Evidence for Super-Alfvénic Oscillations in Solar Type III Radio Burst Sources

At the site of their origin, solar meter-wave radio bursts contain pristine information about the local coronal magnetic field and plasma parameters. On its way through the turbulent corona, this radiation gets substantially modified due to propagation effects. Effectively disentangling the intrinsic variations in emission from propagation effects has remained a challenge. We demonstrate a way to achieve this using a snapshot spectroscopic imaging study of weak type III bursts using data from the Murchison Widefield Array. Using this study, we present the first observational evidence for second-scale quasi-periodic oscillations (QPOs) in burst source sizes and orientation with simultaneous QPOs in intensity. The observed oscillations in source sizes are so fast and large that they require two orders of magnitude larger Alfvén speed than the typical local value of 0.5 Mm s^-1 at the burst generation heights, if interpreted within an MHD framework. These observations imply the presence of a quasi-periodic regulation mechanism operating at the particle injection site, modulating the geometry of energetic electron beams that generate type III bursts. In addition, we introduce a method to characterize plasma turbulence in the mid-coronal ranges. We also detect evidence for a systematic drift in the location of the burst sources superposed on the random jitter induced by scattering. We interpret this as the motion of the open flux tube within which the energetic electron beams travel.