Geoelectric Field 1-Minute (Empirical EMTF - 3D Model)

The near real-time E-field mapping project is a joint effort between NOAA/SWPC, the USGS geomagnetism group, NRCAN Space Weather, and the NASA/CCMC.

Background

Potentially hazardous geoelectric fields can be induced during geomagnetic storms. These geomagnetic storms are a form of space weather driven by enhanced currents in Earth's magnetosphere and ionosphere and are observed at ground level as a time-varying magnetic field. As is well known from Faraday's law, a time-varying magnetic field induces currents along natural and artificial conducting pathways by means of an induced electric field. This geoelectric field product combines information about the time-varying magnetic field together with Earth-conductivity information to estimate regional geoelectric fields on a geographic grid over the lower 48 states. The amount of current induced in an artificial conductor may be calculated by integrating the electric field along the conducting pathway. When currents are induced in artificial conductors, unexpected and sometimes problematic effects can occur in the operation of the affected equipment. Please see our article about the effect this has on electrical power systems at https://www.swpc.noaa.gov/impacts/electric-power-transmission. Please see also the article Modeling geomagnetically induced currents, by Boteler and Pirjola in Space Weather (31 January 2017), for an up-to-date description of this phenomena.

Versions and Caveats:

The first version of this product, released in 2017, used 1D physiographic conductivity models published by Fernberg (2012) and is available on SWPC's website. The empirical EMTF-3D model uses an improved earth conductivity description, based on Empirical Magnetotelluric Transfer Functions (EMTF's) (see Kelbert et al., 2011 for details), which incorporate the full 3D effects of Earth conductivity structures. These EMTF's are a data product from a magnetotelluric survey, and are publicly available thanks to the EMTF data service of IRIS (Incorporated Research Institutions for Seismololgy - please see their website for more information). In this particular implementation, the transfer functions for each survey site are used to calculate E-field values on an irregular grid (nominal spacing is about 70 km). Those results are then re-sampled using interpolation to a regular 0.5 degree geographical grid over the survey coverage area. Note that coverage for the empirical EMTF 3D model is only possible in areas where surveys have been completed. In addition, only transfer functions with a quality rating of 3 or higher are used. We expect to expand the coverage area over time as more MT survey results are completed.

Future upgrades to the E-field model are anticipated depending on the development of state-of-the-art three-dimensional (3D) electrical conductivity models obtained with magnetotellurics (Meqbel et al., 2014; Yang et al., 2015; Murphy & Egbert, 2017) and global electromagnetics (Sun et al., 2015). The upgraded versions of the maps will follow the same output format as the existing Geoelectric Field Maps, and the newer services will be provided experimentally before final operational release to facilitate scientific research, validation, and initial familiarization for the operators.

Users should note specifically that the Geoelectric Field Maps are in need of validation against geoelectric field or geomagnetically induced current measurements. Some initial, limited validation work has been done (Sun & Balch, 2019) showing improvement from the 1D Fernberg model to the empirical EMTF 3D approach, but much more validation work is needed to understand the application of these results over a more complete range of space weather and geological situations.  Generally speaking it is expected that the EMTF 3D empirical results will be more accurate than the 1D Fernberg results.

An extensive comparison of the two different models has been carried out which helps to illustrate the similarities and differences between the two approaches. A summary of this analysis was presented to the Fall 2019 AGU meeting and may be viewed here. Further details of this analysis can be found at https://www.swpc.noaa.gov/products/regional-geoelectric-validation.

We welcome collaborations from the user community to participate in the ongoing validation analysis that is needed. Retrospective E-field maps are available, for example, for the September 07-08, 2017 geomagnetic storm (for both the 1D Fernberg model and for the newer EMTF model) which can be tested using systems models and GIC measurements. SWPC is also retaining the 1D Fernberg gridded E-field output results (starts in October 2017) as well as the EMTF empirical 3D results (starts in July 2019). It is also possible for us to run simulations retrospectively using different conductivity models for earlier periods of interest to our industry collaborators. 

At this time, we advise caution in the utilization of the Geoelectric Field Maps for operational mitigation of geomagnetic hazards without prior investment in a validation study. We hope, however, that the release of this product will facilitate additional research on geomagnetic hazards and validation activities within the power-grid industry and will help operators have better situational awareness during geomagnetic storms. 

