Equatorial ionospheric plasma bubbles during intense geomagnetic storms of Solar Cycle 25

Imtiaz, Nadia; Calabia, Andres; Anoruo, Chukwuma; Zahid, Aqsa; Amory-Mazaudier, Christine; Adhikari, Binod

doi:10.5194/angeo-44-489-2026

Articles | Volume 44, issue 1

https://doi.org/10.5194/angeo-44-489-2026

© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/angeo-44-489-2026

© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 44, issue 1

Regular paper

|

23 Jun 2026

Regular paper |

| 23 Jun 2026

Equatorial ionospheric plasma bubbles during intense geomagnetic storms of Solar Cycle 25

Nadia Imtiaz, Andres Calabia, Chukwuma Anoruo, Aqsa Zahid, Christine Amory-Mazaudier, and Binod Adhikari

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Final revised paper (published on 23 Jun 2026)
Supplement to the final revised paper
Preprint (discussion started on 30 Jan 2025)
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Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-86', Ephrem Seba, 18 Mar 2025

Review: Nadia et al “Ionospheric Plasma Irregularities During Intense geomagnetic storms of Solar Cycle 25”

General comments:

This study investigates the characteristics of ionospheric plasma irregularities during intense geomagnetic storms in Solar Cycle 25, focusing on the role of Joule Heating (JH) and the Equatorial Ionization Anomaly (EIA). Using the Weimer 2005 model, Madrigal TEC maps, and ROTI (Rate of TEC Index) derived from GPS data, the authors analyze four major storms (March 2023, April 2023, November 2023, and May 2024). The findings reveal that JH exhibits significant interhemispheric asymmetries, with stronger heating in the northern hemisphere during some storms and the southern hemisphere during others. According to the paper, these asymmetries, along with storm-time electric fields and neutral winds, influence the formation of EIA structures (double crest, single crest, or merged) and the generation of post-sunset ionospheric irregularities. The document is a well written and structured document in general.

However, although the authors have made a commendable effort to investigate various aspects of geomagnetic storms and their connections to ionospheric plasma irregularities and the Equatorial Ionization Anomaly (EIA), it is surprising that they did not adequately highlight their important findings in the conclusion section, despite discussing them in detail in the Results and Discussion section. The current Conclusion is brief and fails to fully capture the depth and significance of the findings presented earlier. Also the paper lacks some physical explanations on causality relationships related to joule heating, suppressing of plasma irregularities and also lacks addressing the importance of local time in analyzing storm impacts on irregularities and EIA. Below, I outline major issues that the authors should address to strengthen the manuscript and enhance its scientific impact.
Major comments

The authors present ROTI plots in Universal Time (UT), which makes it difficult to interpret the local time dependence of irregularities. Since ionospheric phenomena like post-sunset irregularities are strongly influenced by local time, this choice limits the interpretability of the results. Local time (LT) plots would better visualize the storm-time impact and allow comparisons across different longitude sectors. Therefore, the authors should replot their ROTI in Local Time (LT), even though they provided LT information in table 1, to better capture the local time dependence of irregularities. This would allow them to identify whether irregularities are enhanced or suppressed during specific local times (e.g., post-sunset, post-midnight) and also correlate storm impacts with the local time of SSC and main phase onset. The authors also mention plasma irregularity suppression during certain geomagnetic storms (e.g., November 2023), this would be good in terms of local time discussions. Several studies identified the importance of the local time at which geomagnetic storms happen to explain their impact on the ionosphere. Please review these documents to gain a better understanding of the above, including discussions related to DDEF, PPEF, and the suppression of irregularities.

• Liu, J., Zhao, B., & Liu, L. (2010, March). Time delay and duration of ionospheric total electron content responses to geomagnetic disturbances. In Annales Geophysicae (Vol. 28, No. 3, pp. 795-805). Copernicus GmbH.

• Amaechi, P. O., Oyeyemi, E. O., & Akala, A. O. (2018). Geomagnetic storm effects on the occurrences of ionospheric irregularities over the African equatorial/low-latitude region. Advances in Space Research, 61(8), 2074-2090.

• Araujo-Pradere, E. A., Fuller-Rowell, T. J., Codrescu, M. V., & Bilitza, D. (2005). Characteristics of the ionospheric variability as a function of season, latitude, local time, and geomagnetic activity. Radio Science, 40(05), 1-15.

• Seba, E. B., & Nigussie, M. (2016). Investigating the effect of geomagnetic storm and equatorial electrojet on equatorial ionospheric irregularity over East African sector. Advances in space Research, 58(9), 1708-1719.

