SIMULATING LOW FREQUENCY RADIO IMAGES OF THE SUN

At low radio frequencies, the strong density gradients in the large scale solar corona lead to significant refraction. This implies that interpretation of solar radio images must account for these refractive effects, and as the ray paths at different frequencies travel through different parts of the corona, multi-frequency observations can help unravel the density and temperature distributions in a range of coronal heights. While the refractive properties of the medium are determined by the electron number density (Ne), the brightness temperature (Tb) distribution in the resulting image is determined by the electron temperature (Te) and the absorption coefficient (κ) distributions. In the solar chromosphere and corona the Ne and Te change by 2, and the κ changes by 7 orders of magnitude, leading to significant changes in the morphology and observed Tb of the solar images. We are developing a flexible and precise tool to simulate brightness temperature, Tb, images of the sun resulting from arbitrary electron density, Ne, and electron temperature, Te, distributions. Although this ray-tracing algorithm is being developed in context of the MWA, a radio interferometer which will operate in the 80-300 MHz band (Oberoi et al., 2009, Rightley et al., 2009), it is of much wider applicability. At high radio frequencies, some rays penetrate into the chromosphere, which has significantly different Ne distribution. This tool implements a mathematical method for smoothly "stitching" the chromospheric and coronal Ne distributions, which are very different functions. As an application of this tool, we present and contrast the brightness temperature images from two well regarded coronal models by Baumbach and Allen (1947) and Saito (1970), and the model for chromosphere by Cillie and Menzel (1935). We also investigate the impact of coronal features like streamers on brightness temperature images.