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G3A13:What does the A-index measure?

G3A13G3A

What does the A-index measure?

Direct answer summary (2023-2027 NCVEC pool)

The official correct answer for G3A13 is The long-term stability of Earth’s geomagnetic field.

The key fact is The long-term stability of Earth’s geomagnetic field. Match that fact to the exact condition in the question; the other choices describe a different rule, band, component, or operating situation.

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