Geomagnetic storms are major disturbances in Earth’s magnetosphere caused by interactions between solar wind plasma, interplanetary magnetic field (IMF) structures, and Earth’s magnetic field. This study investigates the relationship between geomagnetic activity, solar wind parameters, IMF variations, and galactic cosmic ray intensity (CRI) during two major geomagnetic storms of solar cycle 24: 14-17 March 2015 and 22-23 June 2015. Hourly solar wind velocity, plasma density, IMF Scalar B, and Bz data were obtained from the OMNI database, while CRI measurements were acquired from the Oulu Neutron Monitor. The Disturbance Storm Time (Dst) index was used as the primary indicator of geomagnetic storm intensity. Correlation and regression analyses were applied to evaluate the geoeffectiveness of CME-driven interplanetary disturbances.

The March 2015 storm was associated with a fast Earth-directed coronal mass ejection (CME) and prolonged southward IMF Bz conditions. Moderate positive correlations were observed between Dst and IMF Bz (r = 0.596) and solar wind proton density (r = 0.575), whereas solar wind plasma velocity showed a moderate negative correlation (r = -0.431). A strong positive correlation between Dst and CRI (r = 0.831) confirmed a significant Forbush decrease during the storm interval. In contrast, the June 2015 storm was dominated by high solar wind velocity and interacting CME-driven shock structures. The strongest relationship was observed between Dst and solar wind plasma velocity (r = -0.914), indicating that solar wind speed was the dominant interplanetary driver. Cosmic ray intensity also showed a strong positive correlation with Dst (r = 0.910), revealing substantial cosmic ray modulation associated with CME turbulence and shock-compressed sheath regions.

The comparative analysis demonstrates that geomagnetic storm intensity during solar cycle 24 depended on IMF orientation, solar wind velocity, plasma density, and CME sheath dynamics. Both storms produced significant Forbush decreases, highlighting the strong coupling between CME-driven interplanetary disturbances and galactic cosmic ray modulation. The findings improve understanding of solar wind-magnetosphere interactions and contribute to space weather forecasting and prediction of geomagnetic storm impacts on near-Earth space environments.