they infer that when abrupt and large changes in the geomagnetic field occur in the magnetograms at TTB, SLZ, and KOU, the changes in the geomagnetic field under the electrojet are even more violent and cause significant GICs. This poses a threat to the local transmission lines.
To investigate the dependencies between electric currents and voltages in the power grid on the one hand and changes in the horizontal component of the geomagnetic field (aka the horizontal intensity) on the other hand, the authors used Pearson's correlation coefficient as a tool. The study separately investigated two sections of the magnetic storm: (1) the initial and main phases and (2) the recovery phase. In the initial and main phases, at all the investigated observatories, the rate of the horizontal intensity change exceeded 36 nT/min, a kind of threshold value for the formation of hazardous GICs. This suggested that changes in electric currents and voltages in the powerlines could be a consequence of GICs. Strong to very strong correlations were found between transmission line parameters (currents and voltages) and horizontal intensity in the initial and main phases of the storm. In the recovery phase, the correlations were less significant (moderate level). The study confirms the authors' assumption that, during a very intense storm, equatorial-electrojet-generated GICs might threaten the powerlines.
The study could be an interesting extension of the literature on GICs in the equatorial region. Regarding the English used in the manuscript, I appreciate that the manuscript is well-written.
Unfortunately, the manuscript lacks a more detailed description of power transmission lines and information about the studied electric current and voltage. Without more detailed information, the results of the study cannot be (or are difficult to be) understood. Moreover, I am not sure that the correlation between powerline current and horizontal intensity is interpreted correctly. Without comparing the time series of storm period current with the normal (quiet) daily time series of current intensity, it is not possible to rule out that the monotonic increase in the current during the main phase of the storm is not just a mere daily recurring deviation in powerline current. In such a case, the variation in the powerline current might be unrelated to the geomagnetic activity.
My conclusion: I suggest that the editor ask the authors for a major revision of the manuscript.
Major/specific comments:
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Figure 4 displays the time series of the currents and voltages for the TMAT-01 and TMAT-02 powerlines during the storm. What do the time series of these variables look like on typical quiet days in May? The authors should show graphs of these variables during typical quiet days.
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I ask this because the graphs for the currents appear to me as if most of the variation is just a kind of diurnal variation; the minima there repeat after 24 hours. If it was just the normal diurnal variation in the powerline current (and I suspect it probably was), then the significant negative correlation between the rising current and the drop in horizontal intensity (both changes happening within the main phase of the storm) is expected. And in that case, the increase in current may have nothing to do with the magnetic storm.
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When considering the typical daily voltage and current variations in powerlines, one might think about different conditions (e.g., consumption of the power) on the network during weekdays (workweek) and over the weekend. Please notice that 10 May 2024 was a workweek day (Friday) and 11 May 2024 was a Saturday.
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The description of the power transmission lines and the provided details of the parameters under investigation (i.e., electric current and voltage) are not sufficient. From the manuscript, the reader will not be able to form an idea of the object under study – the power transmission line. In connection with this comment, I have the following questions:
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The voltage values in Figures 6 to 11 are respectably large (usually, typical transmission line voltages tend to be roughly up to 1200 kV). Also, the electric currents are orders of magnitude larger than usual. Obviously, these are very powerful powerlines. I therefore assume that they are DC (direct current) transmission – it is more efficient over long distances compared to AC (alternating currents) transmission because there are fewer losses. Is that right or am I wrong? Please add some comments about such large voltages.
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Both the currents and voltages in Figure 4 are approximately two orders of magnitude smaller than in Figures 6 to 11. Why?
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Between what points is the voltage measured? Is it the voltage between the ends of the powerline? Or is it the difference in electric potential between the cable and ground? If it's DC transmission, I think it's the difference in electrical potentials between the ends. Am I wrong?
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I would recommend adding a picture, sort of a sketch of the situation, showing the orientation of the powerlines, the location of the power plant and substations, etc., and also the points between which the voltage was measured and where the electric current was measured.
