Observations of the Quiet Sun at Decameter Wavelengths - Effects of Scattering on the Brightness Distribution

The emission from the quiet Sun has been observed at 60 MHz, 36.9 MHz and 29.3 MHz with the large Arecibo radiotelescope. The flux densities and dlmensions of the source have been determined, enabling us to deduce the brightness temperature in the direction of the center of the Sun. The values obtained decrease rapidly with frequency (Tb = 0.60 10 K, 0.50 10 0K and 0.38 10 0K at 60, 36.9 and 29.3 MHz, respectively). We show that these values cannot be explained either by the generally adopted models of Ne and Te in an homogeneous corona, or by changing the gradient of Ne (Q) or T,(Q). To explain the observed values, we introduce the effect of scattering by an irregniar corona. This does indeed lead to a decrease in the flux density and an increase in the size of the source, as compared with the values obtained without scattering. Consequently the apparent brightness temperature decreases. It is shown that the scattering effects increase when the frequency decreases and can explain the observed characteristics of the emission from the qulet Sun at low frequencies, although these effects are negligible at shorter meter wavelengths. Key words: decameter qulet Sun - scattering