This paper has analyzed the characteristic features of a major two-step geomagnetic storm during 23–24 April 2023 based on the data available at the INTERMAGNET magnetometer network. This study is interesting and gets meaningful conclusions. This observation of the substorm current wedge phenomenon provides the basis for studies utilizing the datasets collected with ground-based magnetometers. Therefore this work can be acceptable after a major revision. Specific issues are as follows:

1. Abstract: The abstract should be organized into five aspects: background, purpose, methods, results, and conclusions. It should particularly emphasize explaining the research background, meaning and main achievements of this manuscript.
2. Introduction: There are numerous instances of multiple references cited together, such as

Line 47: “… a large number of studies (see, e.g., (Gonzalez et al., 1994; Laštovička, 1996; …”

Line 53: “… affect human health (Daglis, 2001; Freeman, 2001; Song et al., 2001; Carlowicz and Lopez, 2002; Moldwin, 2008)”

…

Please provide a detailed explanation of the inspiration each of these references brings to the manuscript.

1. Introduction: To explain more clearly, the organizing structure of this paper should be explained at the end of the Introduction.
2. Figure 1: To highlight and avoid confusion, it is recommended to represent the site in different colors.
3. Current Section 2 only introduces data sources and observation stations, the title and text is inconsistent. Therefore, I suggest revising the title “2 Instrumentation and techniques” as “2 Data and materials”. And a detailed observed instrumentation and its working parameters should be added.
4. Section 3 presents the results of the second section, and it is recommended to merge them into one section.
5. The main achievement of this study is the issues concerning the threshold condition for the formation of the substorm current wedge; this accomplishment is at the end of the paper as a conclusion. To highlight this, I suggest renaming the paper as, for instance, “A two-step geospace storm as a new tool for experimentally estimating the threshold condition for the formation of a substorm current wedge”.
6. Section 4 “Analysis of magnetometer data” is suggested to be moved to the Appendix. This rearrangement puts the principle accomplishment of this study at the center of the text.
7. Figures 3-11: Most of the data in the figures are difficult to see clearly. It is recommended to revise the drawing method to be easily seen by readers.
8. Throughout the text two designations of universal time can be found, UT and UTC. This should be fixed somehow. Figures 3-11, horizontal axis: UT (hours) is written, while (hh:mm) is indicated. Figures 12: horizontal axis: Universal Time (hours) is written, while (hh:mm) is indicated.
9. Extensive English editing is required. Such as：

Line 13: replace “show” with “shows”

Line 40: replace “in Global Positioning System and in VLF navigation” with “in the Global Positioning System and VLF navigation”

Line 46: replace “storm” with “storms”

line 95: replace “coordinares” with “coordinates”

…

I am pleased to recommend the acceptance of this manuscript for publication. The author has presented a thorough and insightful analysis of the geomagnetic storm that occurred on April 23–24, 2023, a significant two-step event within Solar Cycle 25. Their use of data from near-meridional chains of magnetometer stations across both hemispheres provides a robust foundation for examining the latitudinal variations and underlying mechanisms of this severe geomagnetic disturbance. The clarity of the results and the depth of analysis reflect a high level of expertise and make a meaningful contribution to our understanding of geomagnetic storm dynamics.

The manuscript effectively highlights the detailed observations of geomagnetic field variations and their implications, particularly during the second step of the storm. The authors’ approach to exploring the spatial and temporal characteristics of geomagnetic field disturbances is commendable. Their findings, which reveal significant increases in geomagnetic field strength and variations with latitude, add valuable insights into how such storms impact different regions of the Earth. The careful presentation and interpretation of these results enhance the manuscript’s scientific value.

To further enrich the manuscript, I suggest incorporating comparisons with recent studies on this extreme space weather event. Kalpesh Ghag et al. (2024) offer a thorough examination of this geomagnetic storm and their analysis attributes the storm's intensity to the transformation of an ICME sheath into quasi-planar magnetic structures, which they demonstrate significantly enhances the southward magnetic field component, thereby intensifying geomagnetic activity [1]. Irina Despirak et al. (2024) further elucidate the sources and behaviors of geomagnetically induced currents (GICs) during this event, highlighting the influence of interplanetary shocks, magnetic clouds, and localized geomagnetic disturbances on GIC intensities [2]. Additionally, Souza et al. (2024) provides a thorough analysis of the effects of storm-time prompt penetration electric fields (PPEF) and traveling atmospheric disturbances (TADs) on TEC, foF2, and hmF2 during this geomagnetic storm, revealing significant shifts in the Equatorial Ionization Anomaly (EIA) and detailed characteristics of TAD propagation. Their findings effectively illustrate how these disturbances impact ionospheric and thermospheric conditions, contributing valuable insights to the understanding of space weather dynamics [3]. Habarulema et al. (2024) report a unique observation of missing high-frequency echoes from ionosondes during the same storm, attributing this anomaly to significant ionospheric depletion and gradients as detected by TIMED/GUVI and simulated by TIEGCM [4]. The references to Kamid Y.’s work, particularly the detailed discussion on the two-step development of geomagnetic storms [5], could provide valuable additional context and further enhance the manuscript's depth and historical grounding.

Overall, this manuscript is a significant contribution to the field of space weather research. The author has provided a detailed and insightful analysis of a complex geomagnetic storm, and their work is of high quality. I strongly support its acceptance for publication, with the aforementioned suggestions for additional context and comparisons to further strengthen its impact.

References  

[1] Kalpesh Ghag et al. (2024) Quasi-planar ICME sheath: A cause of the first two-step extreme geomagnetic storm of the 25th solar cycle observed on 23 April 2023. Advances in Space Research 73(12), 6288-6297. https://doi.org/10.1016/j.asr.2024.03.011 

[2] Irina Despirak et al. (2024) Geomagnetically induced currents (GICs) during strong geomagnetic activity (storms, substorms, and magnetic pulsations) on 23–24 April 2023. Journal of Atmospheric and Solar–Terrestrial Physics 261, 106293. https://doi.org/10.1016/j.jastp.2024.106293 

[3] Souza,J.R., et al. (2024). Impacts of storm electric fields and traveling atmospheric disturbances over the Americas during 23-24 April 2023 geomagnetic storm: Experimental analysis. Journal of Geophysical Research: Space Physics, 129, e2024JA032698. https://doi.org/10.1029/2024JA032698 

[4] Habarulema,J.B., et al. (2024).Absence of high frequency echoes from ionosondes during the 23–25 April 2023 geomagnetic storm; whathappened? Journal of Geophysical Research: SpacePhysics, 129, e2023JA032277. https://doi.org/10.1029/2023JA032277

[5] Kamid, Y. et al. (1998). Two-step development of geomagnetic storms. JOURNAL OF GEOPHYSICAL RESEARCH, 103(A4), 6917-6921.  https://doi.org/10.1029/97JA03337