Frequency-distance structure of the solar radio sources observed by LOFAR

Mykola Gordovskyy, University of Manchester

Low-frequency radio observations make it possible to study the solar corona at distances up to 2–3 solar radii. Frequency of plasma emission is a proxy for electron density of the emitting plasma and, therefore, observations of solar radio bursts can be used to probe the density structure of the outer corona. In this study, positions of solar radio sources are investigated using LOFAR spectral imaging data in the frequency range 30-50 MHz. We show that there are events where apparent positions of the radio sources cannot be explained using the standard coronal density models (such as Newkirk, Baumbach-Allen and Saito models). Namely, the apparent heliocentric positions of the sources are 0.1–0.7 solar radii further from the Sun compared with model predictions, and these shifts are frequency-dependent. We discuss several possible explanations for this effect, including enhanced density in the flaring corona, as well as scattering and refraction of the radio waves due to the plasma turbulence.