This is an image of the Sun's chromosphere, from chromos, the Greek word for color. This layer of the Sun can be detected with a telescope that looks for the red light it gives off, called Hydrogen-alpha light. The temperature of the chromosphere is about 6000 to 50000 degrees C. In this image cooler features, like filaments, appear darker because they do not give off as much red light.The chromosphere is a 2000 km thick layer of gas just above the photosphere. The chromosphere is most easily viewed by special telescopes that filter out all other wavelengths of light from our Sun and only letting the red light from the chromosphere through.
Image Credit: NOAA/SEL/USAF