Earth and Planetary Science

Cosmic Ray Modulation Across Solar Cycles 23 and 24: Phase Dependent Variability and Geomagnetic Storm Effects

Chika Onuchukwu

Industrial Physics, Chukwuemeka Odumegwu Ojukwu University, 54 Egbu Road, Uli Campus, Awka 420102, Anambra, Nigeria

Dio Edwin

Industrial Physics, Chukwuemeka Odumegwu Ojukwu University, 54 Egbu Road, Uli Campus, Awka 420102, Anambra, Nigeria

DOI: https://doi.org/10.36956/eps.v4i2.2207

Received: 24 May 2025 | Revised: 2 August 2025 | Accepted: 18 August 2025 | Published Online: 2 September 2025

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Abstract

This study investigates the modulation of cosmic ray (CR) intensity in response to solar and heliospheric variability across the ascending (ASC) and descending (DSC) phases of Solar Cycles (SCs) 23 and 24. Using daily‑averaged data, we analyze sunspot number (SSN) as a proxy for solar activity, solar wind parameters ‑ interplanetary magnetic field (IMF), solar wind plasma density (SWPD), speed (SWS), temperature (SWT) ‑ and geomagnetic indices (Kp, Dst, ap). Geomagnetic storms were categorized by intensity based on Dst index thresholds. Distribution analyses reveal broadly consistent trends for SSN, IMF, SWPD, SWS, and geomagnetic indices across both SC phases. However, CR intensity and SWT exhibit significant phase‑dependent discrepancies, with CR fluxes more suppressed during ASC phases, perhaps due to increased solar magnetic complexity and related solar activities. Average parameter values also diverge across storm intensity levels, indicating the modulation role of transient solar phenomena. Correlation coefficient analyses indicate stronger positive and negative associations between CR intensity and solar wind parameters during DSC phases compared to ASC phases, suggesting enhanced coupling between heliospheric conditions and CR flux during the declining solar activity. During weak storms, SWPD and SWT have a minimal influence on CR modulation, whereas their role becomes more pronounced during intense geomagnetic activity. The anomalously subdued solar activity in the ASC phase of SC 24, characterized by lower SWPD and IMF magnitudes, further emphasizes inter‑cycle/phase differences in heliospheric shielding efficiency. These findings highlight the importance of phase‑ and storm‑dependent modulation in space weather forecasting.

Keywords: Cosmic Rays; Geomagnetic Storms; Dst Index; Solar Cycle Phase

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