NASA explains that the very first description in the West of the monstrously beautiful solar corona – the glorious plasma aura that surrounds our Sun – probably comes from the writer, speaker, and satirist Lucien in the year 932 BCE. He described the stellar shimmer as “a kind of light [that’s] visible about the rim, which keeps the shadow [of the Moon] from being profound and absolute.”
How times have changed. From text to sketches to photographs, our understanding of this awe-inspiring phenomenon has been transformed over the centuries. Now, as revealed in a new study, we’ve taken some of the best images of it to date, those that have revealed new structures in the corona; hidden filament-like dancers that improve our understanding of the physics of the Sun.
The corona (“crown”), first named during the 1806 eclipse by Spanish astronomer José Joaquin de Ferrer, is mesmerizing. It’s the outermost segment of the Sun’s weird atmosphere, one with multiple sources of light – but light so dim that we can only properly see it when the Moon conveniently blocks out the majority of the Sun’s light.
Extending millions of kilometers into space, its temperature is of the order of several million degrees, far hotter than the surface of the Sun. It’s a ghostly hell, but if you moved through it, its extremely low density means you wouldn’t feel the heat.
There are indubitably more questions than answers at present.
“In deep space, the solar wind is turbulent and gusty,” Craig DeForest – solar physicist at Southwest Research Institute (SwRI), explained in a press release. “But how did it get that way? Did it leave the Sun smooth, and become turbulent as it crossed the Solar System, or are the gusts telling us about the Sun itself?”
Enter, NASA’s STEREO spacecraft, a solar observation mission comprised of two segregated telescopes. The SwRI at Boulder-led team turned to STEREO-A – part one of two – which took long exposures of the Sun while behind it. Longer exposures boosted dimmer details, but the team still had to deal with the inherently fuzzy resolution of the snaps.
Using some novel computational wizardry, the team managed to filter out some of the additional noise, produced by the instrument itself, the Sun, and other electromagnetic interference from elsewhere in space. The team also had to take into account the hyperfast motions of the solar wind emerging from the Sun too in order to make sure their individual images matched up and didn’t have a pesky blur.
Et voila: those fine structures were recorded. They’ve been rather beautifully described as the “cars” of the corona, compared to the “freeway traffic” of the solar wind.
Previously suspecting the outer corona to be a same-same smudge of material, the team were surprised to see any wavering patterns in the miasma at all. It appears that they’re individual parts of larger structures named coronal streamers, closed magnetic loops that are often fairly luminous.
Another revelation involved the Alfven surface, a boundary beyond which any solar material is lost to space forever. The spectacular new images and processing, allowing scientists to track the evolution of the corona like never before, found that the boundary isn’t clean cut, but a wavering, blurrier zone.
Perhaps most enigmatic of all is that, at a distance of around 10 solar radii, their image processing can’t resolve details anymore. Weirdly, at greater distances, they can again. This isn’t likely a computational problem, but rather something far more exciting: a region of the outer corona whose physics we have no understanding of whatsoever.
This is just the beginning. New images, more teams, and new spacecraft will peer into the Sun’s crown, attempting to peel back the enigmatic layers.
This new paper, though – published in The Astrophysical Journal – represents our greatest awareness of the corona to date, many millennia after we first described it shattering the lunar shadow.