Observation of spontaneous decay of Alfvénic fluctuations into co- and counter-propagating magnetosonic waves in a laboratory plasma

We show experimentally that Alfvénic fluctuations can spontaneously decay into a fast and a slow magnetosonic wave in an inhomogeneous plasma. The fast wave of higher frequency propagates in the same direction, while the slow wave of lower frequency propagates in the opposite direction with the pump wave. Both of the daughter waves are characterized by strong parallel but modest perpendicular fluctuations. The measured frequencies and wavenumbers are found to satisfy the energy and momentum conservation conditions for a nonlinear three wave interaction. The evidence of energy flowing from pump fluctuations into daughter waves is also presented in this paper. The results may shed light on the origin of inward Alfvén waves observed in the solar corona and chromosphere and how shear Alfvén waves deposit its energy by driving compressional perturbations.