During an eclipse, when the moon temporarily obscures the overflowing light from the solar photosphere, astronomers can perform unique measurements.
For example, they analyze the normally invisible red halo of the chromosphere, the layer of the solar atmosphere above the turbulent surface of the photosphere. This type of image can be obtained with the last and the first light of the solar limb – just before and after the totality of the eclipse, respectively -, reason why has been given the name of spectrum of the ‘flash’, since the measurements must be made in a matter of seconds. At that time, the emission of the Sun can be divided into a spectrum of colors that shows the traces of different chemical elements.
The colorful image that accompanies this news shows a ‘specter of the flash’ of the chromosphere of the Sun captured during the total eclipse that occurred throughout the United States on August 21, 2017. It is the work of the team of the expedition of the European Space Agency (ESA) that monitored it from Casper (Wyoming). The spectrum of the flash shown here occurred with the first solar limb observable after the whole. The exposure to capture this image was exactly 1/30 s. On the right is an image of the eclipsed Sun and, on the left, a superimposed spectrum of each point of the Sun.
The most powerful emission is due to hydrogen, including the emission of hydrogen-alpha red at the right end and blue and violet at the left. In between, the bright yellow corresponds to helium, an element discovered in a spectrum of flash caught during the total eclipse of August 18, 1868, although at that time did not know what it was. Almost three decades later, the element was discovered on Earth and today is known to be the second most abundant element of the Universe, after hydrogen.