  The observed properties (i.e., source size, source position, time duration,
decay time) of solar radio emission produced through plasma processes near the
local plasma frequency, and hence the interpretation of solar radio bursts, are
strongly influenced by propagation effects in the inhomogeneous turbulent solar
corona. In this work, a 3D stochastic description of the propagation process is
presented, based on the Fokker-Planck and Langevin equations of radio-wave
transport in a medium containing anisotropic electron density fluctuations.
Using a numerical treatment based on this model, we investigate the
characteristic source sizes and burst decay times for Type III solar radio
bursts. Comparison of the simulations with the observations of solar radio
bursts shows that predominantly perpendicular density fluctuations in the solar
corona are required, with an anisotropy factor $\sim 0.3$ for sources observed
at around 30~MHz. The simulations also demonstrate that the photons are
isotropized near the region of primary emission, but the waves are then focused
by large-scale refraction, leading to plasma radio emission directivity that is
characterized by a half-width-half-maximum of about 40~degrees near 30~MHz. The
results are applicable to various solar radio bursts produced via plasma
emission.
