What does the A-index measure?

The A-index is a daily measure of the long-term stability of the Earth's geomagnetic field. It is derived from a 24-hour average of the 3-hour K-index readings, providing a macroscopic view of planetary geomagnetic conditions. A lower A-index (typically 15 or below) indicates quiet geomagnetic conditions, which are generally ideal for reliable HF radio propagation. Conversely, a higher A-index signals geomagnetic instability or storms, leading to degraded band conditions and signal absorption. Common distractors on this question often refer to solar radio emissions or sunspot positions. Solar radio flux is measured by the Solar Flux Index (SFI) and measured at Boulder, Colorado, which is completely different from the A-index.