Langmuir electrostatic decay in magnetized plasmas

Recent STEREO data often display strong perpendicular polarization of Langmuir waves associated with type III solar radio bursts. Such polarization implies wave numbers substantially lower than those of beam-driven Langmuir waves. One mechanism which is able to downshift Langmuir wave number is electrostatic (ES) decay, in which a Langmuir wave decays into a backscattered Langmuir wave and an ion sound wave. However, for an unmagnetized plasma a sufficient reduction in Langmuir wave number is kinematically prohibited. We demonstrate fundamental modifications to ES decay when magnetization effects are included, using kinematic analyses and quasilinear simulations. We calculate the dispersion relation for Langmuir waves in a warm magnetized plasma and show that the background magnetic field changes the kinematics. Specifically, nonzero magnetization permits decay to much lower wave numbers than in an unmagnetized plasma, allowing a low wave number condensate to be formed. These changes are due to magnetization leading to z-mode character of the Langmuir mode at low wave numbers, allowing wave frequencies less than the plasma frequency. We then employ these kinematics in quasilinear simulations of wave-particle interactions and for solar wind parameters find appreciable energy transfer to these small wave numbers, consistent with STEREO data. These fundamental modifications to the kinematics of ES decay may be important in other space physics contexts such as Langmuir turbulence.