We came across an oddity this week that our spacecraft has rarely observed before: a dark filament encircling an active region (Oct. 29-31, 2017). Solar filaments are clouds of charged particles that float above the sun, tethered to it by magnetic forces.
They are usually elongated and uneven strands. Only a handful of times before have we seen one shaped like a circle. (The black area to the left of the brighter active region is a coronal hole, a magnetically open region of the sun).
Credit: Solar Dynamics Observatory, NASA.https://sdo.gsfc.nasa.gov/gallery/potw/item/854
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