Sunday, October 10, 2010

A Southern Active Region, but No Southern Polar Coronal Hole?

As active region 11112 rotates into view we are seeing Solar Cycle 24 continue to increase in activity in both hemispheres. But why is there a dark hole in the Sun's north pole when we look at AIA 193 images but not in the south? These dark patches are called Polar Coronal Holes and are places where the Sun's magnetic field emerges from the Sun into the heliosphere.

They are best seen at solar minimum and disappear at solar maximum. But there may be a hole in the south that we can't see.

The Sun rotates on an axis that is tilted from the Earth's orbit by about 7 degrees. From August to October people watching the Sun from the Earth see the Sun's north pole and the Sun's south pole is invisible. From February to April we can see the south pole but not the north. To see if there is a southern polar coronal hole you need to look at the Sun from another position. We happen to have two STEREO spacecraft almost 90 degrees from the Earth doing just that. If you go to the STEREO webpage you will see there is still a southern polar coronal hole!