Interplanetary Gas. XVIII. The Mean Free Path of Protons at i Astronomical Unit

Velocity distribution functions of solar-wind protons obtained by the Vela 3 satellites have been analyzed to obtain a microscopic determination of the momentum flux along magnetic field lines with respect to a reference frame moving at the bulk speed. The determination from macroscopic parameters allows the calculation of an effective mean free path for protons at 1 a.u. which averages 0.06 a.u. and is relatively independent of solar-wind velocity w. For quiet times when 300 km 1 < w < 400 km s ', the experimental mean free path is 2 to 3 times smaller than the value from plasma theory. For w > 400 km s', the experimental value is 10 to 100 times smaller than the theoretical value. The shorter mean free path for quiet times has been incorporated into the model by Wolff, Brandt, and Southwick by use of a coefficient of viscosity reduced by a factor of 3 at Earth. The resulting model is in better agreement with the values of the quiet solar wind at Earth suggested by Hundhausen than is the model with full viscosity. Subject headings: interplanetary medium - solar wind