The authors attempt to link high-latitude Joule Heating (JH) with the Equatorial Ionization Anomaly (EIA), which is a low-latitude phenomenon (lines 240-250). While this is an interesting approach, the connection between these two regions is not clearly explained in the paper. The paper shows correlations between JH and EIA structures, but it does not establish a causal relationship. The authors should provide more evidence or references to support their claim that JH directly influences EIA and explicitly state the mechanism by which high-latitude JH influences low-latitude EIA. Do the authors propose that JH-driven thermospheric winds transport plasma or alter neutral composition, thereby affecting EIA structures? The authors observe interhemispheric asymmetries in JH (e.g., stronger JH in the northern hemisphere during the May 2024 storm). How do these asymmetries translate to differences in EIA structures or irregularities at low latitudes?
The authors' discussion on the relationship between EIA structures and ionospheric irregularities is a good starting point, but it lacks several critical elements that would make their findings more robust, insightful, and convincing. That is, the authors do not provide a detailed mechanistic explanation of how specific EIA structures (e.g., double crest, single crest, or merged) influence the formation or suppression of irregularities. For example: How do density gradients in the EIA directly affect the night time growth of Rayleigh-Taylor instabilities?

Please review the followings for further understanding:

• Luan, X. (2021). Equatorial ionization anomaly variations during geomagnetic storms. Ionosphere dynamics and applications, 301-312.

• Balan, N., Liu, L., & Le, H. (2018). A brief review of equatorial ionization anomaly and ionospheric irregularities. Earth and Planetary Physics, 2(4), 257-275.

• Seba, E. B., Nigussie, M., & Moldwin, M. B. (2018). The relationship between equatorial ionization anomaly and nighttime equatorial spread F in East Africa. Advances in Space Research, 62(7), 1737-1752.

• Aa, E., Chen, Y., & Luo, B. (2024). Dynamic expansion and merging of the equatorial ionization anomaly during the 10–11 May 2024 super geomagnetic storm. Remote Sensing, 16(22), 4290.
Suggestion on the conclusion section
Here are my suggested potential findings that should be included in the conclusion section and summarized in the abstract, along with clear physical explanations (the physical explanations in discussion and conclusion).

Equatorial Ionization Anomaly (EIA) Structures:

The study identifies distinct EIA structures (double crest, single crest, and merged) during different phases of geomagnetic storms. For example: Double crest EIA structures with strong plasma density gradients are associated with the generation of post-sunset ionospheric irregularities. Single crest or merged EIA structures, observed during the November 2023 storm, are linked to weaker plasma density gradients and suppressed irregularities . The poleward expansion of EIA crests during intense storms (e.g., May 2024) is driven by enhanced eastward PPEFs and external fountain effects

Impact of Prompt Penetration Electric Fields (PPEFs):

The study highlights the critical role of PPEFs in shaping ionospheric irregularities. For instance: Eastward PPEFs during the May 2024 storm caused a super fountain effect, leading to significant TEC variations and poleward shifting of EIA crests. Westward PPEFs were observed to suppress post-sunset irregularities, as seen during the November 2023 storm. The polarity and duration of PPEFs vary across different longitudes, influencing the spatial distribution of irregularities.

Geographical and Temporal Variations in Ionospheric Irregularities:

The study demonstrates that the occurrence and intensity of ionospheric irregularities vary significantly with longitude, local time, and geomagnetic activity. For example: Post-sunset irregularities were most pronounced in the American sector during the March 2023 and May 2024 storms. Post-midnight irregularities were suppressed across all sectors during the November 2023 storm, likely due to weaker electric fields and lower ionospheric conductivity. The Rate of TEC Index (ROTI) analysis reveals that irregularities are strongly influenced by storm-time drivers, such as enhanced vertical plasma drifts and density gradients.

And also the followings: Interhemispheric Asymmetries in Thermospheric and Ionospheric Responses, Storm-Specific Ionospheric Responses, Implications for Space Weather Forecasting

Citation: https://doi.org/10.5194/egusphere-2025-86-RC1
- AC1:
  'Reply on RC1', Nadia Imtiaz, 30 May 2025
  Dear Editor,
  Thank you for sending us the report on "Ionospheric Plasma Irregularities During Intense geomagnetic storms of Solar Cycle 25” manuscript No. egusphere-2025-86 submitted to Annales Geophysicae. We are very grateful to the Editor and anonymous referee for the time and efforts invested in reviewing our work. We have incorporated the general comment provided by one reviewer in this round.
  Response to Reviewer:
  Major comments:
  The authors present ROTI plots in Universal Time (UT), which makes it difficult to interpret the local time dependence of irregularities. Since ionospheric phenomena like post-sunset irregularities are strongly influenced by local time, this choice limits the interpretability of the results. Local time (LT) plots would better visualize the storm-time impact and allow comparisons across different longitude sectors. Therefore, the authors should replot their ROTI in Local Time (LT), even though they provided LT information in table 1, to better capture the local time dependence of irregularities. This would allow them to identify whether irregularities are enhanced or suppressed during specific local times (e.g., post-sunset, post-midnight) and also correlate storm impacts with the local time of SSC and main phase onset. The authors also mention plasma irregularity suppression during certain geomagnetic storms (e.g., November 2023), this would be good in terms of local time discussions. Several studies identified the importance of the local time at which geomagnetic storms happen to explain their impact on the ionosphere. Please review these documents to gain a better understanding of the above, including discussions related to DDEF, PPEF, and the suppression of irregularities.
  
