cloud droplets that produced Amber Rahm's corona were 24 micron
The inner red edge of the first ring is 9.5 moon disks in diameter
or 4.25°. This IRIS simulation for 24 micron diameter droplets
matches the ring size.
Lunar coronae are much more familiar
than those around the sun.
They are seen when the clouds are thin enough that each single corona
light ray reaching the eye is scattered or diffracted by only one
droplet. Of course, the whole corona is made by a great many droplets
scattering the moonlight.
Sometimes as clouds pass over the moon the corona shrinks and swells
as different sized droplets mould it. Small droplets make the largest
coronae with aureoles a few moon diameters across.
A 22° halo
can also surround the moon but that is much larger.
Any small particle can make a corona. Ice crystals in high clouds
form them. Even wind drifting pollen grains make them.