Question
What does the A-index measure?
Answer Options
- A) The relative position of sunspots on the surface of the Sun
- B) The amount of polarization of the Sun’s electric field
- C) The long-term stability of Earth’s geomagnetic field
- D) The solar radio flux at Boulder, Colorado
Correct Answer: C
Explanation
The A-index is the daily average of the K-index (short-term stability index). By averaging the K-index values from eight three-hour periods over a day, the A-index provides a measure of the overall level of geomagnetic disturbance.
Therefore, the A-index is used to characterize the long-term stability of the Earth’s geomagnetic field over a 24-hour period. Similar to the K-index, a low A-index (below 10) indicates generally good propagation conditions, while a high A-index (above 20) indicates a more disturbed ionosphere and degraded HF propagation.
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