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:
- 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.
- 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|
|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|
- 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.
- This rough estimate for the solar-quiet variation is then smoothed. This can be seen for a geomagnetically active day in the following plot:
- 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.
- 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.