Question
What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
Answer Options
- 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
- D) The position of the Moon in its orbit
Correct Answer: C
Explanation
The rotation of the Sun is responsible for a recurring variation in HF propagation conditions. The Sun’s surface and atmosphere do not rotate as a single rigid body; rather, the rotation period varies with latitude, taking approximately 26 to 28 days to complete one rotation near the equator.
Because persistent active regions on the Sun (like sunspots or coronal holes) that cause radio disturbances are fixed to the Sun’s surface, they appear to recur on a 27-day cycle as the Sun rotates. This brings the same active regions back into position to bombard the Earth with solar wind and radiation, causing predictable recurring changes in propagation conditions.
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