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G3A10
What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
Pick an answer to check it.
Answer: C. Rotation of the Sun's surface layers around its axis
The Sun rotates about once every 27 days, so active regions face Earth again on a roughly 26- to 28-day cycle. That causes propagation conditions to repeat on that schedule.
Tip: Sun spins about every 27 days.