The Sun’s magnetic field lines can cause dramatic events like sunspots and solar flares. For a coronal hole, however, the reason it looks darker is because this region is cooler and less dense than other parts of the Sun. So it appears less bright in this extreme ultraviolet view from the SDO.
“This open, magnetic field line structure allows the solar wind to escape more readily into space, resulting in streams of relatively fast solar wind,” notes the Space Weather Prediction Center.
This particular hole was associated with some powerful aurorae earlier this month, which were seen as far south as Nebraska in the US. The additional particles from the Sun flow into the poles of our planet, exciting particles and causing the bright colors associated with aurorae. The Sun is now heading towards a period of solar minimum, when the number of sunspots and solar flares should decrease. Coronal holes, however, should become more prevalent.
“During times of low solar activity, coronal holes cover the north and south polar caps of the Sun,” notes the Encyclopedia of Astronomy and Astrophysics. “One of the reasons that coronal holes are important to study is that they are thought to be the primary sites of acceleration for the high-speed solar wind.”
So, there’s a hole on the Sun, but there’s nothing to worry about. Yet…