Coronal hole

A coronal hole is a region in the solar corona where the magnetic field lines are open rather than closed, allowing the solar wind to escape freely into interplanetary space at velocities of 600–800 km/s. These dark, cooler regions in extreme ultraviolet images of the Sun are the primary source of the fast solar wind, contrasting with the closed-field regions near the magnetic equator that produce the slower, denser streamers. Coronal holes are not static features: they rotate with the Sun, expand and contract with the solar cycle, and migrate from the poles toward the equator as solar minimum approaches. Their persistence over multiple solar rotations creates recurrent high-speed streams that interact with preceding slow wind to produce corotating interaction regions — compressive boundaries that can trigger geomagnetic disturbances even without coronal mass ejections.

Coronal holes are the solar system's open windows — not defects in the corona but functional outlets through which the Sun breathes its magnetic breath into the heliosphere.