Radio2space.com's The Radio Sun says:
At very low frequencies (below 0.1 GHz) and therefore at very long wavelengths (> 3m) the solar disk appears much bigger and brighter in the center, and its brightness gradually decreases and vanishes after several solar radii.
and I guess if you sell dishes and feed horns, then < 100 MHz may seem like "very low frequencies".
This page shows an image of the Sun at 4 GHz and it's relatively dark but lined with bright "spots" in two bands on either side of the equator.
But what does the Sun "look like" below 100 MHz? Is it just a fuzzy, unresolved blob that fades to zero at several solar radii, or can one resolve the Sun's edge.
Different but related:
- Do stars have "radio photospheres"? Are they different from their optical photospheres?
- What will it finally take to accurately measure the distance to Betelgeuse? (includes links to radio images of other stars)
The radio Sun: radio image of the Sun recorded by VLA. The brightest regions are part of corona nearby but beyond sunspots. Courtesy (NRAO/AUI)