The Langmuir waves associated with the 1 December 2013 type II burst

The Langmuir waves associated with an interplanetary type II source region are presented. The type II burst was first observed on 29 November 2013 by STEREO A and B, with the shock crossing STEREO A on 1 December 2013. In the foreshock region upstream of the shock, 11 Langmuir-like waveforms were recorded by STEREO A's Time Domain Sampler on three orthogonal antennas. The observed Langmuir wave electric fields are of large amplitude and aligned with the background magnetic field. Some of the waveforms show evidence of electrostatic decay, and several are consistent with Langmuir eigenmodes of density wells. Harmonic electric fields are observed simultaneously with the Langmuir waveforms and are consistent with fields produced by nonlinear currents. The beam speeds v(b) exciting the Langmuir waves are estimated from the waveform data, yielding speeds v(b) approximate to(0.01-0.04)c. These are consistent with previous observations. The beam speeds are slower than those associated with type III solar radio bursts, consistent with the Langmuir wave electric fields being field aligned. The evidence found for electrostatic decay and against strong perpendicular fields, and so low-wave number Langmuir/z-mode waves, suggests that the dominant emission mechanisms for this type II foreshock involve electrostatic decay and nonlinear wave processes, rather than linear-mode conversion. Harmonic radio emission via antenna mechanisms involving Langmuir waves remains possible.