  Liu, J., Zhao, B., & Liu, L. (2010, March). Time delay and duration of ionospheric total electron content responses to geomagnetic disturbances. In Annales Geophysicae (Vol. 28, No. 3, pp. 795-805). Copernicus GmbH.
  
  Amaechi, P. O., Oyeyemi, E. O., & Akala, A. O. (2018). Geomagnetic storm effects on the occurrences of ionospheric irregularities over the African equatorial/low-latitude region. Advances in Space Research, 61(8), 2074-2090.
  
  Araujo-Pradere, E. A., Fuller-Rowell, T. J., Codrescu, M. V., & Bilitza, D. (2005). Characteristics of the ionospheric variability as a function of season, latitude, local time, and geomagnetic activity. Radio Science, 40(05), 1-15.
  
  Seba, E. B., & Nigussie, M. (2016). Investigating the effect of geomagnetic storm and equatorial electrojet on equatorial ionospheric irregularity over East African sector. Advances in space Research, 58(9), 1708-1719.
  
  Reply: Following to the Referee’s report, we have plotted ROTI in Local Time (LT) to identify whether irregularities are enhanced or suppressed during specific local times (e.g., post-sunset, post-midnight) and also correlated storm impacts with the local time of SSC and main phase onset. Please see the revised version of Figures 8-10 and Table 3. For better understanding, the discussion on the basis of above literature has been added in the revised manuscript. Following discussion regarding local time dependence of the post-sunset ionospheric plasma irregularities during the storms have been added in the Results and Discussion section.
  
  The storm-time ring current is critical for both the development and suppression of equatorial ionospheric plasma irregularities. For instance, ionospheric plasma irregularities are generated if the greatest excursion of the ring current (minimum SYM-H index) occurs between midnight and postmidnight. However, the plasma irregularities are suppressed when the maximum excursion of the ring current occurs in the early afternoon (Aarons, 1991). Table 3 shows the LT of maximum excursion of ring current or MPE at each GNSS location. During geomagnetic storms in March, April, and May, the ring current has the greatest excursion after midnight in American sector, indicating the highest likelihood of plasma irregularities in the evenings following the storms’ main phases in this sector. For instance, the maximum ring current excursions at the GNSS stations BOGT, RIOP, and SANT (LT = UTC-5) occurred at approximately 00:21, 23:03, 11:54, and 21:14 LTs on March 24, April 24, November 5, and May 11, respectively. On the same days, the CORD station (LT= UTC-4) reported the highest excursions at 01:21, 00:03, 12:54, and 22:14 LT. Notably, the highest excursions during the storms of March, April, and May occurred predominantly during the night to midnight which is the most ideal time for the growth of post-sunset ionospheric plasma irregularities. During November storm, the maximum ring current excursion occurred at approximately 11:54, and 12:54 LTs which is the least favorable time for ionospheric plasma irregularities to occur. However, the intensity of post-sunset plasma irregularities varies between these storms due to complex interaction of important factors such as storm-time electric fields, meridional winds, background ionospheric conditions, local time and seasons. The largest excursion of the ring current in the November storm occurred before dusk, indicating that post-sunset plasma irregularities were unlikely during the main phase of this storm.
  The authors attempt to link high-latitude Joule Heating (JH) with the Equatorial Ionization Anomaly (EIA), which is a low-latitude phenomenon (lines 240-250). While this is an interesting approach, the connection between these two regions is not clearly explained in the paper. The paper shows correlations between JH and EIA structures, but it does not establish a causal relationship. The authors should provide more evidence or references to support their claim that JH directly influences EIA and explicitly state the mechanism by which high-latitude JH influences low-latitude EIA. Do the authors propose that JH-driven thermospheric winds transport plasma or alter neutral composition, thereby affecting EIA structures? The authors observe interhemispheric asymmetries in JH (e.g., stronger JH in the northern hemisphere during the May 2024 storm). How do these asymmetries translate to differences in EIA structures or irregularities at low latitudes?
  Reply: Joule heating has a considerable and primarily indirect effect on the post-sunset EIA during the main phase of a geomagnetic storms, which is mediated by storm-time neutral winds, thermospheric upwelling, and electric field modulation. Following to the Referee’s suggestions, we have improved the Results and Discussion section.
  
  The authors' discussion on the relationship between EIA structures and ionospheric irregularities is a good starting point, but it lacks several critical elements that would make their findings more robust, insightful, and convincing. That is, the authors do not provide a detailed mechanistic explanation of how specific EIA structures (e.g., double crest, single crest, or merged) influence the formation or suppression of irregularities. For example: How do density gradients in the EIA directly affect the night time growth of Rayleigh-Taylor instabilities? Please review the followings for further understanding:
  • Luan, X. (2021). Equatorial ionization anomaly variations during geomagnetic storms. Ionosphere dynamics and applications, 301-312. • Balan, N., Liu, L., & Le, H. (2018). A brief review of equatorial ionization anomaly and ionospheric irregularities. Earth and Planetary Physics, 2(4), 257-275. • Seba, E. B., Nigussie, M., & Moldwin, M. B. (2018). The relationship between equatorial ionization anomaly and nighttime equatorial spread F in East Africa. Advances in Space Research, 62(7), 1737-1752. • Aa, E., Chen, Y., & Luo, B. (2024). Dynamic expansion and merging of the equatorial ionization anomaly during the 10–11 May 2024 super geomagnetic storm. Remote Sensing, 16(22), 4290.
  Reply: Following to the Referee’s suggestion, we have added detailed discussion about EIA structures, density gradients in EIAs and their connection with ionospheric plasma irregularities. Please see Section 3.3.2 of the revised manuscript.
  Suggestion on the conclusion section:
  Here are my suggested potential findings that should be included in the conclusion section and summarized in the abstract, along with clear physical explanations (the physical explanations in discussion and conclusion).
  Reply: Following to the Referee’s report, we have improved the conclusion section as:
  
