Estimating Coronal Propagation Effects in the Quiet Sun Corona Using Low Radio Frequency Observations

Abstract:

It has long been appreciated that, on its way to the observer, the low radio frequency coronal emissions get substantially modified due to propagation through the magnetized, inhomogeneous and turbulent corona. The scattering and refraction suffered by this radiation make it hard to estimate the intrinsic properties of the coronal radio sources. On the other hand, though, the distortion and smoothening imposed by these propagation effects can also potentially serve as a probe of the nature of coronal inhomogenities and turbulence. Here we present our attempt to characterize coronal propagation effects using this approach. For this we compare high quality radio maps from the Murchison Widefield Array (MWA) at multiple frequencies in the 108-240 MHz range with the corresponding simulated radio maps from the FORWARD package. FORWARD uses self-consistent Magnetohydrodynamic Algorithm outside a Sphere coronal model to simulate thermal bremsstrahlung images, but does not include propagation effects. We attribute the observed differences between the simulated and observed MWA radio maps, made over short temporal and narrow spectral spans, to propagation effects. Though there is good general correspondence between the predicted and observed brightness distributions, there are significant differences as well. Clear evidence for presence of significant propagation effects, including anisotropic scattering is observed. We find that the observed radio size of the Sun is 25–30% larger in area than that in the simulated images. The peak of the emission associated with the only radio bright active region in the observed maps is found to shift by 8’-11’ from its location in the simulated maps in a non-radial direction, and its size is seen to increase by 35-40%. We use simple models to estimate the fraction of scattered flux density and always find it to be larger than few tens of percent, though it varies significantly between the representative coronal hole, quiet sun and active sun regions studied. We also estimate the coronal density inhomogeneities to lie in the range 1–10%.