Angular scattering of solar radio emission by coronal turbulence

With the advent of large synthesis imaging radio telescopes (e.g., the Westerbrook Synthesis Radio Telescope (WSRT) and the very large array (VLA), many solar observations have been made with a high degree of angular resolution at microwave and decimeter wavelengths. These have revealed a distinct absence of structure on small angular scales. I show that scattering on a turbulent spectrum of fluctuations in the coronal electron number density offers a plausible explanation for the absence of fine structure at radio wavelengths. I review the theory of scattering in an inhomogeneous medium as it pertains to angular broadening and describe recent work on the nature of turbulence in the Sun's outer corona and the solar wind. I then reassess the problem of scattering as it applies to the angular broadening of radio sources embedded in the solar corona in the limit of small-angle scattering. It is pointed out that angular broadening due to scattering represents a fundamental limit on the degree of angular resolution with which solar radio emission can be mapped. The techniques of speckle interferometry are of no help, with the possible exception of observations of millisecond spike bursts. Implications are briefly discussed.