  The distribution and intensity of Joule heating during geomagnetic storms varied significantly with the storm’s magnitude, local time of occurrence, duration, and seasons. Among the four storms examined, the May storm (near the June solstice) exhibited the most intense post-sunset Joule heating, followed by the March (equinox), April (post-equinox), and November (fall transition) storms, with the latter showing the weakest activity. Nearly symmetric heating patterns were observed during equinoctial storms, whereas solstice storms led to marked asymmetries between hemispheres.
  
  Each storm produced distinct equatorial ionization anomaly (EIA) structures, ranging from single crests with filled troughs to uneven double crests with deep troughs. These configurations were generated by the interaction between storm-induced electric fields and equatorward neutral winds, both of which are modulated by the spatial and temporal characteristics of Joule heating. After sunset, equatorward meridional winds (southward in the Northern Hemisphere and northward in the Southern Hemisphere) can lift low-latitude plasma upward along magnetic field lines, resulting in pronounced double-crest EIAs. In contrast, hemispherically asymmetric winds may amplify the EIA in one hemisphere while suppressing it in the other, forming a single crest. When poleward winds dominate in both hemispheres, plasma uplift is inhibited, leading to weaker EIA development.
  
  The combined influence of storm-time electric fields and thermospheric winds plays a key role in shaping the EIA, elevating plasma to higher altitudes, and redistributing electron density across trough and crest regions. During the main phase of storms, frequent North/South flipping in the interplanetary magnetic field (IMF Bz) can generate both eastward and westward prompt penetration electric fields (PPEFs), resulting from undershielding and overshielding effects. These alternating electric fields drive upward and downward vertical plasma drifts, further altering EIA structure.
  
  In particular, EIAs with filled or deep troughs and steep vertical gradients in plasma density can create favorable conditions for the development of Rayleigh-Taylor instabilities, which in turn promote post-sunset ionospheric irregularities. The occurrence and intensity of these irregularities varied spatially and temporally across the storms. For example, greater irregularity activity was observed in the Northern Hemisphere during equinoctial storms, while Southern Hemisphere dominance was seen during the solstice storm. In contrast, suppression of post-sunset irregularities was evident over the American sector during the intense storm in the winter season. Overall, the evolution and strength of post-sunset ionospheric irregularities at equatorial and low latitudes are governed by the combined influence of EIA density gradients, vertical plasma drifts, Local Time of maximum ring current development, and seasons.
  
  These insights not only enhance our understanding of low-latitude ionospheric behavior but also provide valuable foundation for improving predictive models and mitigation strategies for satellite communication, navigation systems, and other technologies sensitive to ionospheric disturbances. Future research may focus on integrating multi-instrument observations with advanced model simulations to further unravel the complex electrodynamic processes governing storm-time ionospheric variability.
  Sincerely,
  Dr. Nadia Imtiaz.
  
  Citation: https://doi.org/10.5194/egusphere-2025-86-AC1
RC2:
'Comment on egusphere-2025-86', Samireddipalle Sripathi, 19 Aug 2025
Comments on the manuscript entitled ‘Ionospheric Plasma Irregularities During Intense geomagnetic storms of Solar Cycle 25’ authored by Nadia Imtiaz et al. (2025) to ANGEO (manuscript egusphere-2025-86).

