Equatorial coronal holes, solar wind high-speed streams, and their geoeffectiveness

Abstract

Context. Solar wind high-speed streams (HSSs), originating in equatorial coronal holes (CHs), are the main driver of the geomagnetic activity in the late-declining phase of the solar cycle.Aims. We analyze correlations between CH characteristics, HSSs parameters, and the geomagnetic activity indices, to establish empirical relationships that would provide forecasting of the solar wind characteristics, as well as the effect of HSSs on the geomagnetic activity in periods when the effect of coronal mass ejections is low.Methods. We apply the cross-correlation analysis to the fractional CH area (CH ) measured between central meridian distances ± 10°, solar wind parameters (flow velocity V , proton density n , temperature T , and the magnetic field B ), and the geomagnetic indices Dst and Ap .Results. The cross-correlation analysis reveals a high degree of correlation between all studied parameters. In particular, we show that the Ap index is considerably more sensitive to HSS and CH characteristics than Dst . The Ap and Dst indices are most tightly correlated with the solar wind parameter B V 2 . Conclusions. From the point of view of space weather, the most important result is that the established empirical relationships provide a few-days-in-advance forecasting of the HSS characteristics and the related geomagnetic activity at the six-hour resolution.