Solar wind high-speed streams and related geomagnetic activity in the declining phase of solar cycle 23

Context. Coronal holes (CHs) are the source of high-speed streams (HSSs) in the solar wind, whose interaction with the slow solar wind creates corotating interaction regions (CIRs) in the heliosphere. Aims. We investigate the magnetospheric activity caused by CIR/HSS structures, focusing on the declining phase of the solar cycle 23 (years 2005 and 2006), when the occurrence rate of coronal mass ejections (CMEs) was low. We aim to (i) perform a systematic analysis of the relationship between the CH characteristics, basic parameters of HSS/CIRs, and the geomagnetic indices Dst, Ap and AE; (ii) study how the magnetospheric/ionospheric current systems behave when influenced by HSS/CIR; (iii) investigate if and how the evolution of the background solar wind from 2005 to 2006 affected the correlations between CH, CIR, and geomagnetic parameters. Methods. The cross-correlation analysis was applied to the fractional CH area (CH) measured in the central meridian distance interval ± 10°, the solar wind velocity (V), the interplanetary magnetic field (B), and the geomagnetic indices Dst, Ap, and AE. Results. The performed analysis shows that Ap and AE are better correlated with CH and solar wind parameters than Dst, and quantitatively demonstrates that the combination of solar wind parameters BV2 and BV plays the central role in the process of energy transfer from the solar wind to the magnetosphere. Conclusions. We provide reliable relationships between CH properties, HSS/CIR parameters, and geomagnetic indices, which can be used in forecasting the geomagnetic activity in periods of low CME activity