TGO
Auroral zone activity last month.
This figure tells the geomagnetic activity as observed from Tromsø (in the auroral zone) during the last 33 days. As a measure of activity we have used an index describing the average deviation of the horizontal field component from its normal value. In mathematical terms the calculation is:

math

Here H(t) is a time series of N = 1440 one minute values of the horizontal component covering the 24 hours before the time T of calculation. H0 is taken as the average of H for the same period.
Note that AZ represents the average deviation for the 24 hours prior to the time of calculation. Recalculation takes place every hour. The time span of 33 days is long enough to cover one solar rotation (typically 27 days as observed from Earth) and thus makes recurrent events visible.

17 August 2011, Truls Lynne Hansen