The (dBdt) indicator estimates the rate of the geomagnetic field change dB/dt and consequently the induced electrical field E~dB/dt.

For each geomagnetic data record B(i), dBdt is calculated as central difference: dB/dt(i) = (B(i+1) - B(i-1)) / (t(i+1) - t(i-1)), according to the approach used in INTERMAGNET.

dBdt is calculated for each component and total intensity, for each magnetic observatory independently. The resulting values of the dBdt indicator are the maximal values of the rate of change for each of the geomagnetic vector components (X, Y, Z, F or H, D, Z, F, depending on the vector magnetometer orientation in reference to geographic or magnetic North) within the corresponding time interval (1 hour or 3 hours). These results are used for calculation of extreme rates of geomagnetically induced currents (GICs).

The results of the application of the dBdt indicator are classified within a 4-grade scale (0–3), where 0 is ‘background’; 1 is ‘weak anomaly’; 2 is ‘anomaly’; 3 is ‘strong anomaly’. The lower threshold value for the 3rd grade is determined as the value that corresponds the 1% of data maximum values for a particular observatory, time resolution and component. This threshold value is rounded to the 2nd significant digit. The lower threshold values for the 1st and the 2nd grades are determined as 1/3 and 2/3 of the lower threshold value for the 3rd grade and are rounded to the 2nd significant digit.

For an observatory or a station with no 1-minute scalar magnetometer data available, the indicator scale for the total field F is set to the ones for the observatories close to it latitudinally. For example, the F scales for KIV and LVV observatories are the same as the one for IRT observatory, and the F scale for AIA observatory is equal to the F scale for YAK observatory (considering the magnetic field symmetry in the Northern and the Southern hemispheres).

An example of 1-hour dBdt indicator grades and corresponding threshold values for the X component data from SPG observatory is given in the following table:

For more details, please check the Data Science Journal paper:

Gvishiani, A et al 2016 Automated Hardware and Software System for Monitoring the Earth’s Magnetic Environment. Data Science Journal, 15: 18, pp.1–24

*The threshold value set to 1% of maximal values was assigned based on the analysis of X or H component data from BOX, KHB, MGD and SPG magnetic observatories during March 2015. Also the threshold values of 10%, 5%, 3% were analyzed.