Direct observation of the latitudinal extent of a high‐speed stream in the solar wind

Abstract

Solar wind speeds measured from the Helios 1 solar probe between 0.31 and 1 AU and the earth‐orbiting Imp 7/8 satellites have been correlated with coronal holes as determined from K coronal brightness measurements. In March 1975 during perihelion passage, Helios 1 traversed the range of heliographic latitudes from −6° to +6° in a period of only 20 days. During that time the space probe crossed the northern boundary of the high‐speed stream associated with an equatorward extension of the south polar coronal hole. While this same stream continued to be observed by Imp satellites at −5° latitude, it was no longer observable from Helios 1 at +5° latitude. The conclusion is that sharp boundaries separate high‐speed flows from the surrounding solar wind. The thickness of the boundary in latitude appears to be narrower than about 10°. The local latitudinal gradient in flow speed is at least 30 km s −1 deg −1 . With less confidence it can be deduced from the Helios 1 data that the actual gradients at 0.31 AU might even exceed 100 km s −1 deg −1 .