This study investigates the long-term modulation of Cosmic Ray Intensity (CRI) in relation to heliospheric parameters during Solar Cycle 24 (2008-2019), using daily averaged neutron monitor data in conjunction with near-Earth solar wind observations. The analysis focuses on the role of the Interplanetary Magnetic Field (IMF) and key solar wind parameters, including solar wind speed (Vsw), dynamic pressure (Pdyn), and proton density (Np), in controlling CRI variability. The results reveal a clear inverse relationship between CRI and IMF magnitude, with a correlation coefficient of r = -0.342 indicating that IMF is the most significant parameter influencing long-term cosmic ray modulation. During solar minimum periods (2008-2009 and 2018-2019), IMF values decreased below 4 nT, corresponding to enhanced CRI levels, whereas during solar maximum (2013-2015), IMF increased to 6-8 nT, leading to a marked suppression of CRI. In contrast, solar wind parameters exhibit comparatively weak correlations with CRI: solar wind speed (r = -0.164), dynamic pressure (r = -0.141), and proton density (r = +0.045), indicating their limited contribution to long-term modulation. Spectral analysis using Fast Fourier Transform (FFT) shows a dominant peak at low frequencies, confirming that CRI variability is primarily governed by long-term solar cycle periodicities (11 years). Cross-correlation analysis further indicates a strong anti-correlation between CRI and IMF with a short response time of approximately 0-3 days, suggesting rapid modulation of cosmic rays by heliospheric magnetic field changes. Overall, the study demonstrates that heliospheric magnetic field conditions, particularly IMF strength, play a dominant role in regulating cosmic ray intensity near Earth, while solar wind plasma parameters contribute only marginally. These findings are consistent with established cosmic ray transport theory and have important implications for space weather forecasting and radiation environment modelling.
Tiwari et al. (Tue,) studied this question.