Coronal holes (CH) are regions of the corona in which the magnetic field lines do not return to the Sun but reach far out into space. The plasma can escape there without hindrance. The corona above a coronal hole is therefore thinner and cooler. In extreme ultraviolet images, coronal holes appear as dark areas.
Fast solar wind at 500 to 800 kilometres per second streams out of coronal holes. The normal solar wind is much slower at about 350 to 400 kilometres per second. If a coronal hole faces the Earth, the fast stream reaches the Earth two to four days later. Coronal holes are often long lived. Because the Sun rotates once in about 27 days, they often return after four weeks.
The fast solar wind catches up with the slow solar wind in front of it and compresses it. A zone of dense plasma and strengthened magnetic field forms. It is called a stream interaction region (SIR). Unlike with a CME, the values do not arrive abruptly but build up slowly. The data of the satellites between the Sun and the Earth show a typical sequence:
- The density of the solar wind rises to a maximum.
- Shortly afterwards the strength of the magnetic field (Bt) increases. Bz often fluctuates strongly between north and south.
- The speed rises while density and magnetic field decrease again.
- The fast solar wind often lasts for several days and then slowly fades.
Coronal holes usually cause weak to moderate geomagnetic storms. The Kp index then often reaches values of 5 or 6. In Germany the aurora on such nights is usually only visible in photos or in the north. However, the continuous observations by AKM members show: coronal holes regularly bring aurora to Germany. This is particularly common in the years after the maximum of a solar cycle. About 40 aurora nights were counted in each of the years 2015 and 2016, and in 2025 even 94.
Around the equinoxes in March and September, the solar wind couples particularly well to the magnetic field of the Earth. Coronal holes therefore cause aurora more often in these months.
