Observations of the parametric decay instability of nonlinear magnetohydrodynamic waves

One of the most important nonlinear processes for Alfven and fast magnetosonic waves is the decay instability in which a forward propagating magnetohydrodynamic (MHD) wave is converted into a forward propagating ion acoustic wave and a backward propagating MHD wave. Despite an extensive theoretical literature and numerous computer simulations of the process, there is minimal experimental or observational evidence for its existence. In this paper we report an extensive search for evidence of the decay instability in the MWD wave field upstream of the Earth's bow shock. Twenty intervals of spacecraft magnetometer and density data with durations between 21 and 168 min were examined. The observational signature of the decay instability sought was a quasi-monochromatic feature in the density power spectrum, attributable to the daughter ion acoustic wave, at a frequency higher than the main wave features in the magnetic power spectra. Such a feature was in fact observed for the interval in which the theoretically predicted instability growth rate was highest, as well as in a second interval for which the instability was permitted with a slower growth rate. However, the data set also contains three long intervals of data in which the ''decay line'' signature is not seen, although theoretically permitted. The decay line is also absent in four shorter intervals in which the plasma beta is less than unity and the instability accordingly facilitated. Possible reasons for the absence of the instability in these intervals are discussed, such as a finite bandwidth for the parent, wave field and plasma kinetic effects. (C) 1997 American Institute of Physics.