F. Malara et al., Nonlinear evolution of parametric instability of a large-amplitude nonmonochromatic Alfven wave, PHYS PLASMA, 7(7), 2000, pp. 2866-2877
The nonlinear evolution of the parametric instability of a finite amplitude
Alfven wave is studied in a one-dimensional geometry. The linear stage of
this instability had been examined in a previous paper [Malara and Velli, P
hys. Plasmas 3, 4427 (1996)], in which it has been shown that even a nonmon
ochromatic broad-band wave is unstable. In the present paper, the time evol
ution is followed up to saturation, by numerically integrating the full set
of magnetohydrodynamic (MHD) equations. Several configurations of the Alfv
enic pump wave, as well as different values of the plasma beta are examined
. The saturation level of the instability does not depend on the spectral w
idth of the pump wave, but it depends on the value of beta. For beta less t
han or similar to 0.5 the final state is characterized by a cross-helicity
similar to 0 and a moderate level of density fluctuations: The Alfvenic cor
relation of the initial wave is completely destroyed, even for a broad-band
initial spectrum. For beta=1 the final cross-helicity is closer to that of
the initial wave, indicating a lower saturation level. The parametric proc
ess appears to be nonlocal in the wave-vector space also in the nonmonochro
matic case, this feature becoming more relevant with increasing beta. (C) 2
000 American Institute of Physics. [S1070-664X(00)03507-2].