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Peer-reviewedCC BY 4.0The Astrophysical Journal

On Modeling the 2017 Galactic Cosmic-Ray Depression

  • O. P. M. Aslam1
  • D. MacTaggart2,1
  • M. S. Potgieter3
  • M. D. Ngobeni4
  • I. I. Ramokgaba5
5 affiliations
  1. 1University of Glasgow
  2. 2University of Trento
  3. 3Christian-Albrechts-University of Kiel
  4. 4Sol Plaatje University
  5. 5North–West University
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Abstract

In the second half of 2017, as Solar Cycle 24 approached its minimum, a pronounced and rigidity-dependent depression in the Galactic cosmic-ray (GCR) proton flux was observed using AMS-02. This event was driven by a sequence of strong coronal mass ejections (CMEs). In this work, we model the modulation of GCR protons during this period using a steady-state Parker transport framework with parameters constrained by AMS-02 observations. Treating the event as a purely long-term modulation interval yields excellent agreement with the observed proton intensities, but implies an inferred rigidity dependence of particle mean free paths that is inconsistent with basic turbulence theory, corresponding to an effective inversion of the scattering regime. We argue that this behavior arises from the ability of highly flexible transport parameterizations to reproduce integrated modulation signatures through correlated adjustments of diffusion coefficients during transient events. To address this inconsistency, we relax the commonly adopted assumption of a time-independent ratio of perpendicular-to-parallel diffusion coefficients and introduce a physically motivated reduction of this ratio during the CME-dominated interval. This modification preserves the quality of the spectral fits while restoring the expected rigidity ordering of mean free paths. Our results indicate that allowing the perpendicular-to-parallel diffusion ratio to vary in time provides an effective means of constraining transport solutions during the 2017 depression and may help to resolve similar inconsistencies reported in other GCR modulation studies.

Versions

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Cite

BibTeXRIS
BibTeX
@article{aslam2026modeling,
  title = {{On Modeling the 2017 Galactic Cosmic-Ray Depression}},
  author = {O. P. M. Aslam and D. MacTaggart and M. S. Potgieter and M. D. Ngobeni and I. I. Ramokgaba},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aea403},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea403},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - On Modeling the 2017 Galactic Cosmic-Ray Depression
AU  - O. P. M. Aslam
AU  - D. MacTaggart
AU  - M. S. Potgieter
AU  - M. D. Ngobeni
AU  - I. I. Ramokgaba
PY  - 2026
DA  - 2026/10/01
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea403
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea403
AB  - In the second half of 2017, as Solar Cycle 24 approached its minimum, a pronounced and rigidity-dependent depression in the Galactic cosmic-ray (GCR) proton flux was observed using AMS-02. This event was driven by a sequence of strong coronal mass ejections (CMEs). In this work, we model the modulation of GCR protons during this period using a steady-state Parker transport framework with parameters constrained by AMS-02 observations. Treating the event as a purely long-term modulation interval yields excellent agreement with the observed proton intensities, but implies an inferred rigidity dependence of particle mean free paths that is inconsistent with basic turbulence theory, corresponding to an effective inversion of the scattering regime. We argue that this behavior arises from the ability of highly flexible transport parameterizations to reproduce integrated modulation signatures through correlated adjustments of diffusion coefficients during transient events. To address this inconsistency, we relax the commonly adopted assumption of a time-independent ratio of perpendicular-to-parallel diffusion coefficients and introduce a physically motivated reduction of this ratio during the CME-dominated interval. This modification preserves the quality of the spectral fits while restoring the expected rigidity ordering of mean free paths. Our results indicate that allowing the perpendicular-to-parallel diffusion ratio to vary in time provides an effective means of constraining transport solutions during the 2017 depression and may help to resolve similar inconsistencies reported in other GCR modulation studies.
SN  - 0004-637X
SN  - 1538-4357
ER  -