On the Backscatter Instability of Solar Wind Alfvn Waves

A coherent Alfv6n wave propagating along the equilibrium magnetic field is unstable against decay into an Alfv6n wave traveling in the opposite direction and a sound wave [Sagdeer and Galeev, 1969]. The stability of incoherent Alfv6n waves against his mode decay is considered, assuming that the sound waves are damped. The criterion for stabilizing waves with power spectral density In o• k -N is N > 1. The results indicate that solar wind Alfv6n waves are stable in the wavelength range considered in Mariner 5 observations, although a mild instability is predicted for longer wavelengths.