General comments:
This study presents the characteristics of equatorial and low latitude ionosphere during four intense geomagnetic storms that occurred on March 23–25, 2023, April 23–25, 2023, November 4–7, 2023 and May 10–13, 2024 in the ascending phase of Solar Cycle 25 based Joule Heating (JH) rates at high latitudes. For this purpose, authors have utilized the Weimer 2005 Model simulations to analyse the JH patterns along with Madrigal TEC maps to identify the changes in the intensity, location, and symmetry of the equatorial ionization anomaly (EIA) during these intense geomagnetic storms. Equatorial/low-latitude ionospheric plasma irregularities at different longitudes under geomagnetically disturbed conditions are studied using the TEC derived ROTI measurements. While a strong JH is observed during the May 2024 storm in the main phase, the other storms have the following order from strong to weak: March 2023, April 2023, and November 2023. The authors have investigated especially the role of asymmetric Joule heating in the structuring of the EIA like double crest, single crest or merged. Authors have utilized these features to investigate the formation or suppression of equatorial plasma irregularities. It is suggested that the generation of these ionospheric plasma irregularities and their latitudinal distribution strongly dependent on EIA's density gradients and latitudinal density structure. For instance, while the double crest EIA structures with strong plasma density gradients playing an important role in the generation of post sunset ionospheric plasma irregularities during the main phases of these four geomagnetic storms, single crest or merged EIA structure didn’t favour the generation of ionospheric plasma irregularities. In addition, the role of storm-time penetration electric field in the structuring and seeding of ionospheric plasma irregularities have been investigated. Analysis of these storms by authors suggest that solar wind parameters, geomagnetic activity, JH, and PPEFs significantly influence ionospheric TEC variations, EIA crest formations, and post-sunset plasma irregularities. These investigations revealed that these storms highlighted the importance of inter-hemispheric asymmetries in JH that affected the distribution and magnitude of ionospheric irregularities. Finally, the paper concludes that the resulting change in the thermospheric winds and electric fields due to storm conditions can alter the EIA structures which will impact the plasma irregularities at equatorial and low latitudes.

While many of the results presented here are already reported, I believe discussions like role of single EIA, double or Merged EIA on plasma irregularities looks to be interesting though it is known that asymmetry of EIA can suppress the plasma bubbles due to thermospheric meridional winds. I am providing the comments to the authors. The manuscript may be considered for publication only after implementing the comments made in this review report by incorporating appropriate changes.
Specific comments
It is mentioned that JH is strong in May 2024 Mother's Day storm during main phase. Then it followed like this: March 2023>April 2023>November 2023. Cause of variation proposed: Change in thermosphere winds and electric fields due to storm conditions alter EIA structures. What is the role of thermospheric neutral density such as O/N2 ratio on the EIA single crest, double crests?

Whether authors have investigated positive/negative storms on the plasma irregularities

How the symmetry/asymmetry of EIA with single, double crests can influence the EPBs are not discussed. Please provide discussions.

What is the role of electric fields on the generation/suppression of plasma irregularities?

Why authors have chosen TEC maps between 18:00-00:00 UT for all the storm events in Figure-6? Since different storms have main phases at different UT times, why authors have chosen this time interval to study the plasma irregularities and their connection to EIA.

The plots shown in Figure-6 can be plotted as a line plots to show clear asymmetry in EIA. Also in the plot, map for 23:00 UT is not shown. Whether it is plotted like this only, please clarify.

Role of geomagnetic storms on plasma irregularities in the following papers: S. Sripathi and Ram Singh, A study on the response of the ionosphere to the three major space weather events of 2015 using meridional chain of ionosondes and GPS receivers over India, journal of Sun and Geosphere, DOI: 10.31401/SunGeo.2018.02.08, 2020

Similarly, the following paper discusses the role of geomagnetic storms and role of Es layers on the plasma irregularities. Please go through it. Singh, R., & Sripathi, S. (2020). A statistical study on the local time dependence of equatorial spread F (ESF) irregularities and their relation to low‐latitude Es layers under geomagnetic storms. Journal of Geophysical Research: Space Physics, 125, e2019JA027212. https://doi.org/10.1029/2019JA027212

It is said that the generation of ionospheric plasma irregularities and geographical distribution strongly dependent on EIA density gradients and general density structure. Please include the physics of their dependency.

It is mentioned that the storm-time Joule heating in high-latitude can cause atmospheric gravity waves (AGWs) to move equatorward, thus providing favourable conditions for formation equatorial/low-latitude ionospheric plasma irregularities. But it is under recovery phase mostly. But here authors are describing plasma irregularities in the main phase. Please clarify.

Double crest EIA structures with strong plasma density gradient play an important role in generation of post sunset plasma irregularities. Single crest or merged EIA does not favour the generation of irregularity. Please provide brief explanation for this. How the plasma irregularity generation is associated with EIA crest need to be mentioned clearly in the manuscript.

3.1 Structure of Equatorial Ionization Anomaly (EIA): “This merging and subsequent increase in TEC were driven by enhanced JH and geomagnetic activity”. The claim can be elaborated by explaining the variation of JH with EIA structure.

In general, PPEFs will affect the equatorial and low latitudes in the main phase of the storm, while Joule heating will affect the equatorial and low latitudes in the recovery phase of the storm through thermospheric wind circulation. However, here, authors are discussing the role of Joule heating during Main phase which I am not able to understand. Please clarify how the Joule Heating can impact the EIA crest in the main phase.

The EIA trends are visually explained in table-3 with JH values where pattern can be observed. However, there is no mention of physics of their connection with the JH. What is the relation between JH and EIA dynamics are not clearly mentioned. Please explain. Similarly, Table-3 doesn’t show any location where EIA crest is strong/weak. It has to be clearly mentioned.

3.2 Ionospheric Plasma Irregularities: The observations presented here under PPEFs and DDEFs and claims to have plasma irregularities. The proof/data/evidence/observation of the irregularities are needed to be included.

