Walter Grotrian

Walter Grotrian was a German astrophysicist known for his work on the solar corona. In 1930, he examined spectra of the corona taken during a total solar eclipse. He observed that while the corona's light had the same colors as the Sun's visible surface (the photosphere), it lacked the absorption lines seen in photospheric light.1

Grotrian hypothesized that this coronal light was scattered by free electrons moving at high speeds, indicating a very hot corona. He also noted unusual bright lines in the corona's spectrum. He speculated these lines might originate from ionized oxygen and nitrogen, similar to lines seen in nebulae, suggesting extremely low density and high temperatures. Collaborating with physicist Bengt Edlén, Grotrian helped identify these lines as originating from elements like iron ionized at temperatures around one million Kelvin. This research significantly advanced the understanding of the Sun's extremely hot outer atmosphere.1