Interferometric imaging of Type III bursts in the solar corona

Pearse Murphy, Trinity College Dublin & Dublin Institute for Advanced Studies

The size of radio emission sources in the solar corona is thought to be fundamentally limited by scattering off of random density inhomogeneities, most prominently at the plasma frequency. Radio interferometers such as the LOw Frequency ARray (LOFAR) have been increasingly used to study radio bursts in the solar corona over the last number of decades. Observations with LOFAR’s tied-array imaging technique suggest that the source sizes of Type III radio burst emission is limited by coronal scattering. However, it has yet to be determined whether source sizes observed with tied-array imaging are a result of this fundamental limit or an effect due to the unique nature of that imaging mode, i.e forming beams at different locations on the Sun to create an image. LOFAR interferometric imaging gives an alternative measure of source sizes with higher spatial resolution than tied-array imaging. Here, a Type III burst was imaged with LOFAR core and remote stations with baselines extending to 86 km (~22 arcseconds at 32MHz). We show that despite a sub-arcminute resolution, coronal source sizes are of the order of ~10 of arcminutes, significantly larger than would be suggested from the spectral fine structure. This further supports the theory of scattering being the limiting factor in coronal radio observations.