The leftmost image shows an active region on the Sun on 1994 October 15
using four different types of radiation. The different types of
radiation, corresponding to different wavelengths, emphasize different
aspects of the region. An active region occurs at a location on the
Sun's
surface where strong magnetic fields rise up from under the surface and
protrude up into the atmosphere. Associated with these magnetic fields
we get strong heating of the upper atmosphere (corona)
which makes an active region much brighter than the surrounding regions
of the solar atmosphere.
The top left panel of the leftmost image (click it to see a larger view) shows a view of the region in soft X-rays, taken with an X-ray telescope on the Earth-orbiting Japanese satellite Yohkoh. In X-rays we see regions of hot dense gas above the surface (at temperatures of millions of degrees), which here can be seen to be following magnetic field lines connecting a sunspot at the front of the actrive region to weaker fields at the back. The top right panel is an optical image of the region's surface (false color): the sunspot shows up as a dark circle because it is cooler than the rest of the Sun's surface. The two bottom panels show the region at two radio wavelengths: 1.5 GHz on the left, where we again see mostly the densest gas in the corona above the surface; and 5 GHz on the right, where the magnetic field of the sunspot makes it a much brighter source than the hot gas to the left.