What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
Long term oscillations in the upper atmosphere
Cyclic variation in Earth’s radiation belts
Rotation of the Sun’s surface layers around its axis✓Bonne réponse
The position of the Moon in its orbit
Explication
An active region on the Sun stays productive for weeks, so as the Sun rotates the same region faces Earth again about every 27 days and similar propagation conditions recur. The period is set by solar rotation, not by the Moon or by terrestrial cycles.