EVOLUTION OF LARGE-AMPLITUDE ALFVEN WAVES IN THE SOLAR-WIND WITH BETA-SIMILAR-TO-1

A new set of equations is obtained which describes evolution of finite amplitude, dispersive, elliptically polarized quasi-parallel Alfven waves, when the plasma and the wave amplitude satisfy the condition, \1-C(S)2/C(A)2\ < deltaB/B0, where C(A) and C(S) are the Alfven and the sound speed, respectively, and deltaB/B0 is the Alfven wave amplitude normalized to the background magnetic field. When this condition is satisfied, the sound wave as well as the right- and left-hand polarized Alfven waves are nearly all degenerated. The solar wind plasma is a typical medium for the obtained set of equations to apply, since the plasma beta is high, and the solar wind Alfven waves have large amplitude. Modulational instability of a circularly polarized parallel Alfven wave is discussed.