Acknowledgements:

Key data provider agencies are gratefully acknowledged for their contributions:
-The U.S. magnetometer observatories are operated and maintained by the U.S. Geological Survey
-The near U.S. Canadian observatories are operated and maintained by NRCAN

The maps use a geomagnetic-field time series interpolation algorithm (Spherical Elementary Current Systems) developed and made available courtesy of the Finnish Meteorological Institute (Amm & Viljanen, 1999; Pulkkinen et al., 2003)

Results from the NSF's EarthScope USArray project with contributions from a survey of Florida by USGS are being used as the source for the development of the improved EMTF-empirical 3D model as well as for doing full 3D Earth conductivity models through inversion and forward calculations. (see Kelbert et al. 2011 and Bedrosian et al. 2015)

Technical advice from David Boteler, NRCAN, is gratefully acknowledged.

References:

Amm, O. & A. Viljanen (1999). Ionospheric disturbance magnetic field continuation from the ground to the ionosphere using spherical elementary current systems, Earth Planets Space, 51, 431-440.

Bedrosian, P.A., A Kelbert, B.L. Burton, J.R. Morris, and C. Blum (2015). Long Period Magnetotelluric Transfer Functions from the Florida Peninsula. doi:10.17611/DP/EMTF/USGS/GEOMAG/FL15

Bedrosian, P. A., & Love, J. J. (2015). Mapping geoelectric fields during magnetic storms: Synthetic analysis of empirical United States impedances. Geophysical Research Letters, 42(23).

Bonner, L. R., & Schultz, A. (2017). Rapid prediction of electric fields associated with geomagnetically induced currents in the presence of three‐dimensional ground structure: Projection of remote magnetic observatory data through magnetotelluric impedance tensors. Space Weather, 15(1), 204-227.

Boteler, D. & R. Pirjola (2017), Modeling geomagnetically induced currents, Space Weather, DOI10.1002/2016SW001499 (31 January 2017).

Fernberg 2012, One-Dimensional Earth Resistivity Models for Selected Areas of Continental United States and Alaska, EPRI Technical Update 1026430, Palo Alto, CA.

Kelbert, A., G.D. Egbert and A. Schultz (2011), IRIS DMC Data Services Products: EMTF, The Magnetotelluric Transfer Functions, https://doi.org/10.17611/DP/EMTF.1 

Kelbert, A., Balch, C. C., Pulkkinen, A., Egbert, G. D., Love, J. J., Rigler, E. J., & Fujii, I. (2017). Methodology for time‐domain estimation of storm‐time geoelectric fields using the 3D magnetotelluric response tensors. Space Weather.

Meqbel, N. M., Egbert, G. D., Wannamaker, P. E., Kelbert, A., & Schultz, A. (2014). Deep electrical resistivity structure of the northwestern US derived from 3-D inversion of USArray magnetotelluric data. Earth and Planetary Science Letters, 402, 290-304.

Murphy, B. S., & Egbert, G. D. (2017). Electrical conductivity structure of southeastern North America: Implications for lithospheric architecture and Appalachian topographic rejuvenation. Earth and Planetary Science Letters, 462, 66-75.

Pulkkinen, A., O. Amm, A. Viljanen, et al. (2003). Separation of the geomagnetic variation field on the ground into external and internal parts using the spherical elementary current system method, Earth Planets Space, 55, 117-129.

Sun, J., Kelbert, A., & Egbert, G. D. (2015). Ionospheric current source modeling and global geomagnetic induction using ground geomagnetic observatory data. Journal of Geophysical Research: Solid Earth, 120(10), 6771-6796.

Sun, R., Balch, C. (2019). Comparison between 1D and 3D Geoelectric Field Methods to Calculate Geomagnetically Induced Currents: A Case Study. IEEE Transactions on Power Delivery, 34, 6, December 2019.

Weigel, R. S. (2017). A comparison of methods for estimating the geoelectric field. Space Weather, 15(2), 430-440.

Yang, B., Egbert, G. D., Kelbert, A., & Meqbel, N. M. (2015). Three-dimensional electrical resistivity of the north-central USA from EarthScope long period magnetotelluric data. Earth and Planetary Science Letters, 422, 87-93.