Solar Wind Dynamic Pressure and Its Correlation with Cosmic Ray Flux Variability at Earth

Abstract

The solar wind dynamic pressure is one of the most fundamental parameters that characterize the energy and momentum flux carried by the solar wind plasma. It directly affects the structure and dynamics of the interplanetary magnetic field (IMF) and modulates the transport of galactic cosmic rays (GCRs) through the heliosphere. Variations in the dynamic pressure alter the level of magnetic turbulence and the extent of heliospheric shielding experienced by cosmic rays as they propagate toward Earth. In this study, we investigate the correlation between solar wind dynamic pressure and cosmic ray flux variability using long-term data from the OMNI database and the Neutron Monitor Database (NMDB) covering the period 2008 to 2025. The analysis spans the declining phase of Solar Cycle 23, the entirety of Solar Cycle 24, and the rising phase of Solar Cycle 25. These intervals include both quiet and disturbed solar conditions, allowing for a comprehensive understanding of how solar wind pressure fluctuations impact cosmic ray modulation. The study finds a strong inverse correlation between dynamic pressure and cosmic ray intensity, particularly during high solar activity periods when frequent coronal mass ejections (CMEs) and high-speed streams enhance solar wind turbulence. The correlation coefficients range from ?0.55 to ?0.73 during solar maxima and from ?0.30 to ?0.45 during solar minima, indicating that the modulation effect is sensitive to heliospheric conditions. Detailed analysis of several Forbush decrease events shows that sharp increases in solar wind dynamic pressure are often followed by a 2–5% drop in cosmic ray intensity, confirming a causal link between heliospheric compression and cosmic ray exclusion. Furthermore, long-term trends reveal 27-day and 13.5-day periodicities related to solar rotation and recurring high-speed solar wind streams. The study concludes that solar wind dynamic pressure is a significant controlling factor in the modulation of cosmic rays at Earth. By influencing the configuration and intensity of the IMF, variations in dynamic pressure govern the diffusion and drift of charged particles through the heliosphere. The findings emphasize the need to incorporate dynamic pressure variability into cosmic ray transport and space weather prediction models to improve forecasting accuracy.

Introduction

The Sun emits a continuous stream of charged particles—the solar wind—which carries the interplanetary magnetic field (IMF) and shapes space weather throughout the heliosphere. Its properties vary with the solar cycle: solar maxima feature frequent high-speed streams and coronal mass ejections (CMEs), while minima produce slower, more uniform outflows. A key parameter describing the solar wind’s influence is its dynamic pressure, which reflects both particle density and velocity and determines how strongly the solar wind can compress planetary magnetospheres.

Cosmic rays originating outside the solar system must pass through this variable solar wind environment. Their intensity at Earth fluctuates depending on diffusion, drift, convection, and energy-loss processes shaped by the heliospheric magnetic field. Strong interplanetary disturbances—particularly CME-driven shocks—cause sudden decreases in cosmic ray intensity known as Forbush decreases. Although many studies have explored links between solar activity and cosmic rays, fewer have specifically examined the role of solar wind dynamic pressure.

During the weak Solar Cycle 24 and into Solar Cycle 25, neutron monitor observations (e.g., Oulu and Newark) showed that increases in dynamic pressure—even without major CMEs—correspond to notable drops in cosmic ray flux. This highlights dynamic pressure as an important factor in cosmic ray modulation. Understanding this relationship improves scientific knowledge of heliospheric physics and supports practical applications such as space weather forecasting and radiation hazard assessment.

The study uses multi-year datasets (2008–2025) from OMNIWeb and neutron monitor stations to analyze correlations between solar wind dynamic pressure and cosmic ray intensity. After data cleaning and normalization, several methods were applied: Pearson correlation, lag analysis, Forbush decrease event alignment, and spectral analysis using wavelet and Fourier techniques. Results reveal a consistent inverse relationship—higher solar wind dynamic pressure leads to lower cosmic ray intensity—across both solar cycles, with recurring periodicities tied to solar rotation. The findings deepen understanding of how dynamic pressure–driven changes in the heliosphere regulate cosmic ray variability.

Conclusion

This comprehensive study establishes a strong and statistically significant relationship between solar wind dynamic pressure and cosmic ray flux variability at Earth over nearly two decades of observations. The analysis confirms that increases in solar wind dynamic pressure correspond to decreases in cosmic ray intensity, particularly during active solar periods when interplanetary disturbances are most frequent. The observed inverse correlation underscores the importance of dynamic pressure as a controlling factor in heliospheric modulation processes. Enhanced dynamic pressure not only compresses the interplanetary magnetic field but also increases its turbulence, thereby limiting the diffusion of high-energy cosmic rays toward Earth. The consistent correlation across Solar Cycles 24 and 25 demonstrates that this mechanism operates universally under varying solar conditions. The results also reveal the temporal sequencing of events during Forbush decreases, with dynamic pressure surges preceding cosmic ray minima by approximately one day. This temporal behavior confirms the causal linkage between solar wind compression and cosmic ray exclusion from the inner heliosphere. Periodicity analysis highlights 27-day and 13.5-day modulations, emphasizing the influence of solar rotation and recurrent solar wind structures.

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Copyright

Copyright © 2025 Ashish Kumar Tiwari, S. Choudhary . 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.