Effects of Geomagnetic Cutoff Rigidity Variations during Forbush Decreases

Jie Feng; Pengwei Zhao

arxiv: 2604.06857 · v1 · submitted 2026-04-08 · 🌌 astro-ph.SR · astro-ph.EP

Effects of Geomagnetic Cutoff Rigidity Variations during Forbush Decreases

Pengwei Zhao , Jie Feng This is my paper

Pith reviewed 2026-05-10 18:13 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.EP

keywords Forbush decreasesgeomagnetic cutoff rigidityneutron monitorsAMS proton fluxcosmic ray modulationICME eventsheliospheric modulationgeomagnetic storms

0 comments

The pith

Geomagnetic cutoff rigidity changes during Forbush decreases bias neutron monitor data down to 1 GV.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Forbush decreases are short-term drops in galactic cosmic ray intensity triggered by interplanetary disturbances such as ICMEs. Neutron monitor count rates used to track these events can also register variations from storm-time shifts in Earth's geomagnetic shielding. Analysis of NM-derived hourly proton spectra shows these cutoff effects can extend to rigidities as low as 1 GV, creating localized anomalies that distort the apparent rigidity dependence. The authors use daily proton spectra from the AMS experiment in space, which are immune to ground-level cutoff changes, to constrain corrections for selected events. After correction the broader FD profile remains intact and daily averages move closer to the direct AMS measurements, indicating that observed short-term variability combines both heliospheric modulation and geomagnetic shielding changes.

Core claim

Storm-time geomagnetic cutoff rigidity variations during ICME events introduce anomalies in neutron monitor observations of Forbush decreases that reach rigidities as low as 1 GV. These effects bias the rigidity dependence extracted from NM-based hourly proton spectra. Constraining corrections with the unaffected AMS daily proton spectrum removes the localized anomalies while preserving the overall FD evolution, and for a representative event the corrected NM daily averages align more closely with AMS data. The results establish that short-timescale cosmic ray variability during FDs reflects both heliospheric modulation and changes in geomagnetic shielding.

What carries the argument

Comparison of NM-reconstructed hourly proton spectra against AMS daily proton fluxes to identify and correct geomagnetic cutoff rigidity variations during disturbed intervals.

If this is right

Localized anomalies in NM-derived proton spectra during disturbed intervals can be removed without distorting the overall Forbush decrease profile.
Corrected spectra for representative ICME events show improved agreement with direct space-based AMS measurements.
The rigidity dependence inferred from NM data during FDs is less biased once geomagnetic cutoff effects are accounted for.
Short-timescale cosmic ray variability is demonstrated to include contributions from both interplanetary disturbances and terrestrial geomagnetic shielding.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The correction approach could be applied to other ground-based cosmic ray detectors to reduce storm-time biases in their records.
Cosmic ray transport models may benefit from explicit inclusion of magnetospheric cutoff variations when simulating short-term solar events.
Combined space-ground datasets could enable better separation of heliospheric and geomagnetic signals in future cosmic ray studies.

Load-bearing premise

The AMS daily proton spectrum supplies an unaffected stable reference that can be used to constrain corrections for neutron monitor data without introducing new biases.

What would settle it

A strong geomagnetic storm event in which the AMS-constrained NM spectra still exhibit the same localized rigidity anomalies as the uncorrected data would falsify the effectiveness of the proposed correction.

Figures

Figures reproduced from arXiv: 2604.06857 by Jie Feng, Pengwei Zhao.

**Figure 1.** Figure 1: Comparison between the uncorrected hourly reconstruction and the AMS daily flux for the 2015 June 25 ICME event. Localized deviations appear during the disturbed interval, including a representative excess above the AMS level in the [9.26, 10.1] GV interval and a representative deficit below the AMS level in the [2.15, 2.4] GV interval. where γ∞ is the high-rigidity limit, A sets the lowrigidity deviati… view at source ↗

**Figure 2.** Figure 2: Correction result for the ICME event of 2015 June 25. The panels show five representative rigidity bins together with the Dst index. Blue, red, and green points denote the original hourly reconstruction, the AMS daily flux, and the corrected hourly values, respectively. The [2.15, 2.4] GV and [9.26, 10.1] GV intervals are shown as representative corrected bins. In the lower-rigidity interval, the original … view at source ↗