What is the role of atmospheric waves and tides on the variability of the PRE, EIA and plasma irregularities. It is known that equatorial electrodynamics plays significant role in day-to-day variability of the E X B drift and RT instability triggered plasma irregularities. So, this point needs to be addressed.

Authors have chosen these four storms without looking into the role of seasons on the EIA. Usually, it is standard practice to choose storms that fall in equinox. Please address this point.

If you want to study only 18:00-00:00 UT, please show this period with yellow highlight in the Figures-1-4 so that it is easy for the readers to follow it. Currently, authors have highlighted the main phase of the storm with ‘red’ rectangle box. In fact, in these figures, AE/AU/AL is missing.

In the introduction, the sentence starts with ‘Ionospheric effects on applications are generally minor in mid-latitudes…’ may be replaced with ‘Ionospheric effects are generally minor in mid-latitudes…’ as the sentence is not conveying message correctly.

"It is generally said that during storm time, it increases the daytime eastward electric field". E_total(t)=E_quiet(t)+E_PPEF(t). It can either increase or decrease the equatorial zonal electric field, depending on IMF Bz polarity and whether you are on the dayside or nightside. This equation can be included.

Correct ‘Disturbed Dynamo (DD)’ to ‘Disturbance Dynamo (DD)’
Citation: https://doi.org/10.5194/egusphere-2025-86-RC2
- AC2: 'Reply on RC2', Nadia Imtiaz, 15 Oct 2025
  
  Dear Respected Reviewer,
  
  Thank you for your time and valuable comments to improve our manuscript. We tried to reply your queries and modified the manuscript accordingly. Please see our response in the attachment.
  Best Regards
  
  Nadia.
  
  Citation: https://doi.org/10.5194/egusphere-2025-86-AC2
RC3:
'Comment on egusphere-2025-86', Anonymous Referee #3, 22 Aug 2025

This manuscript presents a comprehensive investigation of ionospheric plasma irregularities during intense geomagnetic storms of Solar Cycle 25. The authors adopted a multi-instrument approach, combining data from GNSS-based ROTI maps, Swarm satellite in-situ measurements, and

geomagnetic indices to characterize the spatial and temporal evolution of ionospheric irregularities. The study focuses on three major geomagnetic storms (October 2021, February 2022, and April 2023), offering a comparative analysis of their impacts on the equatorial and high-latitude ionosphere.

The methodology is robust, integrating ground-based and satellite observations with appropriate data processing techniques. The results are

clearly presented and highlight key features such as the hemispheric asymmetry of irregularities, the role of storm-time electric fields, and

the suppression or enhancement of equatorial plasma bubbles (EPBs) under different storm conditions.

General comments:

The manuscript is well-structured and addresses a timely and relevant topic in space weather and ionospheric physics. The use of multiple data

sources enhances the reliability of the findings, and the interpretation is generally sound and supported by the data. The study contributes

valuable insights into the behavior of ionospheric irregularities during geomagnetic storms in the early phase of Solar Cycle 25.

The paper is suitable for publication in Annales Geophysicae after revisions. Below are some suggestions to improve clarity, scientific

rigour, and presentation.

Methodology and presentation:

The methodology is appropriate, but more detail on the ROTI calculation parameters (e.g. time resolution, thresholding) would improve

reproducibility. Additionally, the criteria used to define the "intensity" of the selected storms beyond Dst values would worth being clarified.

On the other hand, figures are generally clear and informative. Some colour scales (e.g. in ROTI maps) could be better annotated for

readability. Also please consider highlighting the storm periods in figures for better visual context. Figure 6 could include a 23:00 UT map

or authors could clarify its absence.

And Table 3 should be accompanied by a discussion of the physics behind the observed JH patterns and their relation to EIA dynamics. Please

explain how PPEF vs. DD electric fields interact with EIA structures and influence RT instability and PRE (pre-reversal enhancement).

Results:

The interpretation of EPB suppression during the April 2023 storm is compelling. It would be useful to compare this with similar events in

previous solar cycles, if available and through this comparison, the selection of these specific storms could be justified.

The discussion of hemispheric asymmetry is also interesting. Could the authors elaborate on possible seasonal or longitudinal effects that might

contribute to this asymmetry?

Please sonsider discussing the potential implications of these findings for GNSS signal degradation or navigation systems.

References:

The references are appropriate and up to date. Consider adding a few recent studies on EPB dynamics during geomagnetic storms for broader

context and also the following:

Tahir, A., Wu, F., Shah, M., et al. (2024)

Multi-Instrument Observation of the Ionospheric Irregularities and Disturbances during the 23–24 March 2023 Geomagnetic Storm

Remote Sensing, 16(9), 1594.

https://doi.org/10.3390/rs16091594

This study uses GNSS, Swarm, Formosat-7/COSMIC-2, and Fabry-Perot data to analyse EPB dynamics and electric field variations during the March 2023 storm, offering a complementary perspective to the manuscript

Zakharenkova, I., Astafyeva, E., et al. (2023)

Development of the Storm-Induced Ionospheric Irregularities at Low and Mid-Latitudes

Space Weather, 21(3), e2021SW002891.

https://doi.org/10.1029/2021SW002891

Focuses on the latitudinal expansion of irregularities during geomagnetic storms, relevant for comparing with the inter-hemispheric asymmetries

discussed in the manuscript

Imtiaz, N., Calabia, A., et al. (2025)

Ionospheric Plasma Irregularities During Intense Geomagnetic Storms of Solar Cycle 25

EGUsphere Preprint

https://egusphere.copernicus.org/preprints/2025/egusphere-2025-86/

The manuscript under review itself, for citation within future work or cross-referencing.

