Ham Radio General Class Practice Test

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Approximately how long does it take the increased ultraviolet and X-ray radiation from solar flares to affect radio-wave propagation on the Earth?

28 days
1 to 2 hours
8 minutes
20 to 40 hours

The correct answer is: 8 minutes

The correct choice is based on the speed at which electromagnetic radiation, specifically ultraviolet (UV) and X-ray emissions from solar flares, travels. These emissions travel at the speed of light, which is approximately 299,792 kilometers per second (or about 186,282 miles per second). When a solar flare occurs, the light and radiation it emits can reach Earth in approximately 8 minutes. This rapid arrival time is crucial for understanding how these solar events affect radio-wave propagation. Often, the initial increase in UV and X-ray radiation can disturb the ionosphere, a region of the Earth's atmosphere that is vital for radio communications, resulting in adverse effects such as signal degradation or loss. The time frame of 20 to 40 hours, while potentially relevant for other effects like solar particle events, is not applicable to the immediate effects of UV and X-ray radiation from flares. Instead, the immediate disruption to radio-wave propagation following a solar flare occurs shortly after the radiation reaches Earth, which is within that 8-minute window. Thus, recognizing the speed of light and its role in the transmission of solar radiation helps explain why the choice of 8 minutes is accurate in this context.