M-region

Name given to a region of activity on the sun when the details and nature of that activity cannot be established.

This term was used in the 1940s to account for recurrent geomagnetic storms with a period the same as the period of solar rotation relative to the earth, that is, 27.3 days. Today these storms are usually associated with high-speed solar wind streams emanating from coronal holes.
See magnetic storm.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.