REVIEW 4 major objections 6 minor 3 references
On the features of great Forbush effect during May 2024 extreme geomagnetic storm
T0 review · 4 major / 6 minor · reviewed 2026-08-10 · deepseek-v4-flash
Pith's one-line read The May 10-11, 2024 Forbush decrease reached 15.7% for 10 GV particles, the largest since October 2003, with record-small anisotropy.
desk verdict Solid IZMIRAN event study with a credible 15.7% Forbush decrease, but the paper must clarify whether the quoted numbers are Dst-corrected. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing tool is the Global Survey Method (GSM), maintained at IZMIRAN, which treats the worldwide neutron monitor network as a single multi-directional detector and produces hourly galactic cosmic ray density and first-harmonic anisotropy for 10 GV particles. The Ring of Stations Method (RSM) provides an independent longitude-time view of the cosmic ray angular distribution and is used to identify precursors and the delayed onset. The FEID database supplies the historical comparison set of Forbush effects since 1957. The magnetospheric correction uses hourly Dst values to remove the geomagnetic influence from the monitor count rates.
What would settle it
Recompute the 10 GV cosmic ray density for May 10-13, 2024 from the same neutron monitor data with an independent implementation of the Global Survey Method or with different coupling coefficients and station weighting; if the recovered magnitude falls outside the method's stated uncertainty around 15.7%, or the hourly decrement no longer reaches 4.4%, the central claim fails.
Extended reading notes
Core claim
The paper's central claim is that the Forbush effect beginning at 17:05 UT on May 10, 2024 had a magnitude of 15.7% for 10 GV particles, making it the largest such decrease in the FEID database since the October 29, 2003 event, and that its development was extreme: a maximum hourly density decrease of 4.4% and a two-hour decrease of 8.8%. It further claims that the event's equatorial and north-south anisotropy values were the smallest among all Forbush effects with magnitude above 10% in the database, indicating that Earth was west of the depletion center, and that a magnetic cloud structure passed Earth from 11:00 to 17:00 UT on May 11. A ground-level enhancement, GLE 74, occurred near the minimum, and the storm's magnetospheric effect reached about 4% in neutron monitor data.
Load-bearing premise
The analysis assumes its standard method for turning neutron monitor counts into cosmic ray density and direction is unbiased for this event.
Editorial extensions
If this is right
- The May 2024 event becomes the reference extreme Forbush decrease of the current solar cycle and the second-largest in the FEID era, after October 2003.
- The unusually low anisotropy implies the true minimum of the cosmic ray density depression was deeper than 15.7%, since Earth likely passed west of the center.
- Extreme geomagnetic storms can add a roughly 4% magnetospheric artifact to neutron monitor data, so uncorrected count rates overstate the cosmic ray decrease during such storms.
- The presence of GLE 74 inside a large Forbush decrease adds a rare case to the small set of events where particle acceleration and depletion overlap.
- The FEID catalog comparison gives a quantitative basis for ranking future extreme events against the historical record.
Reading between the lines
- If the GSM-derived numbers survive independent recalculation, single-point near-Earth measurements may systematically underestimate the depth of the largest Forbush decreases; multi-point observations at L1 or STEREO should see a deeper depletion.
- The record-low anisotropy could be tested against magnetohydrodynamic simulations of the interacting CME pileup; a simulated observer west of the flux-rope axis should reproduce the observed density and anisotropy time series.
- Because the method depends on fixed coupling coefficients, future extreme storms with different spectral shapes could bias GSM-derived magnitudes; a spectral inversion for May 2024 would bound this bias.
- The event's slow recovery and multi-step onset suggest that catalog definitions of Forbush effect onset and duration need to accommodate compound disturbances, which could affect FEID rankings.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper analyzes the May 8–13, 2024 period of intense solar activity and reports cosmic ray density and anisotropy variations derived with the IZMIRAN Global Survey Method and Ring of Stations Method using global neutron monitor data. The central result is a Forbush decrease magnitude of 15.7% for 10 GV particles on May 10–11, 2024, with a maximum hourly decrement of –4.4%/h and a two-hour decrement of –8.8%, which the authors identify as the largest since the October 2003 event. The paper also reports anomalously small north–south and equatorial anisotropy components compared with other Forbush effects with magnitude above 10%, a ~4% magnetospheric effect during the extreme geomagnetic storm, and evidence for a magnetic cloud structure on May 11. The solar, interplanetary, and geomagnetic context is documented in detail and compared with the FEID database.
Significance. If the results are sound, the paper provides the most detailed cosmic ray characterization of the May 2024 superstorm, identifying a Forbush decrease that is exceptionally deep and has strikingly small anisotropy, and it places the event in the context of the FEID database back to 1957. The paper benefits from the long-standing IZMIRAN methods and the maintained FEID catalog, which are well suited for such comparisons. However, the absence of uncertainty estimates and the unstated convention regarding the magnetospheric correction leave the headline claims less robust than they could be.
major comments (4)
- [Section 3.2, Table 3, Figures 5 and 6] The paper never states whether the headline values (15.7% magnitude, Dmin = –4.4%/h, two-hour decrement –8.8%, and the anisotropy values in Table 3) are obtained from the Dst-corrected or the uncorrected GSM density. Since the difference between the two curves reaches 4% (Figure 6), which is comparable to the difference between events near the top of the FEID ranking, the 'biggest in the last 20 years' claim is ambiguous. The authors should state explicitly which curve underlies Table 3 and Figure 5, report both corrected and uncorrected values, and indicate which convention is used for the historical FEID comparisons.
