Sun in helium D3 (Coronal Hole!)
Taken by Richard Schrantz on September 11, 2025 @
Nicholasville, Kentucky, USA
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Here is a comparison image of a helium D3 spectroheliogram vs. a Solar Dynamics Observatory satellite image at extreme ultraviolet 211 Angstroms (both from this morning). Note the large coronal hole in the SDO image. It also shows up in the helium D3 image, because helium details are created by extreme UV radiation shining down from the corona above. Where there is a coronal hole, there is less EUV.....and correspondingly less helium detail. It is a way to detect coronal holes from earth's surface indirectly! Aren't spectroheliographs cool? Yes they are. 102mm f/7 SHG, ASI183mm camera.
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