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December 17: Three Years of SWAP Data

  December 17: Three Years of SWAP Data  
  Three Years of SWAP Data  


Click on the above image movie showing solar activity over the course of three years


There are a lot of different ways to track the evolution of the solar activity cycle. One of the most common ways to do this is to track the sunspot number. Since sunspots are caused by strong concentrations of magnetic field at the solar surface, and solar activity is driven by the energy stored in these magnetic fields, sunspots serve as a pretty good proxy for the level of solar activity. When the Sun has only weak fields, sunspots disappear and so does solar activity. As the Sun’s differential rotation scrambles the Sun’s magnetic field, complex regions called active regions begin to poke through the surface, creating sunspots, solar flares, and other coronal activity.
But there are other ways to track solar activity as well. In this paper, led by SWAP principal investigator Dan Seaton, the PROBA2 team explored what we can learn about solar activity by studying the evolution of the brightness of the corona in the extreme ultraviolet far from the surface of the sun where only SWAP, with its large field of view, can see it. The plot above, one of several from the paper, breaks down the sunspot number, the brightness of the solar disk, and the brightness of the corona at heights above 1.3 solar-radii by solar hemisphere.  The yellow plots show the northern hemisphere, while the blue shows the southern hemisphere.
The SWAP team found that the brightness of the corona in EUV does capture some of the same evolution as the sunspot number, but in a very different way. The coronal brightness at large heights increases quite a lot as activity increases, but the increase has a saw-toothed shape, meaning that whatever causes the brightening is a relatively transient phenomenon. The SWAP team speculated that the presence of huge, bright fan-like structures that form near solar active regions was responsible for this brightening. As they rotate across the face sun the curve dips, then jumps as the reach the solar limb and appear high above the sun.
In the specially processed movie linked above, which shows the evolution of the corona over the course of the first three years of SWAP’s mission, you will see several of these structures clearly as they rotate around and around.  Watch especially for structures associated with the strong brightening in the northern hemisphere near the end of 2011, the period of maximum activity of this solar cycle so far.