Filaments are huge ‘clouds’ of hydrogen plasma held above the solar chromosphere by magnetic fields. They are cooler and therefore appear darker than the underlying chromosphere (or lighter when the surface has been inverted, as here).

As the Sun rotates, filaments are carried across to the western limb. Here their elevated nature becomes evident because the appear to extend off the edge of the Sun as a prominence.

Amateur solar observers coined the term ‘filaprom’ to describe the transition of a filament into a prominence. In this image careful processing has been used to show the transition effect.

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