Explanation of the colors used for SOHO extreme ultraviolet images of the Sun (that is, at wavelengths much shorter than “normal” ultraviolet wavelengths). Such radiation is also sometimes referred to as “soft” X-rays, since the boundary between X-rays and UV is rather arbitrarily set at about 100 Ångstroms.
All the images shown above were taken at wavelengths ten times shorter than the shortest visible wavelengths, imaging photons with ten times more energy than the most energetic visible photons. Since in the visible spectrum, shorter wavelengths correspond to blue and green light, and longer wavelengths to yellow, orange and red light, the shorter wavelength images are arbitrarily colored blue or green, and the longer wavelength images are colored amber or red.
The atoms (or, more accurately, since the atoms are missing one or more electrons, the ions) responsible for the wavelengths involved are indicated by Roman numerals which are one unit greater than the number of missing electrons. Hence, He II means helium ions which are missing one electron, and Fe XV means iron atoms which are missing fourteen electrons. The He II image looks different from the others, because it shows radiation from moderately hot helium atoms in the chromosphere; while the highly ionized iron atoms responsible for the other emissions are located in the lower corona.
Credit: SOHO - EIT Consortium, ESA, NASA, Courtney Seligman