Modeling geomagnetically induced currents using geomagnetic indices and data

The possibilities of forecasting geomagnetically induced currents (GIC) in power transmission networks are dependent on the success in modeling these currents. To provide a valuable user-oriented forecast, modeling and proper evaluation of the models using GIC data is important. Many forecasts of geomagnetic storms are presented in terms of geomagnetic indices. Using the GIC data from measuring sites on three power systems in aurora and subauroral regions we estimate the correlation of 3-hourly peak GIC with global geomagnetic indices (3-h ap) and 1 h peak GIC with hourly magnetic range and peak dB/dt values. Geomagnetic 1-min data were used with physics-based and empirical models of the earth and power system response to calculate GIC. These calculated GIC were tested by determining the correlation with measured GIC. Our results show that local geomagnetic indices are better correlated with peak GIC values than are global indices in describing GIC. Correlation coefficients for local (global) indices are 0.9 (0.8) for two subauroral sites and 0.8 (0.7) for an auroral site. Tests of the correlation between 1 min dB/dt or calculated electric field values with measured GIC show a strong directional sensitivity. The direction of peak correlation is different at different sites and is consistent with the direction of power lines. Correlation coefficients for datasets of peak 1-h or 3-h values were higher than for 1-min datasets. This shows that there is a closer relationship between the "envelopes" of geomagnetic disturbances and GIC than between the detailed variations themselves.