What is a coronal hole?

When we look at imagery from SOHO or SDO at a wavelength of 28,4 nanometer, it shows us the hot outer layers of the atmosphere of the Sun, to be specific: the corona. The magnetic field of the Sun plays a great part in how we see this image. The bright areas shows us hot and dense gas that's captured by the magnetic field of the Sun. The dark and empty areas are places where the magnetic field of the Sun reaches into space so that hot gas can escape. These dark areas are coronal holes. They get their dark colour because there isn't enough hot material.

Coronal Hole

Image: A typical coronal hole as seen by NASA's Solar Dynamics Observatory.

The magnetic field around a coronal hole is different than the rest of the Sun. Instead of returning to the surface, these magnetic field lines stay open and stretch out into space. At the moment we do not yet know where they reconnect. Instead of keeping the hot gas together, these open magnetic field lines cause a coronal hole to form, where solar wind can escape at high speeds. When a coronal hole is positioned near the centre of the Earth-facing solar disk, these hot gasses flow to Earth at a higher speed than the regular solar wind and cause geomagnetic disturbances on Earth with enhanced auroral activity on high latitudes. Depending on the size and location of the coronal hole on the disk, more or less auroral activity can be expected. Coronal holes are usually not interesting for middle latitude sky-watchers and often do not even produce geomagnetic storming conditions.

How do I recognize a coronal hole stream?

Other than a coronal mass ejection, a coronal hole high speed stream (short: CH HSS) arrives slowly with first a steady increase in the solar wind density. This increase of the solar wind density occurs because the faster solar wind bunches up the slower solar wind particles in front of it. This phenomenon is also called a co-rotating interaction region (short: CIR) and is often accompanied by an increased strength of the IMF. Only after this co-rotating interaction region we will see that the speed of the solar wind increases while the solar wind density decreases.

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