Depolarization of radio bursts due to reflection off sharp boundaries in   the solar corona
  It is argued that depolarization of solar radio bursts requires reflection off boundary layers no thicker than about a wavelength (a few meters at most) between regions with large density ratios. The implied inhomogeneities suggest that the corona is much more highly and sharply structured than can be resolved from observations at other wavelengths. A simplified version of magnetoionic theory is used to derive a depolarization coefficient: the effect of the magnetic field is ignored in treating the dispersion, but taken into account in treating the (circular) polarization. Plots of the depolarization coefficient are used to infer conditions under which effective depolarization occurs, and it is concluded that favorable conditions require total internal reflection. For type I sources away from the central meridian, effective depolarization requires reflection off an overdense structure with a density ratio $\xi\approx10$. For type III bursts, a density ratio $\xi\approx2$ suffices, with at least two reflections off walls of ducts at $\approx20^\circ$ to the radial. 
D.B Melrose