Coronal Hole
Definition
A dark region of the corona where the Sun's magnetic field opens into space, releasing fast solar wind.
In plain language
In extreme-ultraviolet images the corona shows dark gaps โ places where magnetic field lines stream outward instead of looping back. These act like nozzles, firing fast wind streams that reach Earth as the Sun rotates them into position.
The science
Coronal holes are open unipolar field regions with reduced density and temperature, emitting high-speed streams (500โ800 km/s). Their effects recur with the ~27-day rotation, producing predictable moderate geomagnetic activity.
Why it matters for aurora viewing
Coronal-hole streams are the reliable workhorse of aurora activity โ predictable days ahead and the main show between big CME events.
See this in action on tonight's data.