  At the site of their origin, solar meterwave 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 the propagation effects. Effectively disentangling the
intrinsic variations in emission from propagation effects has remained a
challenge. We demonstrate a way to achieve this, using snapshot spectroscopic
imaging study of weak type III bursts using data from the Murchison Widefield
Array (MWA). We use this study to 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 so large that, they would require two orders of
magnitude larger Alfven speed than the typical 0.5 Mm/s, if interpreted within
a 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 mid
coronal ranges, using such frequent weak bursts. 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.
