The Helioseismic Magnetic Imager (HMI) aboard the Solar Dynamics Observatory takes a series of images every 45 seconds in a very narrow range of wavelengths in visible light of the solar photosphere.
The wavelengths correspond to a region around the 6173 Ångstroms (617.3 nanometers) spectral line of neutral iron (Fe I). From this series of images, it constructs a set of images which extract other characteristics of the photosphere. For this dataset, it measures the shifting of the spectral lines to determine the velocity of gas flows on the solar surface. This spectral line shift is due to the Doppler effect. Blue represents motion towards the observer. Red indicates motion away from the observer.