- [Section 3.2, Table 3 and FEID comparisons] All quoted quantities are given as exact numbers (15.7%, –4.4%/h, –8.8%, Axy max = 1.91%, Axy mean = 0.9%, Az range = 3.1%) with no uncertainties or error propagation. Because the claims 'smallest Axy max of all FEs >10%' and 'biggest for the last 20 years' are statements about ordering, the authors should provide at least standard errors or a sensitivity analysis to show that the ordering is robust to GSM systematic uncertainties and to the choice of Dst correction.
- [Abstract and Section 3.2, Figure 6] The statement that a 'significant magnetospheric effect observed in the data of neutron monitors (~4%)' is not supported by direct observations; the 4% is the difference between two GSM outputs, one with and one without a Dst-index-based model correction (Belov et al., 2015). The text should be rephrased to say that a Dst-based model correction changes the GSM-derived density by up to ~4%, rather than implying the effect is directly observed in the neutron monitor count rates.
- [Section 3.2, first paragraph] The paper asserts that the May 10–11 event is 'the biggest one for the last 20 years' while also stating that the October 29, 2003 FE (magnitude ~26%) 'still holds the record for all the period of observations'. Since October 2003 lies only about six months before the starting point of a 20-year window ending in May 2024, the claim as phrased is potentially misleading. The authors should state explicitly that this event is the second largest in the FEID database and the largest since October 2003.
minor comments (6)
- [Section 2] In the data availability list, 'Shockvawes' is a typo and should read 'Shockwaves'.
- [Section 3.2, Figure 5] The caption and text use the symbol A0 without defining it; please define A0 (likely the cosmic ray density) and clarify the units and scaling of the anisotropy vectors in Figure 5.
- [References] The reference to Burlaga et al. (1981) lacks a title; please complete the bibliographic entry.
- [Section 1] The sentence 'It’s based on data from all available ground detectors' should read 'It is based on data from all available ground detectors'.
- [Abstract] The phrase 'as an extreme geomagnetic storm was recorded there was a significant magnetospheric effect' is grammatically awkward; suggest rephrasing to, for example, 'a significant magnetospheric effect (~4%) was present in the neutron monitor data during the extreme geomagnetic storm'.
- [Section 3.2, Figure 4] The longitude-time plot from the Ring of Stations method would be easier to interpret if the caption explained the color scale and the meaning of the circle sizes.
Circularity Check
No construction-level circularity; the 15.7% magnitude is a measurement from a previously published IZMIRAN method, with self-citation but no definitional reduction.
full rationale
The paper's central claims are measurements, not derivations. The Forbush-effect magnitude of 15.7%, the hourly decrement of -4.4%, and the anisotropy values are obtained by applying the previously published Global Survey Method and Ring of Stations Method to neutron monitor data; no parameter is fitted to the target claims, and the FEID database is used as a comparison catalog rather than as a constraint. The magnetospheric-effect correction (Belov et al. 2015) is model-dependent, and the paper does not explicitly state whether the quoted curves are Dst-corrected, which is a reproducibility and accuracy concern but not a circular reduction. The main self-citation aspect is that the GSM, RSM, and FEID are all maintained by the same IZMIRAN group, so independent verification is limited; however, this does not make the measurement equivalent to its own inputs by construction. No equation in the paper defines the derived quantity in terms of itself, and no fitted parameter is renamed as a prediction.
Assumptions & free parameters
assumptions (4)
- domain assumption The Global Survey Method linear-response model, with coupling coefficients and station normalization from Belov et al. 2018, recovers unbiased 1-hour galactic cosmic ray density and anisotropy for 10 GV particles.
- domain assumption The Dst-index-based magnetospheric correction of Belov et al. 2015 correctly removes the geomagnetic effect on neutron monitor count rates, so the remaining ~4% difference is a true magnetospheric modulation.
- domain assumption The FEID database, maintained by the same group, is complete and methodologically consistent enough that the absence of an FE larger than 15.7% since 2003 justifies the 'biggest in 20 years' claim.
- domain assumption Changes in galactic cosmic ray anisotropy, namely an equatorial component direction reversal and north-south sign flips, can be used to infer magnetic cloud passage when solar wind signatures are ambiguous.
Cite this review
Pith. "Pith review of On the features of great Forbush effect during May 2024 extreme geomagnetic storm." pith.science (2026). https://pith.science/paper/7KDOJMUG
@misc{pith2026250108029,
author = {Pith},
title = {Pith review of: On the features of great Forbush effect during May 2024 extreme geomagnetic storm},
year = {2026},
howpublished = {\url{https://pith.science/paper/7KDOJMUG}},
note = {Machine review of arXiv:2501.08029}
}
read the original abstract
The work investigates the features of galactic cosmic ray density and anisotropy behavior and their relation to solar sources, interplanetary and geomagnetic disturbances from May 8 to May 13, 2024. During this time, powerful solar flares and fast CMEs were recorded, leading to registration of an extreme geomagnetic storm along with one of the most significant Forbush effects for the entire observation period. All the calculations of cosmic ray characteristics are made using the data of global neutron monitor network and unique methods maintained at IZMIRAN: the Global Survey Method and the Ring of Stations Method. It is determined that the magnitude of Forbush effect under study was 15.7% (for particles with 10 GV rigidity) and as an extreme geomagnetic storm was recorded there was a significant magnetospheric effect observed in the data of neutron monitors (~4%).
Figures
Reference graph
Works this paper leans on
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Reviewed August 10, 2026 · model on record in the stance chip above.
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