Multi-scale intense geoelectric and geomagnetic field perturbations observed after an interplanetary magnetic field turning

Intense geoelectric fields during geomagnetic storms generate geomagnetically induced currents in power grids and other infrastructure, necessitating an understanding of their causes, for example, through coordinated space and ground observations. This study investigates localized intense geoelectric (E) and geomagnetic (B) field perturbations following an Interplanetary Magnetic Field (IMF) turning during a geomagnetic storm on 25 October 2011. Observations from EarthScope magnetotelluric sites in the upper Midwest United States revealed shorter period (∼1 min) ultra-low-frequency (ULF) waves superimposed on longer period (∼10 min) perturbations in both E and B fields. These sites, located at ∼19 hr magnetic local time and 56 −57 ° magnetic latitude, recorded large amplitude E and B perturbations. Ground-based all-sky imagers showed auroral brightening with sunward and poleward propagation, while upstream spacecraft linked the perturbations to an IMF turning and solar wind dynamic pressure impulse. The longer-period E and B field perturbations likely stem from localized ionospheric currents tied to substorm auroral activity post-IMF turning. The combination of ionospheric currents, ULF waves, and the Earth's varying conductivity produces intense geoelectric fields of ≥2 V/km in the upper Midwest. A comparison using input data and software compatible with the NOAA/USGS geoelectric field nowcast model revealed its limitations in capturing such events due to the temporal and spatial resolution of the underlying data. Using 1-s geomagnetic field data can improve geoelectric field models by capturing short-period and large spatial scale waves, although localized magnetic perturbations remain underestimated due to insufficient ground magnetometer density.