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 an important role in terms of what we see on these images. The bright areas on these images show us hot and dense gas that is captured by the magnetic field of the Sun. The dark and in a way almost empty looking areas are places where the magnetic field of the Sun reaches into space so that these hot gases can escape. These areas look so dark because there is very little hot material compared to their surroundings.
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 towards Earth at a higher speed than the regular solar wind and cause geomagnetic disturbances on Earth with enhanced auroral activity. Depending on the size and location of the coronal hole on the disk, more or less auroral activity can be expected. Large coronal holes often result in faster solar wind than smaller coronal holes. Coronal holes are usually not interesting for aurora watchers at the middle latitudes and only occasionally cause geomagnetic storm conditions.
Other than a coronal mass ejection, a coronal hole high speed stream (CH HSS) arrives slowly with first a steady increase in the solar wind density over the course of a couple of hours. 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 often referred to as a Stream Interaction Region (SIR) or as a Co-rotating Interaction Region (CIR) and is almost always associated with an increase in the total strength (Bt) of the interplanetary magnetic field. When this compressed solar wind boundry has passed Earth, we will see that the solar wind speed starts to increase while the total strength (Bt) of the interplanetary magnetic field and the solar wind density decreases.
Image: Geometry of the interaction between fast solar wind and ambient solar wind.
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