On the Viscous Attenuation of Alfvénic Fluctuations in the Solar Wind

The possibility of the viscous attenuation of the Alfvénic fluctuation in the solar wind is discused with a viscous WKB solution of incompressible Alfvén waves, the wavelength of wich are larger than the mean free path. Because the proton gyro-frequency is much larger than the collision frequency in the solar wind, viscous attenuation should be propotional to the component of the fluctuation parallel to the average magnetic field vector. If the ratio between the amplitudes of the parallel component and the perpendicular component is a constant with respect to the heliocentric distance r, no viscous attenuation occurs for 0.3 AU < r < 1.0 AU, whereas for r<20 R_⊙, the Alfvenic fluctuation will suffer some viscous attenuation, that may accelerate the solar wind. If, however, there is not parallel component for r < 20R_⊙, the fluctuation will not suffer any viscus attenuation in the interplanetary space.