The energy spectrum of spacecraft radio signals in the caustic shadow zone of the Sun: A new diagnostic of the solar coronal plasma

An integral representation of the wave field in the form of an interference integral is used to investigate the effects of near-solar plasma inhomogeneities on the propagation of radio waves. Asymptotic formulas are derived for the energy spectrum of spacecraft radio signals in the illuminated region and in the neighborhood of a caustic. Numerical simulations of the energy spectrum are performed for different parameters of the plasma inhomogeneities in the solar corona. In the caustic shadow region, where direct and Sun-reflected rays are formally excluded, it is shown that the form of the energy spectrum depends directly on the scattering inhomogeneity parameters. The additional information about the wave field structure in the presence of caustic focusing is a promising new diagnostic for coronal radio sounding investigations.