Geomagnetically induced currents (GICs) flow in grounded technological conductors when time-varying ionospheric and magnetospheric currents induce geoelectric fields in the Earth. A 2020 review chapter by Piersenti and Carter, built around the June 22-23, 2015 geomagnetic storm, identified four unresolved issues in the physics of GIC generation: the unconstrained spatial structure of the equatorial electrojet response at the Huancayo station during the storm's sudden impulse; the unknown mechanism and relative roles of auroral-electrojet and equatorial-electrojet currents in the near-simultaneous high- and low-latitude dB/dt enhancements at sudden impulses and their dependence on solar wind dynamic pressure; the unrecognized generation of extreme, localized geoelectric-field enhancements by small (~100 km) spatial-scale ionospheric structures; and the unquantified contribution of turbulent interplanetary magnetic field (IMF) fluctuations to storm-time GIC. The problem remains open: subsequent work has produced statistical scalings of the equatorial electrojet response to interplanetary shocks, climatologies of localized dB/dt spikes tied to omega bands, auroral streamers, and substorm onsets, and confirmation that global magnetohydrodynamic models cannot yet reproduce sub-100 km disturbances, but no study has quantitatively separated the auroral-electrojet and equatorial-electrojet contributions to the sudden-impulse equatorial dB/dt spike, and no direct quantification of the turbulent-IMF contribution to GIC exists.

Background

Geomagnetically induced currents (GICs) are quasi-direct currents induced in long grounded conductors (power grids, pipelines, railways) by geoelectric fields that Faraday induction generates in the conducting Earth when the geomagnetic field varies rapidly; the rate of change of the horizontal magnetic field, dB/dt, is the standard observational proxy for GIC activity. The strongest GICs occur at high magnetic latitudes beneath the auroral electrojets, the intense east-west ionospheric current bands that flow within the auroral oval, but the 2010s established that low and equatorial latitudes are also at risk. At the magnetic equator, the equatorial electrojet (EEJ) - a narrow (~100 km wide in latitude), intense eastward dayside ionospheric current sustained by the enhanced Cowling conductivity (the condition, arising when a vertical Hall current is blocked by the poorly conducting atmosphere above and below the conducting E layer, that boosts effective conductivity perpendicular to both electric field and magnetic field) - amplifies magnetic disturbances. Carter et al. (2015, 2016) showed that interplanetary shocks and storm sudden commencements produce amplified equatorial GIC activity, and Zhang et al. (2015) recorded SSC-driven GICs in China's low-latitude 500 kV grid larger than those of the storm main phase. A sudden impulse (SI) is the step-like geomagnetic disturbance caused by a sharp increase of solar wind dynamic pressure (the mass density times velocity squared) when an interplanetary shock or discontinuity compresses the magnetosphere; when followed by a storm it is called a storm sudden commencement (SSC). In the standard physical model of the sudden commencement (Araki's SC model), the ground signature combines a magnetopause current contribution (the DL field) with twin-vortex ionospheric current systems: a preliminary impulse (PI) and a main impulse (MI), each carried by a pair of field-aligned currents and ionospheric vortices. That the PI appears at the dayside dip equator essentially simultaneously with high latitudes is explained by near-instantaneous transmission of the polar electric field to the equator through the Earth-ionosphere waveguide, formalized by Kikuchi and Araki (1979) and Kikuchi (2014) as a TM0-mode transmission-line model, and confirmed in three-dimensional global magnetohydrodynamic simulations reviewed by Fujita (2019); this establishes the transmission pathway, but the mechanism by which it couples the auroral-electrojet and equatorial-electrojet currents, and their relative roles, remain unresolved. Piersenti and Carter (2020, in The Dynamical Ionosphere) analyzed the June 22-23, 2015 geomagnetic storm, whose sudden impulse at 18:34 UT on 22 June followed an interplanetary shock with a dynamic-pressure jump of roughly 45 nPa (about twice that of the March 2015 St. Patrick's Day storm). Using the inversion of Piersanti and Villante (2016) to separate magnetospheric from ionospheric (PI_IC and MI_IC) contributions at 49 stations, the chapter found that the SI was the dominant GIC-producing phase across all latitudes: the equatorial station Huancayo (HUA) recorded dB/dt of about 130 nT/min at shock arrival - exceeding the mid-latitude stations at magnetic latitudes 30-50 degrees, though below the >600 nT/min reached at the strongest high-latitude stations - and argued that a coupling between the auroral electrojets and the EEJ was active at the moment of shock impact, as previously reported for the St. Patrick's Day storm. Separately, Ngwira et al. (2015) and Pulkkinen et al. (2015) had documented that extreme geoelectric fields during major storms are often localized to single stations, with structures differing greatly from regional (few-hundred-km) averages, while Huttunen et al. (2008) showed statistically that the turbulent sheath regions ahead of interplanetary coronal mass ejections - rather than the smoother ejecta - drive the largest GICs of a solar cycle, implicating fluctuating (not average) interplanetary magnetic field conditions. Against this background, the chapter closed by listing four unresolved scientific issues.

Problem Statement

Establish a quantitative, mechanistic account of how ionospheric current systems produce the ground electromagnetic disturbances that drive geomagnetically induced currents during storms, closing four linked gaps left by the Piersenti and Carter (2020) analysis of the June 22-23, 2015 storm. (1) Determine the spatial structure of the equatorial electrojet response during a sudden impulse and its contribution to the equatorial dB/dt spike (the single-station HUA analysis could not separate the electrojet from background ionospheric currents); this spatial determination is independent of the cross-latitude relative-role attribution requested in (2). (2) Identify the physical mechanism coupling high-latitude and low-latitude ionospheric currents at sudden impulses - quantitatively establishing the relative roles of the auroral electrojets and the equatorial electrojet in the near-simultaneous high- and low-latitude dB/dt enhancements, and how this coupling depends on the solar wind dynamic-pressure change. (3) Determine whether and how small spatial-scale (of order 100 km) ionospheric current structures generate extreme geoelectric fields (local geoelectric-field magnitudes exceeding the 99th percentile of the regional, few-hundred-km distribution) and localized GIC enhancements. (4) Quantify the contribution of turbulent interplanetary magnetic field fluctuations, as distinct from the average IMF and dynamic pressure, to GIC generation during storms. Together they contribute distinct parts of one objective: a physically resolved, quantitatively validated chain from solar wind driver (pressure impulse, IMF turbulence) through ionospheric current systems (global auroral-equatorial coupling down to ~100 km structures) to ground dB/dt, geoelectric field, and GIC, evaluated at both equatorial and high latitudes.

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