Origin of the slow solar wind

Abstract

The origin and location of the slow wind are interesting subjects on their own and also they are important in the matter of the origin of the fast wind especially if the slow wind originates directly from a coronal hole. We have been studying the solar wind whose speed is comparative to or a little slower than that measured in the heliospheric plasma sheet. We derived solar wind velocity distribution maps on the source surface using an interplanetary scintillation tomography analysis, and low speed regions in them were investigated in relation to a coronal hole using potential field magnetic field lines. Slow solar wind was found to originate from an equatorial coronal hole located in the vicinity of active regions and also from a polar coronal hole which was about to disappear at solar activity maximum. The properties of the slow solar wind from an equatorial coronal hole were investigated using spacecraft in situ measurements. Magnetic polarity in this wind was uniform, as expected from a potential field analysis. The helium relative abundance was as large as the fast wind, and variances of density, velocity, and helium abundance were as small as the fast wind from a large coronal hole. In spite of the small equatorial coronal hole origin, the density and ion freeze-in temperature were as large as observed for the slow wind in the heliospheric plasma sheet.