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The authors should add some commentary on why the correlation is negative for line current and positive for line voltage. Is it related to the geometry of those powerlines? For example, does it depend on which side of that powerline is the power generation plant?
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The 10-11 May 2024 storm was the most intense in the last two decades. Did it cause any documented damage to the power grid studied? Or did this recent severe storm merely indicate that some subsequent and even more severe storm will indeed cause real damage?
Technical corrections and minor comments:
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The Em Dash (i.e., “—”) is used in many places in the manuscript. The authors have placed spaces before the Em Dashes everywhere, which is incorrect.
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Line 122: What is Hx component?
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In the captions of Figures 6, 7, and 8: it should be added that this is about the first section of the magnetic storm (i.e., the initial and main phases).
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In the captions of Figures 9, 10, and 11: it should be added that this is about the second section of the magnetic storm (i.e., the recovery phase).
Citation: https://doi.org/10.5194/egusphere-2025-2381-RC1 -
AC1: 'Reply on RC1', Éfren Mota de Souza, 07 Aug 2025
reply
We appreciate the reviewer’s comment and the opportunity to clarify the points raised in this work, which addresses the analysis of two power transmission lines during the major geomagnetic storm that occurred between May 10–11, 2024.
Below, we address each specific comment raised by the reviewer:
- Figure 4 shows the time series of current and voltage for the TMAT-01 and TMAT-02 Figure 4 displays the time series of the currents and voltages for the TMAT-01 and TMAT-02 powerlines during the storm. What do the time series of these variables look like on typical quiet days in May? The authors should show graphs of these variables during typical quiet days.
1. I ask this because the graphs for the currents appear to me as if most of the variation is just a kind of diurnal variation; the minima there repeat after 24 hours. If it was just the normal diurnal variation in the powerline current (and I suspect it probably was), then the significant negative correlation between the rising current and the drop in horizontal intensity (both changes happening within the main phase of the storm) is expected. And in that case, the increase in current may have nothing to do with the magnetic storm.
2. When considering the typical daily voltage and current variations in powerlines, one might think about different conditions (e.g., consumption of the power) on the network during weekdays (workweek) and over the weekend. Please notice that 10 May 2024 was a workweek day (Friday) and 11 May 2024 was a Saturday.
Response:
Indeed, due to limitations in the availability of data provided by the electric system operator responsible for the analyzed transmission lines, the records used in this study do not include geomagnetically quiet periods in a representative manner.However, in the next revision of the manuscript, based on the currently available data, we intend to include a time series of current and voltage parameters recorded in the hours preceding the onset of the geomagnetic disturbance. This series has already been prepared and will be incorporated into the revised version. This will allow the results and interpretations regarding the possible occurrence of GICs in the lines to be reassessed with greater caution, in order to adopt a more conservative stance in interpreting the findings.
- The description of the power transmission lines and the provided details of the parameters under investigation (i.e., electric current and voltage) are not sufficient. From the manuscript, the reader will not be able to form an idea of the object under study – the power transmission line. In connection with this comment, I have the following questions:
1. The voltage values in Figures 6 to 11 are respectably large (usually, typical transmission line voltages tend to be roughly up to 1200 kV). Also, the electric currents are orders of magnitude larger than usual. Obviously, these are very powerful powerlines. I therefore assume that they are DC (direct current) transmission – it is more efficient over long distances compared to AC (alternating currents) transmission because there are fewer losses. Is that right or am I wrong? Please add some comments about such large voltages.
Response:
We deeply appreciate your attention to this point. Indeed, this is a specific error related to the inversion of axes in the figure, which unfortunately went unnoticed during the review process. The correction is straightforward and will be promptly made in the next version of the manuscript. Thank you for this valuable contribution to improving the work.The transmission lines discussed, TMAT–01 and TMAT–02, operate on alternating current (AC) due to both physical and economic factors. In Brazil, the majority of transmission lines use AC, although there are also some DC lines.
2. Both the currents and voltages in Figure 4 are approximately two orders of magnitude smaller than in Figures 6 to 11. Why?
