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Abstract
AbstractDuring magnetic storms, solar‐magnetosphere‐ionosphere‐Earth interactions give rise to geomagnetically induced currents (GICs) in man‐made technological conductors such as power grids, gas pipelines, and railway networks with potentially damaging outcomes. Generally, electrically conductive regions of the Earth are assumed to be less at risk to GICs than resistive ones, since induced electric fields associated with GICs are linearly related to given magnetic source fields via Earth's impedance. Here, we show that magnetic source fields associated with storms can be enhanced by secondary electromagnetic (EM) induction in Earth's electrically conductive asthenosphere and that this previously neglected effect can give rise to larger electric fields close to the lithosphere‐asthenosphere boundary in regions where the conductance of the asthenosphere is higher. Our analysis of data from the 30 October 2003 “Halloween” and 8 September 2017 storms shows that the magnitudes of electric fields from both storms are affected by lithospheric plate thickness and asthenosphere conductance (conductivity‐thickness product) and that they are 5 times larger in southern Sweden (>5 V/km for the 30 October 2003 “Halloween” storm) than in central Scotland. Our results provide insight into why Sweden experienced a storm‐related power outage in 2003, whereas Scotland did not.
Highlights
Magnetic storms occur when high-velocity plasma from solar flares or coronal mass ejections (CMEs) interacts with Earth’s magnetosphere (Chapman and Ferraro, 1931)
We investigate how the regionally-variable thickness of Earth’s quasirigid outer, electrically-resistive shell – the “tectonic plate” or “lithosphere” – and regiondependent conductance of the underlying electrically conductive layer known as the “asthenosphere” modify surface electric fields making some regions of the world inherently more vulnerable to extreme space weather events than others
For τ = τ0, electromagnetic induction in the Earth increases the horizontal magnetic field observed at the surface by a factor of 2 relative to the external source field, whereas τ > τ0 leads to horizontal magnetic fields that are more than twice the external magnetic source field and vice versa for τ < τ0
Summary
Magnetic storms occur when high-velocity plasma from solar flares or coronal mass ejections (CMEs) interacts with Earth’s magnetosphere (Chapman and Ferraro, 1931). Magnetotellurics (MT) is a passive electromagnetic technique that harnesses natural fluctuations in the electric and magnetic fields induced in Earth due to solar-magnetosphereionosphere interactions to study the electrical conductivity of Earth’s crust and mantle (e.g., Simpson and Bahr, 2005). By sampling these electric and magnetic fields simultaneously at a fixed sample rate, a quasi-continuous time series is recorded, which can be decomposed into its constituent electric and magnetic spectra at discrete frequencies via Fourier transform. Cross-correlation of these spectra for each frequency allows computation of a frequencyand direction-dependent impedance tensor, Z (Cagniard, 1953; Simpson and Bahr, 2005): Ex Ey
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