The Geoelectric Field is a measure of the induction hazard to artificial conductors, such as electrical power lines, that results from Geomagnetic Activity, and can be used to estimate the amount of current induced by integrating along the conducting pathway.
Potentially hazardous geoelectric fields can be induced during geomagnetic storms. These geomagnetic storms are a form of space weather driven by greatly enhanced currents in Earth's magnetosphere and ionosphere and are observed at ground level as a time-varying magnetic field. As is well known from Faraday's law, a time-varying magnetic field induces currents along natural and artificial conducting pathways. This geoelectric field product combines information about the time-varying magnetic field together with available Earth-conductivity information to estimate regional geoelectric field levels on a 2 degree by 2 degree geographic grid over the lower 48 states. The amount of current induced in an artificial conductor may be calculated by integrating along the conducting pathway. When currents are induced in artificial conductors, unexpected and sometimes problematic effects can occur in the operation of the affected equipment. Please see our article about the effect this has on electrical power systems at http://www.swpc.noaa.gov/impacts/electric-power-transmission. Please see also the article Modeling geomagnetically induced currents, by Boteler and Pirjola in Space Weather, DOI10.1002/2016SW001499 (31 January 2017) for an up-to-date description of this phenomena.
The local geoelectric field is specified in millivolts per kilometer and is based on convolving a geomagnetic time-series signature with an Earth-response function, where the response function depends on the local Earth conductivity. In this initial prototype, the nearest observatory to each of the 2 degree by 2 degree grid points is used as the input geomagnetic time-series signal. The Earth conductivity is determined, based on the physiographic region that the grid point lies in and the associated, one-dimensional conductivity profile as compiled by Fernberg (EPRI Technical Update 1026430, 2012). Active work is in progress by USGS to assist SWPC in the replacement of these very simple conductivity models with more up-to-date Earth-Response functions based on modern magneto-telluric surveys. Active research is also in progress to replace the 'nearest observatory' approach with an interpolated time-series instead.