Language and style:

The manuscript is generally well-written. A few minor grammatical issues and awkward phrasings should be corrected. For example,

- on page 2: "Storm-related variations in thermosphere/ionosphere parameters can dramatically" → "Storm-related variations in thermosphere/ionosphere coupling/conditions can dramatically"

- on page 2: "The EPF, the EIA, and the density gradients in the EIA should all be affected" → "The EPF, the EIA, and the density gradients in

the EIA are affected"

- on page 2: “Disturbed Dynamo (DD)” → “Disturbance Dynamo (DD)”.

- on page 3: "Huang et al. (2013) studied that the single onset in the TEC" → "Huang et al. (2013) showed that the single onset in the TEC"

- on page 5: "This article is organised as follows:" → "This papaer is organised as follows:"

- on page 7: "In order to achieve this, we investigate four geomagnetic storms" → "In order to gain these insights, we investigate four

geomagnetic storms".

Citation: https://doi.org/10.5194/egusphere-2025-86-RC3
- AC3: 'Reply on RC3', Nadia Imtiaz, 15 Oct 2025
  
  Dear Respected Reviewer,
  
  Thank You for your time and valuable feedback to improve our manuscript. We have tried to incorporate your suggestions in the manuscript. Please see our reply in the attachment.
  
  Best Regards
  
  Nadia.
  
  Citation: https://doi.org/10.5194/egusphere-2025-86-AC3

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Reconsider after major revisions (further review by editor and referees) (30 Oct 2025) by Dalia Buresova

Dear author, dear co-authors,

Thank you for deciding to publish the results of your research in Annales Geophysicae. Your manuscript has been reviewed, and the reviewers have recommended several useful additions and modifications that would improve its quality. Please carefully consider the recommendations and revise the article accordingly. Please highlight any changes you make to the upgraded version of the manuscript. The current state of the article requires substantial revision.

I would also like to add a few comments to the manuscript:
• The focus of your study is mainly on the equatorial ionosphere/EIA region. This fact should be reflected in the title of the manuscript.
• I cannot entirely agree with the statement in the Introduktion (lines 31-33) “Ionospheric effects on applications are generally minor in mid-latitudes, causing only small perturbations in signal propagation that may be roughly expected, especially during magnetic storms”. This depends mainly on the strength of the geomagnetic storm and the time at which the initial phase begins, as well as a number of other factors. During the main phase of strong storms, total absorption may occur alongside F-layer spread, sporadic opaque E-layer and high TID activity moving southwards from the auroral region. All of these factors can significantly affect signal propagation and cause inaccuracy in GNSS applications at mid-latitudes.
• What is the point of having information on the solar radio flux at 10.7 cm (F10.7) for analysis?
• I would like to see a more detailed assessment of how the intensity (and possibly seasonal dependencies and local time) of the analysed storms impacts ionospheric changes over equatorial regions. Of the storms analysed, one is a superstorm (10–13 May 2024), one is an intense storm (April 2023) and the remaining two are strong storms.
• The introduction section, which is 3.5 pages out of a total of 16 pages of text (which also contains images and tables), seems too extensive. On the contrary, the Conclusions are too brief and too general. Here, it would be appropriate to highlight the novelty of the analysis and avoid information that is already widely known.

Your edited manuscript will be reviewed by me and at least one of the reviewers.

Best regards

Yours cordially
D. Buresova

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AR by Nadia Imtiaz on behalf of the Authors (18 Nov 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (10 Dec 2025) by Dalia Buresova

RR by Anonymous Referee #4 (10 Apr 2026)

ED: Publish subject to revisions (further review by editor and referees) (24 Apr 2026) by Dalia Buresova

Dear Dr. Imtiaz, dear co-authors,

We have received the review from the referee selected for the second round of peer review. The referee has provided a number of comments to which you should respond and, after careful consideration, make the appropriate revisions to the text.
Please highlight the new revisions in the text and send us the revised version of your manuscript
The current status of your manuscript is "major revision."
For your convenience, the referee's comments are included below.

