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 3 sigma.
- 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 |
- 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.
- 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 on the web page
