About this forecast

Chart above is the long term Ap-forecast published daily by US Air Force, just shown in different format. Forecast is updated weekly, typically very early on each Monday. Original forecast can be found from here. The forecast is based on coronal holes on the surface of the sun. Those holes stay open typically about five rotations of the sun (one takes about 27 days). During that time these coronal holes can grow or shrink, or even disappear. On the other hand also new holes can form. In practise this means that the forecast is based on those coronal holes, that have existed on previous rotation(s) of the sun, and the predicted Ap-value is based on the measurements and effects of the solar wind during that time. This forecast also cannot predict any possible coronal mass ejections (CME).

Ap-value varies from 0 to 400. 0 equals average geomacnetic conditions ("quiet" background), and 400 means "giant geomagnetic storm". Ap-value can be converted from Kp-values by taking each of the eight daily Kp-values (not shown in the chart above) and using conversion table to find equivalent Ak-value. The average of the sum of those eight values is daily Ap-value. Main difference between these two values is that Kp-value means highest predicted Kp for given day, and the Ap-value means estimated average value of geomagnetic disturbances of the same day

The colored bars on the charts represent highest forecasted Ap-value for each day. The higher the bar is, the higher the forecasted value and thus also the chance for aurora to occur. However, auroras can be seen also when the forecasted value is weak, because values are planetary, which basically means that those are averaged values for the whole planet. Local values can be something very different.