Coronal cells spotted on Sun
Researchers have for the first time observed a pattern of cells with bright centres and dark boundaries occurring in the sun’s atmosphere, the corona.
Neil Sheeley, a solar scientist at the Naval Research Laboratory in Washington, D.C, looked through the images of the sun from NASA’s Solar Dynamics Observatory (SDO).
The cells looked somewhat like a cell pattern that occurs on the sun’s surface – similar to the bubbles that rise to the top of boiling water – but it was a surprise to find this pattern higher up in the corona, which is normally dominated by bright loops and dark coronal holes.
Sheeley discussed the images with his Naval Research Laboratory colleague Harry Warren, and together they set out to learn more about the cells.
Their search included observations from a fleet of NASA spacecraft called the Heliophysics System Observatory that provided separate viewpoints from different places around the sun.
They have described the properties of these previously unreported solar features, dubbed “coronal cells”.
The coronal cells occur in areas between coronal holes – colder and less dense areas of the corona seen as dark regions in images and “filament channels” which mark the boundaries between sections of upward-pointing magnetic fields and downward-pointing ones.
Understanding how these cells evolve can provide clues as to the changing magnetic fields at the boundaries of coronal holes and how they affect the steady emission of solar material known as the solar wind streaming from these holes.
“We think the coronal cells look like flames shooting up, like candles on a birthday cake,” said 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.”
The researchers used time-lapse sequences obtained from the three satellites to track these cells around the sun. When an observatory looked down on one of these areas, it showed the cell pattern that Sheeley first noticed.
But when the same region was viewed obliquely, it showed plumes leaning off to one side. Taken together, these two-dimensional images reveal the three-dimensional nature of the cells as columns of solar material extending upward through the sun’s atmosphere, like giant pillars of gas.
To round out the picture even further, the team turned to other instruments and spacecraft.
The original SDO images were from its Atmospheric Imaging Assembly, which takes conventional images of the sun. Another instrument on SDO, the Helioseismic and Magnetic Imager (HMI), provides magnetic maps of the sun.
The scientists superimposed conventional images of the cells with HMI magnetic field images to determine the placement of the coronal cells relative to the complex magnetic fields of the sun’s surface.
The team also constructed Doppler images – images that show how quickly and where solar material in the sun’s atmosphere moves toward the viewer – of the coronal cells using the Extreme-Ultraviolet Imaging Spectrometer (EIS) on the Japanese Hinode spacecraft.
These images show that the centres of the cells move upward faster than their boundaries, further rounding out the physical image of these giant candles with a section rising from the middle.
The study has been published online in The Astrophysical Journal.