Question
What effect does a sudden ionospheric disturbance have on the daytime ionospheric propagation?
Answer Options
- A) It enhances propagation on all HF frequencies
- B) It disrupts signals on lower frequencies more than those on higher frequencies
- C) It disrupts communications via satellite more than direct communications
- D) None, because only areas on the night side of the Earth are affected
Correct Answer: B
Explanation
A Sudden Ionospheric Disturbance (SID) is caused by a burst of X-ray radiation from a solar flare hitting the Earth’s atmosphere. This event causes massive ionization, primarily in the D-region of the ionosphere (the lowest layer). During daylight hours, the D-region absorbs HF radio signals, particularly at the lower frequencies.
Because the D-region is responsible for absorbing signals below about 10 MHz, an SID dramatically increases this absorption. The result is a sudden and severe disruption, often a complete blackout, of signals on the lower HF bands (like 80m and 40m), while signals on the higher HF bands (like 20m and 15m) are affected less.
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