Second, the scientists learned more about how the coronal cells are related to other structures on the Sun, in their location between a coronal hole and a nearby filament channel. The cells consistently occurr in areas dominated by magnetic fields that point in a single direction, either up or down. In addition, the fields of the nearby coronal hole are known as "open," extending far into space without returning to the Sun. On the other hand, the field lines in the cells are "closed," looping up over the filament channel and connecting back down to the Sun.
The side-by-side nature of these open and closed magnetic fields — open in the coronal holes, and closed in the coronal cells — led to another scientific insight. In some of the movies, a large loop of solar material called a "filament" erupted from the adjacent filament channel. The coronal cells, with their closed field lines, disappeared and were replaced with a dark coronal hole and its associated open field lines.
"Sometimes the cells were gone forever, and sometimes they would reappear exactly as they were," said 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."
It has long been known that isolated plumes occur intermittently inside coronal holes when very small active regions erupt there. Presumably, these eruptions are providing glimpses of discrete coronal structures similar to the more permanently visible candles adjacent to the holes. When a portion of a hole closes, the candle-like structure is suddenly lit up by the appearance of cells.
In addition to SDO and STEREO, the team went back to historical data on the European Space Agency’s (ESA) and NASA's Solar and Heliospheric Observatory (SOHO), which provided observations since the previous sunspot minimum in 1996. They did not find coronal cells in 1996 or in the years around the recent sunspot minimum in 2008-2009, but they did find numerous examples of cells in the years around the intervening sunspot maximum in 2000. The recent increase in sunspot activity together with the improved observations from STEREO and SDO may explain why the cells were discovered in 2011.
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 on the Japanese Hinode spacecraft. These images show that the centers 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.
"One of the wonderful things about SDO is the way the observations can be combined with other instruments," said Dean Pesnell from NASA's Goddard Space Flight Center in Greenbelt, Maryland. " Combining data from SDO, STEREO, SOHO, and Hinode lets us paint a picture of the whole Sun in ways that one instrument can't."
The discovery of coronal cells has already increased astronomers’ 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.Solar Coronal Cells as Seen by STEREO
changes of a coronal cell region as solar rotation carries it across
the solar disk as seen with NASA's STEREO-B spacecraft. The camera is
fixed on the region (panning with it) and shows the plumes changing to
cells and back to plumes again -- based on the observatory's perspective
-- during the interval June 7-14, 2011.