Reexamination of the Viscous Effect in the Solar Wind

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 in the condition of collision-dominated plasma is not suitable to be used to describe 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 the resulting force, which approximates to the solar gravity in value between 0.3 AU and 1 AU. The radial dependence of the real viscous force in the streams is deduced in the present work and the corresponding viscosity is derived by assuming the trend of the real viscosity approaching to the classical viscosity as the helicentric distance decreasing. It is found that the ratio of the real viscosity to the classical one at 1 AU would be equal to or greater than one tenth.