Why The A-index and K-index are both measures of geomagnetic activity, derived from magnetometer readings of how much Earth's magnetic field is being disturbed. The K-index is a quasi-logarithmic 0-9 value computed every three hours; the A-index is a daily linear average derived from the K values. When either number climbs, the geomagnetic field is being shaken by solar wind and coronal mass ejections, which typically degrades HF paths, especially those crossing high latitudes, while sometimes enhancing auroral propagation.
Watch out Solar UV levels and the ionospheric critical frequency track with the solar flux index (10.7 cm flux) and sunspot number, not with A or K; a high flux with low A and K is the ideal HF condition.
A and K = magnetic Agitation/Kick. Low is good for HF. Flux and sunspots high, A and K low is the dream.