Regional estimation of geomagnetically induced currents based on the local magnetic or electric field

Previous studies have demonstrated a close relationship between the time derivative of the horizontal geomagnetic field vector (d H /d t ) and geomagnetically induced currents (GIC) at a nearby location in a power grid. Similarly, a high correlation exists between GIC and the local horizontal geoelectric field ( E ), typically modelled from a measured magnetic field.Considering GIC forecasting, it is not feasible to assume that detailed prediction of time series will be possible. Instead, other measures summarising the activity level over a given period are preferable. In this paper, we consider the 30-min maximum of d H /d t or E as a local activity indicator (|d H /d t | 30 or | E | 30 ). Concerning GIC, we use the sum of currents through the neutral leads at substations and apply its 30-min maximum as a regional activity measure (GIC 30 ).We show that |d H /d t | 30 at a single point yields a proxy for GIC activity in a larger region. A practical consequence is that if |d H /d t | 30 can be predicted at some point then it is also possible to assess the expected GIC level in the surrounding area. As is also demonstrated, | E | 30 and GIC 30 depend linearly on |d H /d t | 30 , so there is no saturation with increasing geomagnetic activity contrary to often used activity indices.