What does the A-index measure?
The correct answer is C: The long-term stability of Earth's geomagnetic field. The A-index measures the long-term stability of Earth's geomagnetic field. A-index is derived from K-index and indicates longer-term geomagnetic conditions. For amateur radio operators, A-index helps assess overall geomagnetic conditions. Understanding this helps when checking propagation forecasts.
Exam Tip
A-index = long-term stability of Earth's geomagnetic field. Think 'A'-index = 'A'verage (long-term) of 'E'arth's 'G'eomagnetic field. Measures long-term stability of Earth's geomagnetic field (derived from K-index). Not sunspots, not Sun's field, not solar flux - just Earth's geomagnetic field.
Memory Aid
"A-index = long-term stability of Earth's geomagnetic field. Think 'A'-index = 'A'verage (long-term). Measures long-term stability of Earth's geomagnetic field (derived from K-index). Provides longer-term view than K-index. Key propagation indicator."
Real-World Application
You check the A-index - it's 12 (quiet conditions). A-index measures the long-term stability of Earth's geomagnetic field, derived from K-index values. Low A (0-7) means quiet conditions, high A (15+) means disturbed conditions. A-index provides a longer-term view than K-index.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. A-index doesn't measure sunspot position - that's not what it measures. A-index measures geomagnetic field stability.
Option B: Incorrect. A-index doesn't measure polarization of the Sun's electric field - that's not what it measures. A-index measures Earth's geomagnetic field.
Option D: Incorrect. A-index doesn't measure solar radio flux - that's the solar flux index. A-index measures geomagnetic field stability.
题目解析
The correct answer is C: The long-term stability of Earth's geomagnetic field. The A-index measures the long-term stability of Earth's geomagnetic field. A-index is derived from K-index and indicates longer-term geomagnetic conditions. For amateur radio operators, A-index helps assess overall geomagnetic conditions. Understanding this helps when checking propagation forecasts.
考试技巧
A-index = long-term stability of Earth's geomagnetic field. Think 'A'-index = 'A'verage (long-term) of 'E'arth's 'G'eomagnetic field. Measures long-term stability of Earth's geomagnetic field (derived from K-index). Not sunspots, not Sun's field, not solar flux - just Earth's geomagnetic field.
记忆口诀
A-index = long-term stability of Earth's geomagnetic field. Think 'A'-index = 'A'verage (long-term). Measures long-term stability of Earth's geomagnetic field (derived from K-index). Provides longer-term view than K-index. Key propagation indicator.
实际应用示例
You check the A-index - it's 12 (quiet conditions). A-index measures the long-term stability of Earth's geomagnetic field, derived from K-index values. Low A (0-7) means quiet conditions, high A (15+) means disturbed conditions. A-index provides a longer-term view than K-index.
错误选项分析
Option A: Incorrect. A-index doesn't measure sunspot position - that's not what it measures. A-index measures geomagnetic field stability. Option B: Incorrect. A-index doesn't measure polarization of the Sun's electric field - that's not what it measures. A-index measures Earth's geomagnetic field. Option D: Incorrect. A-index doesn't measure solar radio flux - that's the solar flux index. A-index measures geomagnetic field stability.
知识点
A-index, Geomagnetic field, Long-term stability, Propagation indicator
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.