The Impact of Magnetic Field Temporal Sampling on Modeled Surface Electric Fields

Abstract

Descriptions of the surface magnetic-to-electric field relationship, using magnetotelluric impedance tensors, have seen refinement in recent years. Correspondingly, models of geomagnetically induced currents flowing in power line conductors using input from more fundamental observables (e.g., the surface magnetic field or solar wind drivers) have become more sophisticated. Magnetometer data used for purposes of surface electric field modeling typically have cadences between 1 s and 1 min. Using 1-D and 3-D magnetotelluric impedance tensors, we quantify the effect of magnetometer cadence on modeled peak surface electric fields during the 22 geomagnetic storms. It is shown that 1-min magnetometer data sampling leads to median biases in the peak electric field of 10-20% magnitude attenuation and 2-17 ∘ in direction (depending on the storm) across the United States using the Fernberg 1-D conductivity models and 11-26% (0.9-8 ∘ ) using the 3-D impedance tensors derived from the USArray magnetotelluric survey. The largest 10% of attenuations almost entirely occur near low-resistivity subsurface structures in the western and eastern United States, reaching a 50% median attenuation during the 17 March 2015 event. It is concluded that cadence, local geology, and the spectral content of the geomagnetic field all play a significant role in the bias.Under the standard magnetotelluric assumptions (neglect of displacement and polarization currents, free-space permeability, and quasi-uniformity of the source field; Chave & Weidelt, 2012, and

Frequency Components of Geomagnetically Induced Currents for Power System Modelling

“…However, the peaks around the time of the SSC, which is the part of the GIC with the fastest rise time but not in this case the largest peaks, are shifted and underestimated by up to 40%. This is in line with the results of Grawe et al [17] regarding the under-sampling of magnetic fields and its impact in the estimation of the peaks of the surface electric field. Even if the physical GIC drivers are not high-frequency in nature, mathematically these high frequencies are often needed to reproduce peaks (peak resolution) and sudden steep changes.…”

Predictability of Geomagnetically Induced Currents as a Function of Available Magnetic Field Information

“…for example, Grawe et al (2018) Figure 4) had lower errors overall. Stronger space weather tends to deposit more high-frequency spectral energy into the surface magnetic field perturbation, leading to a generally stronger surface electric field and subsequently, larger peak GIC.…”

“…where g j,ωp (t) is the ideal S2S GIC prediction at scale ω p , and g j,ω B (t) is the ideal S2S GIC prediction at the baseline scale ω B >> ω p . We chose a baseline of ω B = 125 mHz based on the results in Grawe et al (2018) and to avoid the need to implement extrapolation methodologies for frequencies above the impedance tensor specification. Figure 4 shows plots of Equation 8 across the considered intervals and regions.…”

“…This can overheat structural parts of the transformer, generate harmonics, and lead to cascading failure through the activation of differential relay thresholds (Girgis & Vedante, 2012). What constitutes a "fast" variation in this context is the subject of current research, but is typically assumed to include temporal scales on the order of ten or hundreds of mHz (Grawe et al, 2018;Oyedokun et al, 2020), and the first part of this study investigates peak GIC error as a function of the rapidity of the magnetic field.…”

Effects of Ionospheric Conductivity and Ground Conductance on Geomagnetically Induced Currents During Geomagnetic Storms: Case Studies at Low‐Latitude and Equatorial Regions

“…The effects of the shallower layers appear as a difference in amplitude in these curves, which control the relative effects of the ground conductance between the stations. This difference in amplitude is related to impedance at frequencies above the Nyquist (∼8.3 mHz in our case), which significantly influences the magnitude of the induced surface electric field (see a more detailed discussion in Grawe et al, 2018).…”