What is indicated by a rising A-index or K-index?

The A-index and K-index are both measurements of the stability (or instability) of the Earth's geomagnetic field. The K-index provides a short-term (3-hour) logarithmic reading from 0 to 9, while the A-index is a daily linear average ranging from 0 to 400. A rising value in either of these indices indicates an increasing disturbance in the geomagnetic field, usually triggered by solar wind or coronal mass ejections. These geomagnetic storms can severely degrade HF propagation, particularly on paths crossing the polar regions. Lower values (a K-index under 3, and an A-index under 15) generally indicate quiet conditions, which are much better for reliable HF sky-wave communications.