A comparison of methods to compute the rate of horizontal geomagnetic field variation

The rate of change of the horizontal external magnetic field is often used as a proxy for space weather activity and in particular for estimating geomagnetically induced currents (GICs) in high voltage power grids. This paper compares two commonly adopted methods for computing this rate of change: (1) the difference between consecutive measurements in the magnitude of the horizontal magnetic field, H′, and (2) the combined difference in the magnitude in the northward and eastward directions, usually denoted R. We find that there can be an absolute difference in the calculations between the two methodologies exceeding 100 nT/min during storm times for observatories in the sub-auroral zone, demonstrating that the choice between R and H′ can make a significant difference to any GIC estimate. We also note an observable difference between the two methodologies during quiet times when the measurements are made close to the agonic line, though this difference does not have a significant impact on the efficacy of either of the two methodologies for GIC studies. Future studies should consider carefully the choice of geomagnetic indicators for estimation of GICs.