Reproducing electric field observations during magnetic storms by means of rigorous 3-D modelling and distortion matrix co-estimation

Abstract

Electric fields induced in the conducting Earth by geomagnetic disturbances drive currents in power transmission grids, telecommunication lines or buried pipelines, which can cause service disruptions. A key step in the prediction of the hazard to technological systems during magnetic storms is the calculation of the geoelectric field. To address this issue for mid-latitude regions, we revisit a method that involves 3-D modelling of induction processes in a heterogeneous Earth and the construction of a magnetospheric source model described by low-degree spherical harmonics from observatory magnetic data. The actual electric field, however, is known to be perturbed by galvanic effects, arising from very local near-surface heterogeneities or topography, which cannot be included in the model. Galvanic effects are commonly accounted for with a real-valued time-independent distortion matrix, which linearly relates measured and modelled electric fields. Using data of six magnetic storms that occurred between 2000 and 2003, we estimate distortion matrices for observatory sites onshore and on the ocean bottom. Reliable estimates are obtained, and the modellings are found to explain up to 90% of the measurements. We further find that 3-D modelling is crucial for a correct separation of galvanic and inductive effects and a precise prediction of the shape of electric field time series during magnetic storms. Since the method relies on precomputed responses of a 3-D Earth to geomagnetic disturbances, which can be recycled for each storm, the required computational resources are negligible. Our approach is thus suitable for real-time prediction of geomagnetically induced currents by combining it with reliable forecasts of the source field.

Observatory geoelectric fields induced in a two-layer lithosphere during magnetic storms

“…Other results reported by Love and Swidinsky, including the average inferred conductivity and fits to the electric field data, are unaffected by this mistake. The elements reported here have been checked carefully for placement and sign; given expected statistical jitter, the half-space elements for G H are certainly reasonably consistent with those estimated by Püthe et al (2014).…”

“…The complicated regional distribution of electrical conductivity is almost certainly responsible for the high degree of polarization in the induced geoelectric field measured at Kakioka. Püthe et al 2014 have noted a seeming inconsistency between their estimate of the Kakioka galvanic distortion tensor, obtained for a spherical model, and that reported by Love and Swidinsky (2014, eq. 29), obtained for a Cartesian half-space.…”

“…In the context of inductionhazard science and the need to develop algorithms for estimating storm-time induction of geoelectric fields, lithospheric conductivity is often modeled as being only depth dependent (e.g., Pulkkinen et al 2007). Recently, however, three-dimensional models have been used in the analysis of storm-time induction (e.g., Püthe and Kuvshinov 2013; Thomson et al 2009;Viljanen et al (2014); in their independent analyses of Kakioka geomagnetic and geoelectric data, Love and Swidinsky Love and Swidinsky (2014) and Püthe et al 2014 each used a time-constant galvanic distortion tensor in their modeling of the polarizing effects of localized three-dimensional lithospheric conductivity. Given our practical goal of obtaining improved estimates of storm-time geoelectric fields, we choose to extend the parameterizations of the preceding analyses in two different ways.…”

Compilation of 3D global conductivity model of the Earth for space weather applications

“…Püthe and Kuvshinov 2013). This model has been shown to be adequate to reproduce large-scale geomagnetic storm electric fields (Püthe et al 2014), but it is most probably not sufficient for accurate characterization of smaller-scale magnetospheric substorm electric fields due to higher frequency content of their spectrum.…”

Introducing inter-site phase tensors to suppress galvanic distortion in the telluric method

“…In particular, the TS and the T-MT method did not allow for a correction of galvanic distortion. However, it is relevant to remark that recently, a time-domain approach was introduced for a direct estimation of the distortion effect (Püthe et al 2014).…”