read the original abstract

Forbush decreases (FDs) are short-term reductions in galactic cosmic ray flux caused by interplanetary disturbances. During some interplanetary coronal mass ejection (ICME) events, neutron monitor (NM) data also contain variations produced by geomagnetic storms. Earlier studies emphasized apparent effects near 10~GV, but storm-time changes in geomagnetic cutoff rigidity can either increase or decrease the ground-level count rate. Using a recently published hourly proton flux reconstructed from NM data for May 2011 through October 2019, the interval covered by the published AMS daily proton fluxes, we show that these localized anomalies can extend to lower rigidities and reach 1~GV in some events. Such effects can bias the rigidity dependence inferred from NM-based hourly proton spectra during disturbed intervals. Because AMS measures proton rigidity directly in space, its daily proton spectrum is not affected by cutoff variations at ground stations and provides a stable reference. We therefore use AMS to constrain corrections for selected events. The correction removes localized anomalies while preserving the broader FD evolution, and for a representative ICME event it brings the corrected daily averages closer to the AMS measurements. Our results show that short-timescale cosmic ray variability during FDs reflects both heliospheric modulation and storm-time changes in geomagnetic shielding.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper shows that geomagnetic cutoff rigidity changes during some Forbush decreases reach down to ~1 GV and can be corrected using AMS daily proton fluxes as a reference, but the support comes from a handful of events and rests on the assumption that AMS introduces no offsetting biases.

read the letter

The key point is that this work pushes the documented range of storm-time geomagnetic effects on neutron monitor data lower than the usual ~10 GV emphasis, down to 1 GV in some ICME cases, and shows a workable correction that removes localized spikes while leaving the broader Forbush decrease intact. They do this by comparing a published hourly NM-based proton reconstruction against the overlapping AMS daily proton spectra, then adjusting the NM side for selected events so the daily averages move closer to the space measurements. That concrete before-and-after comparison is the clearest contribution. It is useful for anyone who converts NM count rates into rigidity spectra during disturbed periods, because it flags a possible contamination source that earlier papers had treated as minor or higher-rigidity only. The approach is straightforward and anchored to two independent published datasets rather than new observations, which keeps the overhead low. The main soft spot is that the correction procedure assumes any NM-AMS mismatch during the chosen events is caused solely by ground-level cutoff variations. Daily averaging in AMS could smooth over rapid changes that the hourly NM data capture, and the NM-to-proton conversion itself carries reconstruction assumptions that are not fully stress-tested here against alternative choices. Without seeing the full event list, error propagation, or a quantitative check on how often the anomalies appear outside the selected ICMEs, it is difficult to gauge how general the 1 GV claim really is. The paper is aimed at heliophysics groups that routinely use NM networks for cosmic-ray monitoring and spectrum reconstruction during solar events. It is narrow but directly actionable for that audience. I would send it to peer review; the data comparison is reproducible from public sources and the practical correction is worth checking even if the authors need to add more validation statistics.

Referee Report

2 major / 1 minor

Summary. The paper claims that during Forbush decreases (FDs) triggered by ICMEs, neutron monitor (NM) data contain localized anomalies from storm-time geomagnetic cutoff rigidity variations that can extend down to ~1 GV. Using published hourly NM-reconstructed proton fluxes over 2011-2019 and AMS daily proton spectra as an independent reference unaffected by ground-level cutoffs, the authors apply event-specific corrections that remove these anomalies while preserving the broader FD evolution, concluding that short-timescale cosmic ray variability reflects both heliospheric modulation and geomagnetic shielding effects.

Significance. If the corrections prove robust, the result is significant for the cosmic-ray and space-weather communities because it demonstrates that geomagnetic cutoff changes can bias NM-derived rigidity spectra at rigidities previously thought unaffected, down to 1 GV. The approach of leveraging independent space-based AMS data to constrain ground-based reconstructions is a constructive contribution that could improve interpretation of NM observations during disturbed intervals. The use of existing published datasets is a strength, though the lack of detailed validation statistics limits immediate impact.

major comments (2)

[Abstract and correction procedure] The central claim that AMS daily proton fluxes serve as an unaffected stable reference for identifying and correcting NM anomalies is load-bearing (see abstract description of the correction procedure and the representative ICME event). However, the manuscript does not address potential biases from daily averaging during rapid FD variations or differences in temporal sampling (daily AMS vs. hourly NM), which could artifactually align the datasets without confirming a geomagnetic origin; explicit quantification of these effects or comparison with any available higher-cadence AMS data is needed to support the extension of anomalies to 1 GV.
[Results for representative ICME event] Soundness of the anomaly removal is difficult to assess without reported error propagation, validation statistics, or quantitative metrics (e.g., pre/post-correction residuals or rigidity spectra in tables/figures) showing how the 1 GV lower limit is determined and that the correction does not overfit to AMS. This directly affects the claim that short-timescale variability reflects both heliospheric and geomagnetic contributions.

minor comments (1)

[Abstract] The abstract uses non-standard notation such as '10~GV'; standard formatting (10 GV) would improve clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which highlight important aspects of our analysis that require clarification and strengthening. We address each major comment below and indicate the changes we will make in the revised manuscript.

read point-by-point responses

Referee: [Abstract and correction procedure] The central claim that AMS daily proton fluxes serve as an unaffected stable reference for identifying and correcting NM anomalies is load-bearing (see abstract description of the correction procedure and the representative ICME event). However, the manuscript does not address potential biases from daily averaging during rapid FD variations or differences in temporal sampling (daily AMS vs. hourly NM), which could artifactually align the datasets without confirming a geomagnetic origin; explicit quantification of these effects or comparison with any available higher-cadence AMS data is needed to support the extension of anomalies to 1 GV.

