Parametric instabilities of Langmuir waves in strong magnetic fields and phase transition of Langmuir turbulence

Parametric instabilities of Langmuir waves in strong magnetic fields such that Ωe≳ωe were investigated, where Ωe(ωe) denotes the electron cyclotron (plasma) frequency. It has been well established that as a plasma is more strongly magnetized, the parametric instabilities of Langmuir waves become more field aligned and thus one dimensional. Here, it is demonstrated that this trend continues only up to Ωe/ωe=1, and that above this value, most Langmuir waves decay to relatively short wavelength oblique-Langmuir and ion waves that are damped rapidly. These new parametric instabilities have growth rates greater than those of the conventional modulational and decay instabilities. Consequently, the scenario of strong Langmuir turbulence in which Langmuir solitons play a major role may not apply to the strong magnetic-field regime. In the weak Langmuir turbulence regime these decay instabilities make the turbulence more isotropic. Therefore, it is concluded that there occurs a type of phase transition in magnetized Langmuir turbulence at Ωe/ωe∼1.