Abstract
Variability of the geomagnetic field induces anomalous Geomagnetically Induced Currents (GICs) in grounded conducting infrastructure. GICs represent a serious space weather hazard but are not often measured directly and the rate of change of the magnetic field is often used as a proxy. We assess the correlation between the rate of change of the magnetic field and GICs during Sudden Commencements (SCs) at a location in New Zealand. We observe excellent correlations (r2 ∼0.9) between the maximum 1‐min rate of change of the field and maximum GIC. Nonetheless, though SCs represent a relatively simple geomagnetic signature, we find that the correspondence systematically depends on several factors. If the SC occurs when New Zealand is on the dayside of the Earth, then the magnetic changes are linked to 30% greater GICs than if New Zealand is on the nightside. We investigate the finding that the orientation of the strongest magnetic deflection is important: changes predominantly in the east‐west direction drive 36% stronger GICs. Dayside SCs are also associated with faster maximum rates of change of the field at a resolution of 1 s. Therefore, while the maximum rates of change of the magnetic field and GICs are well correlated, the orientation and sub‐1‐min resolution details of the field change are important to consider when estimating the associated currents. Finally, if the SC is later followed by a geomagnetic storm, then a given rate of change of the magnetic field is associated with 22% larger GICs, compared to if the SC is isolated.
Plain Language Summary
Changes in the Earth's magnetic field will drive electrical currents that can flow in infrastructure, such as power networks and pipelines. These currents can pose a hazard to their operation and safety. We often do not have access to direct measurements of the currents that flow within our infrastructure, so we typically report and forecast magnetic perturbations to infer when we are likely to see large currents. In this work, we investigate the link between the magnetic changes and currents that are observed when the Earth is impacted by a sharp change in the solar wind dynamic pressure, that is, a shock. We also have access to direct measurements of current in infrastructure from New Zealand. In general, we find excellent correlations between the two parameters. However, we find that the type of shock event during which they are observed is important as is the location of the observations relative to the day/nightside of the Earth. We find that the orientation of the rate of change of the magnetic field as well as high time resolution (i.e., subminute resolution) information are both important to consider when attempting to estimate the currents that will be generated, even with relatively simple processes.
Key Points
The rate of change of the magnetic field during Sudden Commencements (SCs) excellently correlates with observed Geomagnetically Induced Currents (GICs)
Storm SCs are associated with 22% larger GICs than Sudden Impulses
SCs that occur when New Zealand is on the dayside of the Earth are associated with 30% larger GICs