*Question G3A13:
What does the A-index measure?*  

*Correct Answer is  C: The long-term stability of Earth’s geomagnetic field*  

### AI Response: (may contain errors)  

The A-index is a measure of the long-term stability of Earth's geomagnetic field. It is calculated as the average of the daily K-index values over a 24-hour period. The K-index measures short-term disturbances in the geomagnetic field, while the A-index provides a broader view of geomagnetic activity over a longer time frame.

### Explanation:

1. **Correct Answer (C):**  
   - The A-index reflects the long-term stability of Earth's geomagnetic field by averaging the K-index values over a day. This helps in understanding the overall geomagnetic conditions and their potential impact on radio wave propagation.

2. **Incorrect Choices:**
   - **Choice A:** The relative position of sunspots on the surface of the Sun is not related to the A-index. Sunspots are tracked separately and influence solar activity but are not measured by the A-index.
   - **Choice B:** The amount of polarization of the Sun’s electric field is not measured by the A-index. Polarization is a different concept related to electromagnetic waves.
   - **Choice D:** The solar radio flux at Boulder, Colorado, is measured by the 10.7 cm solar flux index, not the A-index. The solar flux index is another indicator of solar activity but is distinct from geomagnetic measurements.

### Useful Information:

- **K-index:** A scale from 0 to 9 that quantifies disturbances in the horizontal component of Earth's magnetic field.
- **A-index:** A linear scale derived from the K-index, providing a daily average to assess geomagnetic conditions.

Understanding the A-index helps amateur radio operators predict conditions for HF radio propagation, as geomagnetic stability can significantly affect signal quality and reach.