Aptly enough, we call these coronal holes. These dark patches in the sun million plus degree corona are areas where the solar atmosphere rapidly streams away from the sun.
This stream of particles from the solar atmosphere is called the solar wind. It comes from all over the sun but the fastest streams of solar wind come from coronal holes. These are regions where the sun’s magnetic field opens into space instead of looping back to the solar surface.
Because there is less atmosphere where the fast wind streams away, there is less material to emit light. So they appear dark compared to the surrounding atmosphere.
The fast solar wind is about twice as fast as the normal or slow solar wind. When it reaches Earth, the fast solar wind pushes harder on the magnetosphere, disturbing it. When the magnetosphere become disturbed it can release particles it has stored up in its tail.
These particles travel down the magnetic field to Earth’s poles (North and South), giving us aurora. High speed solar wind from the coronal hole facing us should reach Earth around June 2 – 3. Then high latitude aurora watchers may get a treat.