Astronomers use certain wavelengths of light and other electromagnetic emissions as "windows" into different regions of the Sun. White light with a wavelength between 400 and 700 nanometers (nm) shows the photosphere, the visible "surface" of the Sun. Red light magnetograms ("magnetic maps") reveal the magnetic field at the photosphere. Infrared emissions from helium, red light from hydrogen, and ultraviolet (UV) "light" from calcium all show features in the chromosphere, the Sun's lower atmosphere, where temperatures rise to tens of thousands of kelvins. Extreme ultraviolet (EUV) emissions from helium at 30.4 nm wavelength reveal the transition region between the lower atmosphere and the much hotter corona (upper atmosphere). High energy EUV and X-ray emissions characterize the corona, where temperatures rise above a million kelvins. Ultraviolet emissions at wavelengths of 17.1, 19.5, and 28.4 nm from iron ions (Fe IX, Fe XII, etc.) reveal hotter regions as our view moves progressively higher above the photosphere. X-rays provide a picture of the upper reaches of the corona at temperatures of several million kelvins.
Original artwork by Windows to the Universe staff (Randy Russell) using images courtesy of SOHO (NASA and ESA), NCAR/HAO/MLSO, Big Bear Solar Observatory, and SDO/AIA.