Effects of Scattering on Radio Emission from the Quiet Sun at Low Frequencies

Abstract

The observations of the quiet Sun at meter and decameter wavelengths show that its brightness temperatures can be ~1 order of magnitude lower than the expected values of 106 K and the apparent diameters can be very large. We examine whether this unusual behavior is due to refraction in the smoothly varying coronal plasma and scattering by random density fluctuations using an improved Monte Carlo simulation technique. We use a three-dimensional model for the electron density, -->Ne, and a power-law spectrum for the density fluctuations with an exponential index of α. We consider two cases: (1) -->α = 11/3 (Kolmogorov spectrum), relative level of density fluctuations --> = Δ Ne/Ne = 0.1, and scale heights ranging from the inner scale --> lo = 106li, and (2) -->α = 3 (flat spectrum), --> = 0.02, and scale heights ranging from 50 to 75 km. We consider the fluctuations to be anisotropic with axial ratios of ~10. The Kolmogorov type of fluctuations yield slightly lower brightness temperatures in comparison with those of a flat spectrum. The brightness temperature distributions, east-west diameters, and central brightness temperatures at various frequencies indicate that the refraction and scattering are probably the underlying reasons for the unusual behavior of the quiet-Sun radio emission. This study clearly demonstrates that Monte Carlo techniques can be very effective in extracting the coronal electron temperatures accurately from the radio data, provided the information about the density distributions and density fluctuations is known by some independent methods.