On the Relative Brightness of Coronal Holes at Low Frequencies

We present low-frequency (80 – 240 MHz) radio observations of coronal holes (CHs) made with the Murchison Widefield Array (MWA). CHs are expected to be dark structures relative to the background corona across the MWA bandwidth due to their low densities. However, we observe that multiple CHs near disk center transition from being dark structures at higher frequencies to bright structures at lower frequencies (\({\lesssim} \, 145~\mbox{MHz}\)). We compare our observations to synthetic images obtained using the software suite FORWARD, in combination with the magnetohydrodynamic algorithm outside a sphere (MAS) model of the global coronal magnetic field, density, and temperature structure. The synthetic images do not exhibit this transition, and we quantify the discrepancy as a function of frequency. We propose that the dark-to-bright transition results from refraction of radio waves into the low-density CH regions, and we develop a qualitative model based on this idea and the relative optical depths inside and outside a CH as a function of frequency. We show that opacity estimates based on the MAS model are qualitatively consistent with our interpretation, and we conclude that propagation and relative absorption effects are a viable explanation for the dark-to-bright transition of CHs from high to low frequencies.