Deep Dive: G3A13
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.
Why Other Answers 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.
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 Example
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.
Source & Coverage
Question Pool: 2023-2027 Question Pool
Subelement: G3A
Reference: 2023-2027 Question Pool · G3 - Radio Wave Propagation
Key Concepts
Verified Content
Question from the official FCC General Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the G3A topic.