Polarization and Fragmentation of Solar Type II Radio Bursts

Abstract

Solar type II radio bursts are the electromagnetic signatures of collisionless shocks propagating radially outward in the solar atmosphere. The Zurich radio spectrograph has detected a fragmented type II burst at high frequencies. The Trieste polarimeter found this event to contain a high degree of left-hand polarization. These observations indicate that (1) the fragmented type II event is probably emitted at the fundamental of the electron plasma frequency fpe as one of the magnetoionic modes, namely, the ordinary (O) mode, (2) the fragmentation is due to fluctuations in the efficiency of linear coupling of Langmuir waves with escaping radiation at sharp density gradients associated with the oncoming shock, and (3) the key role of backscattering Langmuir waves in the direction of the oncoming shock is probably played by one of the saturation mechanisms, such as induced scattering of Langmuir waves on thermal ions or the electrostatic decay instability.