MAGNETOHYDRODYNAMIC PARAMETRIC-INSTABILITIES DRIVEN BY A STANDING ALFVEN-WAVE IN THE PLANETARY MAGNETOSPHERE

The parametric instabilities driven by a standing Alfven wave of circu lar polarization are studied. Above a certain threshold amplitude, a s tanding Alfven wave can generate convective or purely growing MHD para metric processes. It is shown that the threshold conditions can be sat isfied by the ULF waves in the planetary magnetospheres. Large density fluctuations and cavities may result from the ponderomotive interacti on of Alfven and acoustic waves. Application of this theory to the obs ervation of Alfven-acoustic turbulence in the Earth's auroral plasma i s discussed.