Dependence of Langmuir wave polarization on electron beam speed in type III solar radio bursts

We present observational evidence that the polarization of Langmuir waves associated with solar type III radio bursts is organized by the speed of the driving electron beam. Seven periods of type III radio burst activity are examined using spectral data and high cadence in-situ waveforms from the STEREO spacecraft. It is found that when associated with faster beam electrons, the waves near the electron plasma frequency (usually called “Langmuir” waves) show strong electric fields perpendicular to the local magnetic field direction. When associated with slower beam electrons, the waves show weak or no perpendicular electric fields. This behavior of these waves near the local plasma frequency is consistent with Langmuir/z-mode waves shifted to small wave number, where the z-mode character is dominant. We discuss possible mechanisms for shifting these waves to small wave number.