Discussion of Solar Image
The solar picture you just saw is an X-ray image, like the one above. That
is, it was made by
focusing the X-rays that are produced by the Sun's
hot outer atmosphere. X-rays are produced where the temperature of the
superhot gas (or "plasma") reaches more than a
million degrees, and where the density of the plasma exceeds a certain
threshold. Since X-rays are invisible, we have digitized
the images and made them false-color so you can see where the hottest,
most dense, plasma is located. The electrically
charged (that is, not neutral) plasma is tied to the magnetic field, so
the brighter condensations in the image are "active regions"
where the magnetic field is concentrated.
NOTE: It's only been about fifty
years that we've known that the Sun even produces
X-rays. Why the outermost atmosphere of the Sun (called the "corona") is
hot enough to do this is still a mystery.