NONLINEAR ALFVEN-WAVE PROPAGATION IN THE SOLAR-WIND (VOL 465, PG 451,1996)

We derive a self-consistent set of equations that describe the propaga tion of nonlinear shear Alfven waves in the solar wind. The equations include the interaction between the waves and wind plasmas and has an exact energy conservation law. Using a truncated Fourier series, we fu rther reduce the system to a simplifed nonlinear model with multiple w aves. We solve the model numerically in a radially expanding box with a finite angular size. We find that for linear waves in a time-varying plasma background, the conditions for the WKB approximation are viola ted in practically the entire radial domain (even at high frequencies) , although the scaling of wave amplitudes (delta B) with radial distan ce and the equipartition between the magnetic and kinetic energy are s imilar to the WKB results. For nonlinear waves, equipartition still ho lds, but nonlinear wave reflection enhances the backward propagating w aves (in the rest frame of the wind), which leads to a faster drop in delta B than the linear waves. As a result, there is a ''soft'' satura tion in which delta B approaches the background magnetic held graduall y at large distance. The simplified nonlinear model produces reasonabl y high wind speed (between 600 and 700 km s(-1)).