Definition:

A 3hourly ”equivalent amplitude” index of geomagnetic activity for a specific station or network of stations (represented generically here by k) expressing the range of disturbance in the horizontal magnetic field. ”ak” is scaled from the 3hourly K index according to the following table:
K
0
1
2
3
4
5
6
7
8
9
ak
0
3
7
15
27
48
80
140
240
400
At NOAA these values are used directly for operational purposes. But to convert the ak values to nanoteslas (nT), a local (stationdependent) conversion factor must be found by dividing the station’s lower limit for K=9 by 250. For example, at Boulder and Fredericksburg the lower limit for K=9 is 500 nT so the factor is 2; therefore the ak values for these stations are in units of 2 nT. (To obtain an equivalent amplitude in nanotesla for Boulder or Fredericksburg, the index value must be doubled.)
