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FMI Method

The FMI method is a method for calculating the k-index of a location, developed by the Finnish Meteorological Institute. The K-index quantifies the level of variation of the horizontal (H) component of the geomagnetic field in three-hour intervals. The index ranges from 0 to 9 on a quasi-logarithmic scale, where K=0 indicates completely quiet conditions and K=9 indicates highly disturbed conditions. Large K-indices can be used to indicate a high chance of observing the aurora borealis(northern lights). This method allows us to estimate and subtract the solar-quiet variations in the magnetometer data. The steps undertaken in this method are as follows:

  1. The raw magnetometer data is binned into average minute values. It is then cleaned with a moving hour long window. Any values that deviate by more than 3σ.
  2. For each 3 hour block (00:00-03:00, 03:00-06:00, etc), the variation between the maximum and minimum of the two horizontal B-field values are compared to the following table to get an initial K-index n;
 K-Index Value  Limit of Range Classes (nT)  BGS Scale  NOAA Scale
 0  0 – 5.4  Quiet  G0
 1  5.4 -10.8  Quiet   G0 
 2 10.8 – 21.6 Unsettled  G0
 3 21.6 – 43.2 Unsettled  G0
4 43.2 – 75.6 Active   G0
 5 75.6 – 129.6 Minor Storm   G1
 6 129.6 – 216.0 Major Storm   G2 – G3
 7 216.0 – 356.4 Major Storm   G2 – G3
 8  356.4 – 540.0  Severe Storm  G4
 9 540.0 + Severe Storm    G4 
  1. For each hour of the day, the average horizontal values for that hour +/- (n + m) minutes are calculated, where n is the initial K-index, and m is a constant which depends on the time of day. Together, these points give a rough estimate of the solar-quiet variation.
  2. This rough estimate for the solar-quiet variation is then smoothed. This can be seen for a geomagnetically active day in the following plot:
  3. The smoothed solar-quiet variation is taken away from the raw data. This is then used as in step 2 to get a secondary K-index.
  4. Steps 3-5 are then repeated using the secondary K-Index to finally calculate the third and final K-index. This last K-index is what is displayed above.