Variation of Solar Wind and Geomagnetic Parameters during Ascending and Declining Phases of Solar Cycles 23 and 24

This study examines the relationship between solar wind dynamics and geomagnetic activity during the ascending (ASC) and declining (DSC) phases of Solar Cycles (SCs) 23 (1996–2008) and 24 (2008–2019), which exhibited contrasting levels of solar activity. High-resolution solar wind parameters—including speed (SWS), plasma density (SWPD), temperature (SWT), and interplanetary magnetic field (IMF)—are analyzed alongside geomagnetic indices (Dst, ap, and Kp) to quantify phase-dependent relationships using correlation analysis and linear regression modeling. The results reveal significant differences in solar-terrestrial coupling between the two cycles. Sunspot number (SSN) and IMF exhibit comparable correlations (r ~0.7) across the ASC and DSC phases of SC 23 and SC 24. However, the correlation between SSN and SWT/SWS is stronger in SC 23 (r ~0.4/0.3) than in SC 24 (r ~0.3/0.2). Additionally, SWPD and SSN display a negative correlation during the ASC phases—more pronounced in SC 23—but no correlation during the DSC phases. SSN also exhibits a mild correlation with geomagnetic indices (r ≥ 0.3). The IMF demonstrates distinct relationships with SWT and SWS, maintaining a positive correlation of varying strength across phases, whereas its correlation with SWPD is negative during the ASC phases but positive during the DSC phases. Moreover, IMF, SWS, and SWT exhibit positive correlations with geomagnetic indices, though with varying strengths. These findings underscore the influence of SC amplitude and phase on the efficiency of solar wind energy transfer into Earth's magnetosphere. The efficiency of this transfer is not uniform but varies depending on the strength of the solar cycle and its phase. Stronger cycles (e.g., SC 23) generally facilitate more efficient energy transfer and enhanced geomagnetic activity. Furthermore, distinct solar wind-magnetosphere coupling mechanisms are evident in different phases, with coronal mass ejections playing a dominant role during the ASC phase and high-speed solar wind streams prevailing during the DSC phase, as suggested by previous studies. By contrasting two solar cycles with distinct characteristics—SC 24 being notably weaker—this study advances the understanding of long-term space weather variability and provides empirical constraints for models predicting geomagnetic responses to evolving solar wind conditions. The results highlight the necessity of phase- and cycle-specific approaches to enhance space weather forecasting and improve resilience across solar maxima and minima.