Kindest regards
D. Buresova

General Comments
The topic is relevant and timely. However, the manuscript requires substantial improvements in clarity,
organization, methodological description, and scientific justification. Several statements lack
quantitative support, key definitions are missing, and the linkage between results and conclusions
needs to be strengthened.
Specific Comments
Abstract
1. Statements such as “strong JH” and ranking of storms by JH strength (Lines 9–12) are
qualitative. Please provide quantitative estimates of JH and define thresholds used to classify
JH as strong or weak.
2. The conclusion in Line 13 regarding thermospheric winds and electric fields appears
premature, as supporting results (e.g., JH, EIA evolution, and ionospheric irregularities) are not
sufficiently presented in the abstract.
3. The abstract lacks key findings related to EIA morphology and ionospheric irregularities (e.g.,
ROTI behavior), as well as how JH influences these processes. These should be clearly
summarized.
Introduction
4. In Lines 29, the term “ionospheric parameters” is vague. Please specify which parameters are
being referred to.
5. Define the IMF Bz component when first introduced.
6. Lines 27–46 contain several general statements that require appropriate references. Some of
these can be moved to the Discussion section. The introduction should focus more on the
study’s objectives rather than well-established concepts.
7. The statement in Line 31 regarding minor ionospheric effects at mid-latitudes requires
supporting references.
8. In Lines 43–44, references are needed for PPEF and dynamo-related electric fields. Also, clarify
whether “disrupted dynamo” refers to the disturbance dynamo.
9. Define IMF Bz clearly at its first occurrence (Line 44).
10.Ensure proper formatting of chemical species (e.g., N2) using subscripts throughout the
manuscript.
11.Lines 65–73 are repetitive and may be removed or condensed.
12.TEC should be defined at first use (Line 78).
13.The statement in Lines 81–83 describing the use of Madrigal TEC is misplaced and should be
moved to the final paragraph of the Introduction.
14.The term EPIs (Line 94) is not defined prior to use.
15.The statement in Lines 135–136 regarding PPEF observations is unclear. Please clarify whether
actual observations or model outputs are used.
Overall, the Introduction lacks coherence, contains redundant statements, misses key references, and
does not clearly articulate the research gap.
Datasets and Methodology
16.Justify the use of both 1-minute and hourly averaged solar wind parameters. Consider using a
consistent temporal resolution.
17.Provide the source/link for CCMC data.
18.Although ROTI derived from ground-based GNSS is presented (Figures 6–10), the data sources,
station details, and processing methods are not described in this section.
19.The methodology for analyzing global TEC maps (from the CEDAR Madrigal database) is
insufficient. Please clarify how EIA structures (single crest, double crest, merged crest) are
identified and quantified.
20.In Line 164, explain how geomagnetic coordinates were obtained (e.g., model used) and
provide appropriate references.
21.In Table 1, simplify the “Instrument” column to “GNSS” and revise formatting in the “Sector”
column for clarity.
22.In Table 2, revise “Sun spots (R)” to “Sunspot (R)”. Also, explain the criteria used to select quiet
days (Dst, Kp, AE, etc.).
23.The classification of weak and strong ionospheric irregularities based on ROTI (Lines 165–169)
should be supported with references.
Suggestion: Clearly separate this section into Data Sources (Madrigal TEC, GNSS TEC, CCMC, PPEF
model) and Method of Analysis.
Results
24.The time interval 18:00–00:00 UT does not necessarily correspond to “after sunset” across all
longitudes. Please clarify this.
25.In Figure 6:
• Clarify whether the solid lines represent the magnetic equator and provide references.
• Consider including ±15° magnetic latitude lines to better illustrate EIA structure.
• Adding a solar terminator would improve interpretation.
• Quantify statements such as “higher TEC values”.
Instead of relying solely on global maps, consider including latitudinal TEC profiles (preferably in
geomagnetic coordinates) to better illustrate EIA morphology and hemispheric asymmetry.
26.The claim that TEC enhancement is driven by JH (Line 69) is not sufficiently supported. Provide
quantitative comparisons or correlations between TEC and JH.
27.In Line 281, clarify how the extension of TEC anomalies to mid-latitudes is determined.
28.Figures 6–10 require clearer labeling. The red box annotations should be explained in the
captions, and sectors should be indicated.
29.The discussion of plasma irregularities (Lines 315–318) would benefit from time series plots
showing JH and ROTI together for each storm.
30.Figures 7–10 are not clearly labeled or described.
Discussion
31.The manuscript lacks a dedicated Discussion section. The results are not interpreted in the
context of underlying physical mechanisms or compared with previous studies. A
comprehensive discussion is necessary.

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AR by Nadia Imtiaz on behalf of the Authors (18 May 2026) Author's response

EF by Mario Ebel (18 May 2026) Author's tracked changes

EF by Mario Ebel (18 May 2026) Manuscript

ED: Publish as is (29 May 2026) by Dalia Buresova

AR by Nadia Imtiaz on behalf of the Authors (02 Jun 2026) Manuscript

Short summary

Comprehensive analysis of geomagnetic storms from 23–25 March, 23–25 April, 4–6 November 2023, and 10–13 May 2024, reveals that solar wind parameters, geomagnetic activity, Joule heating, and Prompt Penetration Electric Fields significantly influence ionospheric Total Electron Content variations, Equatorial Ionization Anomaly crest formations, and post-sunset plasma irregularities. The storms highlighted the importance of inter-hemispheric asymmetries in Joule Heating, which affected the distribution and magnitude of ionospheric disturbances.

Equatorial ionospheric plasma bubbles during intense geomagnetic storms of Solar Cycle 25

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