Authors: We agree that the difference in temporal sampling between daily AMS and hourly NM data merits explicit discussion to exclude possible artifacts. In the revised manuscript we will add a dedicated paragraph in the Methods section that quantifies the effect of daily averaging on the NM data: we show that the localized low-rigidity anomalies remain visible when the hourly NM series is compared with its own daily mean, and that the main FD evolution occurs on multi-day timescales for the events studied. Published AMS proton spectra for 2011–2019 are available only at daily cadence, so a direct higher-cadence comparison is not feasible; however, the persistence of the rigidity-localized features in the hourly NM record (independent of the AMS reference) supports their geomagnetic origin rather than an averaging artifact. revision: partial
Referee: [Results for representative ICME event] Soundness of the anomaly removal is difficult to assess without reported error propagation, validation statistics, or quantitative metrics (e.g., pre/post-correction residuals or rigidity spectra in tables/figures) showing how the 1 GV lower limit is determined and that the correction does not overfit to AMS. This directly affects the claim that short-timescale variability reflects both heliospheric and geomagnetic contributions.

Authors: We acknowledge that the original submission lacked quantitative validation metrics. In the revised version we will expand the Results section for the representative ICME event to include: (i) propagation of uncertainties through the correction procedure, (ii) a table of pre- and post-correction residuals at selected rigidities, and (iii) an additional figure panel displaying the rigidity spectra before and after correction with the 1 GV threshold indicated where the NM–AMS difference exceeds the combined uncertainty. These additions demonstrate that the correction removes only the localized anomalies while leaving the broader FD shape unchanged, thereby confirming that the short-term variability contains both heliospheric and geomagnetic contributions. revision: yes

Circularity Check

0 steps flagged

No significant circularity; central result uses external AMS reference

full rationale

The paper's derivation identifies geomagnetic cutoff effects by comparing NM-reconstructed hourly proton fluxes against independent AMS daily proton spectra measured in space. AMS is invoked explicitly as an external, unaffected benchmark because it is not subject to ground-level cutoff rigidity variations. No step reduces a claimed prediction or result to a fitted parameter, self-definition, or load-bearing self-citation chain; the NM reconstruction serves as input data while AMS supplies the independent constraint. The overall argument therefore remains self-contained against external benchmarks rather than internally forced.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the domain assumption that AMS rigidity measurements are free of geomagnetic cutoff effects and can serve as an unbiased reference. No free parameters or invented entities are described in the abstract.

axioms (1)

domain assumption AMS proton spectrum measured in space is unaffected by geomagnetic cutoff rigidity variations at ground stations
Stated directly in the abstract as the reason AMS provides a stable reference.

pith-pipeline@v0.9.0 · 5520 in / 1287 out tokens · 39622 ms · 2026-05-10T18:13:16.971113+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

16 extracted references · 16 canonical work pages

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ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month number organization pages publisher school series title misctitle type volume year version url label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts ...

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[2]

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[3]

MBQ rM [ݯ

thebibliography [1] 20pt to REFERENCES 6pt =0pt \@twocolumntrue 12pt -12pt 10pt plus 3pt =0pt =0pt =1pt plus 1pt =0pt =0pt -12pt =13pt plus 1pt =20pt =13pt plus 1pt \@M =10000 =-1.0em =0pt =0pt 0pt =0pt =1.0em @enumiv\@empty 10000 10000 `\.\@m \@noitemerr \@latex@warning Empty `thebibliography' environment \@ifnextchar \@reference \@latexerr Missing key o...

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V., Wawrzynczak , A., Sdobnov , V

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, " * write output.state after.block = add.period write newline

ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month number organization pages publisher school series title misctitle type volume year version url label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts ...

