Centennial evolution of monthly solar wind speeds: Fastest monthly solar wind speeds from long-duration coronal holes

摘要

High speed solar wind streams (HSSs) are very efficient drivers ofgeomagnetic activity at high latitudes. In this paper we use a recentlydeveloped $\Delta{H}$ parameter of geomagnetic activity, calculated from thenight-side hourly magnetic field measurements of the Sodankyl\"a observatory,as a proxy for solar wind (SW) speed at monthly time resolution in 1914-2014(solar cycles 15-24). The seasonal variation in the relation between monthly$\Delta{H}$ and solar wind speed is taken into account by calculating separateregressions between $\Delta{H}$ and SW speed for each month. Thereby, we obtaina homogeneous series of proxy values for monthly solar wind speed for the last100 years. We find that the strongest HSS-active months of each solar cycleoccur in the declining phase, in years 1919, 1930, 1941, 1952, 1959, 1973,1982, 1994 and 2003. Practically all these years are the same or adjacent tothe years of annual maximum solar wind speeds. This implies that the mostpersistent coronal holes, lasting for several solar rotations and leading tothe highest annual SW speeds, are also the sources of the highest monthly SWspeeds. Accordingly, during the last 100 years, there were no coronal holes ofshort duration (of about one solar rotation) that would produce faster monthly(or solar rotation) averaged solar wind than the most long-living coronal holesin each solar cycle produce.