Note that the ring around the sun in this picture is created by a lens. You wouldn't see it without a camera.
Agreed.
Thing is, it doesn't help much to decide what we should render.
What you
actually see looking into the sun is... complicated. Part of it is simply oversaturation of the light-sensitive cells in the eye - which is what fades immediate brightness and creates dark after-images later as the cells aren't ready to receive light for a while. Part of it is deviation from logarithmic Weber-Fechner intensity perception - actual light intensity isn't perceived on the typical log scale. Part of it is color de-saturation of the scene surrounding the glare - color contrasts seem to go away to some degree. Part of it is spikes/rays generate in the lens of the eye.
Admittedly I can't render most of that - largely because the screen simply doesn't produce anything like the real intensity and will never blind you.
So, having said all that - I think the lens think looks
more similar to the impression I have looking into the sun than what we have now, despite of it
not being actually correct. I'd say it is less wrong perhaps.
Also, the way it's implemented, it's supposed to represent a degree of blinding glare. I understand the wide halo in the current implementation is supposed to represent Mie, so it's not simply a replacement of an existing texture, it's a fourth component to the Sun disc rendering. And you can give it a texture you consider more appropriate for the physiology of the eye if you have one. Maybe there's one which is even less wrong. Your halo.png looks fairly promising as well - so I'll give that a try.