It is shown both experimentally and theoretically that a Bernstein wave can decay parametrically into another Bernstein wave and a whistler-mode wave. The process only occurs in a highly collisionless plasma and it is suggested that this is the dominant mechanism for producing whistler-mode noise in the auroral ionosphere.

• Received 11 August 1976

DOI:https://doi.org/10.1103/PhysRevLett.39.277