A ground-based image of the total solar eclipse on Aug. 21, 2017 (gray, middle ring), is superimposed over an image of the Sun’s atmosphere, called the corona (red, outermost ring), as seen by ESA (the European Space Agency) and NASA’s Solar and Heliospheric Observatory (SOHO), which watches the Sun from space. At center is an image of the sun’s surface as seen by NASA’s Solar Dynamics Observatory in extreme ultraviolet wavelengths of light.
During a total solar eclipse, ground-based telescopes can observe the lowest part of the solar corona in a way that can’t be done at any other time, as the dim corona is normally obscured by the bright light of the Sun. The structure in the ground-based corona image — defined by giant magnetic fields sweeping out from the Sun’s surface — can clearly be seen extending into the outer image from the space-based telescope. The more scientists understand about the lower corona, the more they can understand what causes the constant outward stream of material called the solar wind, as well as occasional giant eruptions called coronal mass ejections.