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[2] [2]

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[3] [3]

MBQ rM [ݯ

thebibliography [1] 20pt to REFERENCES 6pt =0pt \@twocolumntrue 12pt -12pt 10pt plus 3pt =0pt =0pt =1pt plus 1pt =0pt =0pt -12pt =13pt plus 1pt =20pt =13pt plus 1pt \@M =10000 =-1.0em =0pt =0pt 0pt =0pt =1.0em @enumiv\@empty 10000 10000 `\.\@m \@noitemerr \@latex@warning Empty `thebibliography' environment \@ifnextchar \@reference \@latexerr Missing key o...

work page arXiv 2017

[4] [4]

V., Wawrzynczak , A., Sdobnov , V

Alania , M. V., Wawrzynczak , A., Sdobnov , V. E., & Kravtsova , M. V. 2013, title Temporal Changes in the Rigidity Spectrum of Forbush Decreases Based on Neutron Monitor Data , Solar Physics, 286, 561, 10.1007/s11207-013-0273-0

work page doi:10.1007/s11207-013-0273-0 2013

[5] [5]

AMS Collaboration . 2021, title Periodicities in the Daily Proton Fluxes from 2011 to 2019 Measured by the Alpha Magnetic Spectrometer on the International Space Station from 1 to 100 GV , Physical review letters, 127, 271102, 10.1103/PhysRevLett.127.271102

work page doi:10.1103/physrevlett.127.271102 2021

[6] [6]

Belov , A., Baisultanova , L., Eroshenko , E., et al. 2005, title Magnetospheric effects in cosmic rays during the unique magnetic storm on November 2003 , Journal of Geophysical Research (Space Physics), 110, A09S20, 10.1029/2005JA011067

work page doi:10.1029/2005ja011067 2005

[7] [7]

Cane , H. V. 2000, title Coronal Mass Ejections and Forbush Decreases , Space Science Reviews, 93, 55, 10.1023/A:1026532125747

work page doi:10.1023/a:1026532125747 2000

[8] [8]

DAMPE Collaboration . 2021, title Observations of Forbush Decreases of Cosmic-Ray Electrons and Positrons with the Dark Matter Particle Explorer , The Astrophysical Journal Letters, 920, L43, 10.3847/2041-8213/ac2de6

work page doi:10.3847/2041-8213/ac2de6 2021

[9] [9]

Ghag , K., Tari , P., Raghav , A., et al. 2023, title The role of extreme geomagnetic storms in the Forbush decrease profile observed by neutron monitors , Journal of Atmospheric and Solar-Terrestrial Physics, 252, 106146, 10.1016/j.jastp.2023.106146

work page doi:10.1016/j.jastp.2023.106146 2023

[10] [10]

Lockwood , J. A. 1971, title Forbush Decreases in the Cosmic Radiation , Space Science Reviews, 12, 658, 10.1007/BF00173346

work page doi:10.1007/bf00173346 1971

[11] [11]

E., & King , J

Papitashvili , N. E., & King , J. H. 2020, OMNI Hourly Data Set . Heliophysics Digital Resource Library (HDRL) dataset 10.48322/1SHR-HT18

work page doi:10.48322/1shr-ht18 2020

[12] [12]

Richardson , I. G. 2018, title Solar wind stream interaction regions throughout the heliosphere , Living Reviews in Solar Physics, 15, 1, 10.1007/s41116-017-0011-z

work page doi:10.1007/s41116-017-0011-z 2018

[13] [13]

I., Danilova , O

Tyasto , M. I., Danilova , O. A., Ptitsyna , N. G., & Sdobnov , V. E. 2013, title Variations in cosmic ray cutoff rigidities during the great geomagnetic storm of November 2004 , Advances in Space Research, 51, 1230, 10.1016/j.asr.2012.10.025

work page doi:10.1016/j.asr.2012.10.025 2013

[14] [14]

V \"a is \"a nen , P., Usoskin , I., & Mursula , K. 2021, title Seven Decades of Neutron Monitors (1951-2019): Overview and Evaluation of Data Sources , Journal of Geophysical Research (Space Physics), 126, e28941, 10.1029/2020JA028941

work page doi:10.1029/2020ja028941 2021

[15] [15]

2023, title Properties of Forbush Decreases with AMS-02 Daily Proton Flux Data , The Astrophysical Journal, 950, 23, 10.3847/1538-4357/acca1b

Wang , S., Bindi , V., Consolandi , C., et al. 2023, title Properties of Forbush Decreases with AMS-02 Daily Proton Flux Data , The Astrophysical Journal, 950, 23, 10.3847/1538-4357/acca1b

work page doi:10.3847/1538-4357/acca1b 2023

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P., & Feng , J

Zhao , P., Yan , J., Leung , A. P., & Feng , J. 2026, title Measurements of Cosmic Proton Flux through Neutron Monitors Using Deep Networks and Imputation Techniques in the AMS-02 Era , The Astrophysical Journal Supplement Series, 282, 5, 10.3847/1538-4365/ae1cc1

work page doi:10.3847/1538-4365/ae1cc1 2026