Science Column: ‘Coronal Cells’ discovered on Sun
By Suzanne Smith
At the end of 2011, Neil Sheeley, a solar scientist at the Naval Research Laboratory in Washington, D.C., discovered something new when looking at the daily images of the sun from NASA’s Solar Dynamics Observatory. A pattern of cells with bright centers and dark boundaries could be seen occurring in the sun’s atmosphere, the corona. The cells looked somewhat like the granules, convection cells, that occur on the sun’s surface, but it is surprising to see the phenomenon to occur higher up in the corona.
Sheeley and his team at the Laboratory set out to learning more about the coronal cells. The team used observations from the NASA spacecraft Heliophysics System Observatory. The report of the research is explained more thoroughly in a paper published this month in the Astrophysical Journal.
The coronal cells occur in areas between coronal holes, colder and less dense areas of the corona, seen as dark regions in images of the sun. 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 streaming from these holes.
The researchers used time-lapse sequences obtained from the three satellites, STEREO A, STEREO B, and SDO, to track the cells around the sun. When an observatory looked down into 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 complete the picture of the coronal cells, 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, 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 discovery of coronal cells has already increased our knowledge of the magnetic structure of the sun’s corona. In the future, studies of the evolution of coronal cells may improve scientists’ understanding of the magnetic changes at coronal-hole boundaries and their effects on the solar wind and Earth’s space weather.