Every 22 years

1908 — THE SUN IS MAGNETIC

Animation of Sun's flipping magnetic polarity

American astronomer George Ellery Hale notices that spectral lines split in regions near a sunspot. The phenomenon, known as the Zeeman effect, was known to occur in the presence of intense magnetic fields. His results suggest that sunspots have magnetic fields over a thousand times stronger than Earth’s.

Over the following decade, Hale and colleagues discover that the changing number of sunspots reflects a larger magnetic process on the Sun known as the solar cycle. Every 22 years — twice the length of the sunspot cycle — the magnetic polarity of the Sun reverses. North becomes south, and vice versa.

https://www.nasa.gov/feature/goddard/2020/spots-waves-and-wind-a-solar-science-timeline-fulltext/