Analysis of Cosmic Ray Intensity Variations with Solar and Solar Wind Parameters in Solar Cycles 23 and 24

Abstract

The intensity of GCRs is strongly modulated by solar and interplanetary conditions and varies systematically with the solar activity cycle. In this study, cosmic ray intensity variations during Solar Cycles 23 and 24 are investigated using neutron monitor data from Oulu and Moscow stations, representing different cutoff rigidities. The observations are analysed in conjunction with solar and Solar wind parameters, including Rz, SRF, VSP, TSP and DSP . GCR intensity and solar activity are found to be strongly anti-correlated, with much greater modulation during Solar Cycle 23 than during the weaker Solar Cycle 24. There are clear stiffness-dependent effects, with increased modulation at lower cutoff rigidity. Transient solar disturbances are also linked to short-term reductions. These findings advance knowledge of cosmic ray modification in the heliosphere that is dependent on the solar cycle.

Introduction

Galactic Cosmic Rays (GCRs) are high-energy charged particles originating outside the solar system. Their intensity near Earth is modulated by the heliosphere and shows a strong inverse relationship with the 11-year solar activity cycle. Solar activity affects GCRs through changes in the solar wind and interplanetary magnetic field (IMF), influencing cosmic ray transport via diffusion, convection, drifts, and adiabatic deceleration. Both long-term (sunspot number, solar radio flux) and short-term (solar flares, CMEs, high-speed solar wind) solar phenomena impact cosmic ray intensity, with transient events often causing Forbush decreases.

This study analyzed GCR modulation during Solar Cycles 23 and 24, using neutron monitor data from Oulu (0.8 GV cutoff rigidity) and Moscow (2.4 GV) to examine rigidity-dependent effects. Solar activity parameters included sunspot numbers (Rz), 10.7 cm solar radio flux (SRF), CMEs, and solar wind parameters (speed, proton temperature, plasma density). Monthly and annual means were analyzed using Pearson correlation and linear regression to quantify relationships.

Key Findings:

1. Cosmic Ray–Sunspot Relationship:

   • Strong negative correlation observed between cosmic ray intensity and sunspot number:

     • Solar Cycle 23: r ≈ −0.85 to −0.86

     • Solar Cycle 24: r ≈ −0.94 to −0.95

   • Regression analysis indicates weaker modulation during Solar Cycle 24, consistent with its lower solar activity.

   • Sunspot numbers explain 72–74% of CRI variability in Cycle 23 and 88–90% in Cycle 24.

2. Cosmic Ray–Solar Radio Flux Relationship:

   • F10.7 cm SRF also shows a strong inverse correlation with CRI:

     • Cycle 23: r ≈ −0.84 to −0.86

     • Cycle 24: r ≈ −0.95 to −0.96

   • Higher SRF values during Cycle 23 correspond to stronger solar modulation, while Cycle 24 shows weaker modulation effects.

Conclusion

This study provides a detailed multi-parameter correlation analysis between cosmic ray intensity and solar–interplanetary features during Solar Cycles 23 and 24. Key conclusions are: 1) CRI shows strong anti-correlation with sunspot numbers, SRF, and CME rates—especially during Solar Cycle 24. 2) Solar wind speed and proton temperature influence CRI moderately in Solar Cycle 23 but insignificantly in Solar Cycle 24. 3) Solar plasma density exhibits weak or no long-term relationship with CRI. 4) Solar activity parameters can explain up to 90% of CRI variability, indicating their dominant role in heliospheric modulation. These findings help improve understanding of cosmic ray transport and provide valuable input for space weather modeling.

References

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Copyright

Copyright © 2025 Jitendra Satnami, Achyut Pandey, Deepak K Chaurasiya, C. M. Tiwari. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.