Areas of minimum intensity of soft X-rays as sources of solar wind high-speed streams

Abstract

All high-speed (v⩾600 km/ s) streams interacting with the Earth magnetosphere during 1995–1996 were analyzed. Analysis of synoptic maps of relative intensity of soft X-rays, compiled from individual YOHKOH soft X-ray images, showed that the substantial majority of those streams were associated with the areas of minimum intensity of soft X-rays. The character of those areas depends strongly on the choice of cut-off level of relative units. The endeavour to find appropriate level of agreement between the coronal holes as published by NOAA and the “coronal holes” on the synoptic maps led in the end to choose 5 relative units as the lowest level. For the very high-speed (v⩾700 km/ s) streams of which there were in the above-mentioned years 11 (10 in 1995; 1 in 1996) we document the dependence of the speed of the analyzed streams on the size, the shape and the location of “coronal holes” on the solar disc.