Coronal holes and solar wind high-speed streams: II. Forecasting the geomagnetic effects

摘要

We present a simple method of forecasting the geomagnetic storms caused by high-speed streams (HSSs) in the solar wind. The method is based on the empirical correlation between the coronal hole area/position and the value of the Dst index, which is established in a period of low interplanetary coronal mass ejection (ICME) activity. On average, the highest geomagnetic activity, i.e., the minimum in Dst, occurs four days after a low-latitude coronal hole ( CH) crosses the central meridian. The amplitude of the Dst dip is correlated with the CH area and depends on the magnetic polarity of the CH due to the Russell - McPherron effect. The Dst variation may be predicted by employing the expression Dst(t) = (- 65 +/- 25 x cos lambda)[A(t*)](0.5), where A(t*) is the fractional CH area measured in the central-meridian slice [- 10 degrees, 10 degrees] of the solar disc, lambda is the ecliptic longitude of the Earth, +/- stands for positive/ negative CH polarity, and t - t* = 4 days. In periods of low ICME activity, the proposed expression provides forecasting of the amplitude of the HSS-associated Dst dip to an accuracy of approximate to 30%. However, the time of occurrence of the Dst minimum cannot be predicted to better than +/- 2 days, and consequently, the overall mean relative difference between the observed and calculated daily values of Dst ranges around 50%.