A solar researcher has spotted a hitherto-unreported feature of the Sun's atmosphere, giving us an insight into the star's magnetic structure.

Neil Sheeley at the Naval Research Laboratory in Washington DC was examining the daily images sent back from Nasa's Solar Dynamics Observatory satellites when he saw a pattern of cells -- with bright centres and dark boundaries. That's normal on the surface of the Sun, but it hasn't previously been documented in its corona, which is normally dominated by loops and holes.

The holes represent colder, less dense areas of the atmosphere, and the loops represent "filament channels", where there's a boundary between sections of differing magnetic fields. The coronal cells appear to pop up in the areas between these features, indicating that their evolution could be tied to how the magnetic fields change on the boundaries of coronal holes, and how they affect the solar wind -- which has a significant impact on the Earth.

"We think the coronal cells look like flames shooting up, like candles on a birthday cake," says Sheeley. "When you see them from the side, they look like flames. When you look at them straight down they look like cells. And we had a great way of checking this out, because we could look at them from the top and from the side at the same time using observations from SDO, STEREO-A, and STEREO-B."

Solar Imagery

As the Sun span through its 27-day rotation period, the coronal cells were snapped by all three satellites from different angles, allowing Sheeley and his colleague Harry Warren to get a three-dimensional perspective on the structure of the cells. Then, using the SDO's Helioseismic and Magnetic Imager, the pair superimposed regular images of the cells with images of the magnetic fields present in the atmosphere. This showed bundles of magnetic field at the centre of the cells, differentiating them from to-called "supergranules", which have enhanced magnetic fields at their edges.

The team were also able to see how the cells relate to other structures in the Sun's atmosphere. In some of the time-lapse imagery, the cells disappeared when filaments lept out of the atmosphere near them, to be replaced by dark coronal holes. However, they didn't always stay gone. "Sometimes the cells were gone forever, and sometimes they would reappear exactly as they were," says Sheeley. "So this means we need to figure out what's blowing out the candles on the birthday cake and re-lighting them. It's possible that this coronal cell structure is the same structure that exists inside the coronal holes -- but they're visible to us when the magnetic fields are closed, and not visible when the magnetic fields are open."

The findings shed light on the little-understood magnetic structure of the Sun's atmosphere, and were reported in the Astrophysical Journal.