Coronal Holes: Why Some Auroras Return Almost Every 27 Days
Not every active auroral night begins with a spectacular solar eruption. Many displays are powered by coronal holes, regions in the Sun’s atmosphere where magnetic field lines open into space and allow faster solar wind to escape.
An open highway in the magnetic field
In ultraviolet images, coronal holes appear darker because their plasma is less dense and cooler than surrounding regions. They release high-speed streams, often called CH HSS, which can reach Earth and compress the magnetosphere.
Why the pattern can repeat
As seen from Earth, the Sun rotates in roughly 27 days. If a coronal hole survives for several rotations, its stream may return on a similar schedule. Each passage is not identical: the hole’s shape, plasma density and magnetic orientation evolve.
Typical signs
- A gradual increase in solar-wind speed.
- Fluctuating Bz with possible prolonged southward intervals.
- Geomagnetic activity that may be less abrupt than a CME but can last longer.
CMEs are like violent space thunderstorms; coronal holes are strong winds that can return with the rhythm of solar rotation.
How to use them for planning
Solar maps can reveal a large coronal hole before its stream reaches Earth. A 27-day recurrence is a useful clue, not a promise. The final decision must still rely on real-time solar-wind data and local cloud conditions.