DECAY INSTABILITY OF INCOHERENT ALFVEN WAVES IN THE SOLAR-WIND

The nonlinear evolution of a large-amplitude incoherent Alfven wave is studied via one-dimensional magnetohydrodynamic simulation. The initial wave magnetic field is given as a superposition of circularly polarized Alfven waves with the same helicity and propagation direction but with different wave numbers. What we observe is contrary to the previous belief that incoherent Alfven waves are stable against decay: In a low beta plasma (beta = 0.2) we can clearly see the growth of backscattered Alfven waves, which are opposite in helicity and propagation direction from the original Alfven waves. In a high beta plasma (beta = 2.0), on the other hand, no backscattered Alfven waves are observed. These results are consistent with the expectation from the theory of the parametric decay instability developed for a coherent Alfven wave. We also show that incoherent Alfven waves in the solar wind can decay parametrically in the region of 4-20 R(S) where beta is sufficiently low. For the decay process to work, the period of Alfven waves should be less than several minutes.