Only 20kw per square meter on the surface of the sun ? How come it is so low ?
We receive about 1kw of sunlight per square meter on Earth, and earth is 149M km from the sun. From napkin math, it should rather be ~45MW/sqm on the sun to receive 1kw/sqm on Earth (surface of the sphere of radius 149M km divided by surface of the sun gives ~45000, so 1 watt from the sun becomes 1/45000 watt when it reaches the Earth)
Where am I wrong ?
I’m astounded by how plain and round the visible light images are. Why is the corona only visible in the UV images, if it is, according to the article, visible from earth?
Corona is very hot (millions of degrees) as opposed to 6000 of the Sun's surface, therefore it has higher contrast over Sun's surface if you go to shorter wavelengths. The reason corona is still visible from Earth is because it you mask the main solar disk (during the eclipse).
It might be that the surface is much brighter in visible light than the corona rather than the corona emits no visible light (as anyone who witnessed the recent total solar eclipse can attest). Since the corona is made up of rarefied high energy particles I would expect it to emit less total, but more short wavelength light.
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