What is a coronal hole?

When we look at solar imagery from NASA's Solar Dynamics Observatory (SDO) at a wavelength of 193 or 211 Ångstöm, we can see the hot outer layers of the Sun's atmosphere. This outermost layer of the Sun is called 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 is captured by the magnetic field of the Sun. The dark and almost empty looking areas are places where the magnetic field of the Sun reaches into space so that hot gas can escape. These areas look so dark because there is very little hot material compared to their surroundings.

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

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

The magnetic field of 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 only occasionally cause a geomagnetic storm.

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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