Response:
We thank you for your detailed observation. This error will also be corrected in the next version of the manuscript, as explained in the previous response.3. Between what points is the voltage measured? Is it the voltage between the ends of the powerline? Or is it the difference in electric potential between the cable and ground? If it's DC transmission, I think it's the difference in electrical potentials between the ends. Am I wrong?
Response:
For this particular study, data were made available from the devices responsible for adjusting current and voltage levels to match the operational parameters required by the grid operators. Regarding the voltage/current measurement points, we clarify that the electrical records are obtained at the substations to which the transmission lines are connected. Each line has four measurement devices—current transformers (CTs) and potential transformers (PTs)—equally distributed at both ends of each line. These devices allow for accurate monitoring of electrical parameters suitable for the analysis conducted. These details are described in lines 86–89 of the manuscript as follows:“They operate at the same voltage level (230 kV) and are equipped with comparable electrical components. Among these components are current transformers (CTs) and potential transformers (PTs), which are responsible for acquiring the operational measurements used in this study.”
We recognize that the manuscript lacks further elaboration on this point raised by the reviewer. This shortcoming will be addressed in the next version of the manuscript.
- I would recommend adding a picture, sort of a sketch of the situation, showing the orientation of the powerlines, the location of the power plant and substations, etc., and also the points between which the voltage was measured and where the electric current was measured.
Response:
Thank you for the suggestion. In the next version of the manuscript, we will incorporate a representative figure showing the measurement points for voltage and current in the transmission lines analyzed in this study.- The authors should add some commentary on why the correlation is negative for line current and positive for line voltage. Is it related to the geometry of those powerlines? For example, does it depend on which side of that powerline is the power generation plant?
Response:
This difference arises from the distinct variations observed in the time series of current and voltage compared to the variation in the H component of the geomagnetic field. By analyzing the series (Figure 4), such behaviors become evident. In a power transmission line, to transmit constant power, one must increase the voltage and consequently reduce the current (since power is proportional to their product). Therefore, fluctuations in these parameters directly affect the direction of the linear relationship between these variables and the H component during the analyzed periods.We acknowledge the absence of a more detailed explanation of this point in the current manuscript and will include such a discussion in the revised version.
- The 10-11 May 2024 storm was the most intense in the last two decades. Did it cause any documented damage to the power grid studied? Or did this recent severe storm merely indicate that some subsequent and even more severe storm will indeed cause real damage?
Response:
The storm did not cause any damage to the power grid analyzed. However, there are records of incidents within the network whose causes are not attributed to technical, accidental, or natural factors, and are thus classified as “undefined.” These reports were provided by the operators of the transmission lines addressed in this study. In this context, considering a potential geomagnetic origin in cases of anomalies observed during geomagnetic storms may help transmission line managers prevent damage to the electrical infrastructure.Technical corrections and minor comments:
- The Em Dash (i.e., “—”) is used in many places in the manuscript. The authors have placed spaces before the Em Dashes everywhere, which is incorrect.
Response:
Thank you for the correction. We will fix this and reduce the number of unnecessary occurrences of this punctuation mark.
- Line 122: What is Hx component?
Response:
The notation Hx used in this study refers to the horizontal component of the geomagnetic field, usually represented as X or Bx. The choice of this designation was made to emphasize the H component throughout the study. However, we acknowledge that this nomenclature was not adequately clarified in the manuscript and will correct this in the revised version.
- In the captions of Figures 6, 7, and 8: it should be added that this is about the first section of the magnetic storm (i.e., the initial and main phases).
Response:
Thank you for the suggestion. The correction will be implemented in the revised manuscript.
- In the captions of Figures 9, 10, and 11: it should be added that this is about the second section of the magnetic storm (i.e., the recovery phase).
Response:
Thank you for the suggestion. The correction will be implemented in the revised manuscript.
We will incorporate the reviewer’s comments during the final revision of the manuscript.
Sincerely,
Éfren MotaCitation: https://doi.org/10.5194/egusphere-2025-2381-AC1
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