Mapping geoelectric fields during magnetic storms: Synthetic analysis of empirical United States impedances

Abstract

Empirical impedance tensors obtained from EarthScope magnetotelluric data at sites distributed across the midwestern United States are used to examine the feasibility of mapping magnetic storm induction of geoelectric fields. With these tensors, in order to isolate the effects of Earth conductivity structure, we perform a synthetic analysis—calculating geoelectric field variations induced by a geomagnetic field that is geographically uniform but varying sinusoidally with a chosen set of oscillation frequencies that are characteristic of magnetic storm variations. For north‐south oriented geomagnetic oscillations at a period of T0=100 s, induced geoelectric field vectors show substantial geographically distributed differences in amplitude (approximately a factor of 100), direction (up to 130∘), and phase (over a quarter wavelength). These differences are the result of three‐dimensional Earth conductivity structure, and they highlight a shortcoming of one‐dimensional conductivity models (and other synthetic models not derived from direct geophysical measurement) that are used in the evaluation of storm time geoelectric hazards for the electric power grid industry. A hypothetical extremely intense magnetic storm having 500 nT amplitude at T0=100 s would induce geoelectric fields with an average amplitude across the midwestern United States of about 2.71 V/km, but with a representative site‐to‐site range of 0.15 V/km to 16.77 V/km. Significant improvement in the evaluation of such hazards will require detailed knowledge of the Earth's interior three‐dimensional conductivity structure.

Geoelectric hazard assessment: the differences of geoelectric responses during magnetic storms within common physiographic zones

“…In both regions considered here (Minnesota and Virginia), we estimated the maximum geoelectric field at two sites to be two orders of magnitude different despite being located in the same physiographic zones and proximal to one another. While Bedrosian and Love (2015) used a simple sinusoidal source, we have shown a similar result derived from observatory geomagnetic fields. Therefore, by considering the measured impedance at each site rather than applying a 1-dimensional conductivity model to a wide region, we can improve estimates of the behavior and amplitude of the geoelectric field and the corresponding risk to electric power grids.…”

“…These 1-dimensional conductivity models encompass large geographic regions of the USA which demonstrate considerable geologic complexity. Bedrosian and Love (2015) have demonstrated that such broad applications of these models to geologically complex regions do not remain consistent when evaluated against EarthScope impedances. These models do not consider the 3-dimensional effects of geoelectric induction when estimating the geoelectric field.…”

“…These include Bedrosian and Love (2015), who use 3-dimensional magnetotelluric EarthScope impedances to estimate the geoelectric response of a simulated time-varying geomagnetic field across northern North America. Love et al (2016a) used these impedances to produce a hazard map of half of the USA by convolving these impedances against latitudedependent maps of extreme geomagnetic activity.…”

Influence of spatial variations of the geoelectric field on geomagnetically induced currents

“…The former is generally the major factor affecting GIC magnitudes (Beggan, 2015). The latter leads to spatial variations of the electric field even if the magnetic field is a uniform plane wave (Bedrosian & Love, 2015). In case of nonuniform magnetic fields, lateral variations of the ground conductivity can locally play a further significant role (Beggan, 2015).…”

“…The geoelectric field varies rapidly temporarily and spatially as demonstrated, for example, by Beggan et al (2013), Bedrosian and Love (2015), Wei et al (2013) and Wang et al (2016) for regional and continental scales, and by Püthe and Kuvshinov (2013) and Ngwira et al (2015) for global scales. Temporal variations are due to time variations of space currents and are connected with time variations of the geomagnetic field by Faraday's law.…”