Why The K-index is derived from magnetometer readings taken every three hours and reported on a quasi-logarithmic scale of 0 to 9, so it is a short-term snapshot of how disturbed or stable Earth's geomagnetic field is. Low values (0 to 1) mean a quiet field and good HF conditions, while values of 5 and up indicate a geomagnetic storm with absorption and auroral disruption, especially on high-latitude paths. The A-index is the related daily figure derived from the day's K values.
Watch out The solar flux measured at 2800 MHz is the solar flux index, historically measured at Ottawa and now Penticton, British Columbia (the Boulder, Colorado tie-in is to the NOAA Space Weather Prediction Center, not the flux observatory), and it describes solar activity rather than Earth's magnetic field.
K for Kompass: the K-index watches Earth's magnetic field, updated every 3 hours, 0 = quiet, 9 = storm.