When solar flares leap outwards from the Sun high-frequency images of them can sometimes look a little like a hand, with several thin streamers reaching upwards, known as a Flare Arcade. In video form flares show the reverse, fingers of darkness curling down towards the Sun. Now astronomers think they have explained these “downward-moving dark voids” as products of the unmixing of fluids of different densities.
Once perhaps the bands of darkness reaching threateningly towards our source of light might have been named after a dark lord from a fantasy novel or some mythical light-vanquishing demon. Having only been discovered in 1999 as extreme ultraviolet and soft X-ray images of the Sun became available, however, they go by the more prosaic name supra-arcade downflows, although at least the acronym is SADs.
It's taken 23 years but astronomers have now offered up in Nature Astronomy an explanation for SADs.
Almost as soon as SADs were seen astronomers had a hypothesis. Prior to solar flares, the Sun's magnetic field gets distorted and condensed. Energy is released as the field reconfigures, causing the flares.
“It’s like stretching out a rubber band and snipping it in the middle. It’s stressed and stretched thin, so it’s going to snap back,” said Dr Kathy Reeves of the Harvard & Smithsonian Center for Astrophysics in a statement.
The idea the SADs are a product of the snap back was an obvious one. The paper begins: “Magnetic reconnection is a universal process that powers explosive energy-release events.” Astronomers think something similar, on a vastly more powerful scale, is responsible for the newly discovered object that produces radio waves like a bright pulsar, but orders of magnitude slower. However, the observations didn't match the theory. Instead of falling quickly, as models suggested, SADs curl down slowly.
The abundance of data produced by the Solar Dynamics Observatory, which takes images of the Sun every 12 seconds at seven wavelengths, provided the tools required to solve the anomaly.
"Those dark, finger-like voids are actually an absence of plasma. The density is much lower there than the surrounding plasma," Reeves explains. We have an everyday example of what happens when fluids of different densities combine when we try to mix oil and water-based liquids, for example olive oil and vinegar for a dressing. No matter how hard we stir them together, the two eventually separate, but the process can be slow.
If this is indeed why SADs occur, it may not just be solar astronomers the explanation will make happy. The paper considers the process “A phenomenon analogous to the formation of similar structures in supernova remnants,” so the work may help explain these as well.