Abstract
The Helios observations of the plasma and magnetic field in the high-speed solar wind streams between 1 AU and 0.3 AU are used to examine the radial distribution of the various forces exerted on the streams. It is shown that the classical expression of the viscosity derived under the condition of collision-dominated plasma is not suitable for describing the viscous effect in the high speed wind streams, and the viscous force appears to exist in order to balance the inertial force of the fluid element with a resulting force, which approximates to the solar gravity between 0.3 AU and 1 AU. The radial dependence of the real viscous force in the streams is deduced and the corresponding viscosity is derived by assuming the trend of the real viscosity to approach the classical viscosity at small heliocentric distances. It is found that the ratio of the real to the classical viscosity at 1 AU should be equal to or greater than one tenth.
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