What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle?
The correct answer is C: Rotation of the Sun's surface layers around its axis. What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle is the rotation of the Sun's surface layers around its axis. The Sun rotates, bringing active regions (sunspots, flares) into and out of view, creating a cycle. For amateur radio operators, this explains why propagation varies on a monthly cycle. Understanding this helps predict propagation patterns.
Exam Tip
26-28 day propagation cycle = Sun's rotation. Think '2'6-28 days = 'S'un 'R'otation 'C'ycle. Rotation of Sun's surface layers around its axis brings active regions into view, creating 26-28 day propagation cycle. Not atmosphere, radiation belts, or Moon - just Sun rotation.
Memory Aid
"26-28 day propagation cycle = Sun's rotation. Think '2'6-28 days = 'S'un 'R'otation. Rotation of Sun's surface layers around its axis brings active regions into view, creating periodic propagation variations. Key cause of monthly propagation cycles."
Real-World Application
A sunspot group appears on the Sun. As the Sun rotates (about 27 days), the sunspot group moves across the face of the Sun, affecting propagation. When it's facing Earth, propagation is better. When it rotates away, conditions may decline. This solar rotation creates the 26-28 day propagation cycle.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Long-term oscillations in the upper atmosphere aren't the cause - the cycle is related to solar rotation, not atmospheric oscillations.
Option B: Incorrect. Cyclic variation in Earth's radiation belts isn't the cause - the cycle is related to solar rotation, not radiation belts.
Option D: Incorrect. The Moon's orbital position isn't the cause - the cycle is related to solar rotation (26-28 days), not the Moon (28-day cycle is coincidental).
题目解析
The correct answer is C: Rotation of the Sun's surface layers around its axis. What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle is the rotation of the Sun's surface layers around its axis. The Sun rotates, bringing active regions (sunspots, flares) into and out of view, creating a cycle. For amateur radio operators, this explains why propagation varies on a monthly cycle. Understanding this helps predict propagation patterns.
考试技巧
26-28 day propagation cycle = Sun's rotation. Think '2'6-28 days = 'S'un 'R'otation 'C'ycle. Rotation of Sun's surface layers around its axis brings active regions into view, creating 26-28 day propagation cycle. Not atmosphere, radiation belts, or Moon - just Sun rotation.
记忆口诀
26-28 day propagation cycle = Sun's rotation. Think '2'6-28 days = 'S'un 'R'otation. Rotation of Sun's surface layers around its axis brings active regions into view, creating periodic propagation variations. Key cause of monthly propagation cycles.
实际应用示例
A sunspot group appears on the Sun. As the Sun rotates (about 27 days), the sunspot group moves across the face of the Sun, affecting propagation. When it's facing Earth, propagation is better. When it rotates away, conditions may decline. This solar rotation creates the 26-28 day propagation cycle.
错误选项分析
Option A: Incorrect. Long-term oscillations in the upper atmosphere aren't the cause - the cycle is related to solar rotation, not atmospheric oscillations. Option B: Incorrect. Cyclic variation in Earth's radiation belts isn't the cause - the cycle is related to solar rotation, not radiation belts. Option D: Incorrect. The Moon's orbital position isn't the cause - the cycle is related to solar rotation (26-28 days), not the Moon (28-day cycle is coincidental).
知识点
Solar rotation, 26-28 day cycle, Propagation cycles, Sun rotation
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.