General exam question G3A10
Subelement G3, RADIO WAVE PROPAGATION· Group G3A
What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
- A.Long term oscillations in the upper atmosphere
- B.Cyclic variation in Earth’s radiation belts
- C.Rotation of the Sun’s surface layers around its axis (correct answer)
- D.The position of the Moon in its orbit
WhyHF propagation conditions vary periodically on a 26- to 28-day cycle because the Sun's surface layers rotate around its axis on roughly that period. The Sun rotates on its own axis roughly once every 26 to 28 days, so the same active regions and coronal holes rotate back into view (and back to facing Earth) on that same cycle, which is why propagation conditions tend to echo themselves on a similar period, answer C. Upper-atmosphere oscillations, radiation belt cycles, and the Moon's orbital position don't drive this particular periodicity.
Published 2026-08-28 · Last updated 2026-08-28 · Question text is verbatim from the 2023-2027 NCVEC Element 3 pool (public domain); the explanation above is original writing, see our methodology.
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