Tom Schuessler, N5HYP of Irving, Texas wrote to ask about the differences between the geomagnetic A and K index. He asks, "I know that both of them are indications of the instability of the geomagnetic field. The K index is logarithmic and the A index is linear, and they track together -- kind of. K indexes are given every three hours while A index readings are for a full 24 hour period. Do the two indexes have different uses or tell a person different things about what to expect on the air?"

Based on three hours of magnetic data, a particular magnetometer or group of them is used to track the change in nanoTeslas, which are the international units for measuring magnetic flux density. K index is based on changes in the flux density over a 3 hour period, and the difference between the highest and lowest values at the magnetometer is converted to a semi-logarithmic scale that runs from 0 to 9, yielding a K index between 0 (very quiet) and 9 (extreme magnetic storm).

Note that if you have K indexes for one day averaging 2, the A index for that day would be 7. But if the average were 3, this corresponds to an A index of 15, and an average of 4 equals 27. You can see a table of three different A and K index readings at,

They are both derived from the same magnetic readings, but the A index is for a whole day, and has an expanded scale. When those numbers are low, we expect less absorption and in general better propagation of radio waves. But sometimes high geomagnetic activity can signal improved VHF conditions, allowing distant propagation of

6 meter signals, for instance. Because there is a new K index every three hours, this gives us a more immediate indication if conditions are changing fast. So if WWV reports a K index of 2, then three hours later reports a K index of 5, this indicates a dramatic event, such as an earth-directed solar flare, or a blast of strong solar wind, and a resulting geomagnetic storm.