About these netCDF files:

These files contain results for the NOAA-USGS empirical EMTF geoelectric field model. The model uses an 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.

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 and Balch et al., 2023), but much more work is needed to understand the application of these results over a more complete range of space weather and geological situations. 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 welcome collaborations from the user community to participate in the ongoing validation analysis that is needed. 

This directory is an archive of the real-time calculations. The results have not been corrected at this time for occasional anomalies in the input data from the magnetic observatories, such as noise spikes, calibration offsets, or other types of input data issues, so users are cautioned that the output results can, on occasion, contain non-physical results because of these anomalies.

Each file contains one day of results, as indicated by the first part of the filename: yyyymmdd. The suffix part of the filename after 'EMTF' contains the year and month for which the transfer functions were downloaded from the IRIS website.

The netcdf files have the following attributes which can be read from the file:
FILENAME, MODEL_TYPE, MODEL_VERSION, CADENCE, NGRIDPTS

The netcdf files make use of three dimensions: time, gridpt, and stationlist (index to an individual station in obslist, see below)

The variables contained in the file are the following:
time - seconds since the start of the UT day (one entry per time step)
nobs - number of observatories used for the magnetometer data input (one entry per time step)
emax - Peak magnitude of the geoelectric field over a single map (one entry per time step)
obslist - string array lists the stations used for the magnetometer data input (one station list per time step)
latitude - the latitude of the gridpoint (one entry per grid point)
longitude - the longitude of the gridpoint (one entry per grid point)
distance - The distance of the gridpoint to the nearest observatory used in the calculation (one entry per grid point)
Ex - The north component of the calculated geoelectric field (one entry per grid point per time step) in mV/km
Ey - The east component of the calculated geoelectric field (one entry per grid point per time step) in mV/km
Quality - A quality indicator for the E-field results (one entry per grid point per time step) 

The time variable has an attribute REFTIME which give the time of reference to which its value is added to determine the time tag for a given time step. REFTIME is a string format time in CCSDS format (yyyy-mm-ddThh:mm:ss.mss)

Data for each time step is added progressively through the current UT day. Files for past days are therefore fixed and are
the basis for the archive repository for these model outputs.

For documentation on netCDF files, please refer to https://www.unidata.ucar.edu/software/netcdf/

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 input magnetic field time series used by the geoelectric field maps is dervied using the method of Spherical Elementary Current Systems, developed and made available courtesy of the Finnish Meteorological Institute (Amm & Viljanen, 1999; Pulkkinen et al., 2003)

The MT survey results were made available through the National Science Foundation (NSF)’s USArray Program and the USMTArray Program, funded in separate phases by the National Aeronautics and Space Administration (NASA) and then the U.S. Geological Survey (USGS). 

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

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

Balch, C., C. Jing, A. Kelbert, P. Arons, K. Richardson, "Geoelectric Field Model Validation in the Southern California Edison System: Case Study". IEEE Energy Conversion Congress and Exposition (ECCE), Nashville, USA, Nov. 2023, in press.

Kelbert, A., G.D. Egbert and A. Schultz, “EMTF, The Magnetotelluric Transfer Functions”. IRIS DMC Data Services Product, 2011.

Pulkkinen, A., O. Amm, A. Viljanen, et al., "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, 2003

Schultz, A., G.D. Egbert, A. Kelbert, T. Peery, V. Clote, B. Fry, staff of the National Geoelectromagnetic Facility, and their contractors, “USArray TA Magnetotelluric Transfer Functions”. IRIS DMC Data Services Product, 2006-2018. 

Schultz, A., L. Pellerin, P. Bedrosian, A. Kelbert, and J. Crosbie, “USMTArray South Magnetotelluric Transfer Functions”. IRIS DMC Data Services Product, 2020-2023.

Sun, R., C. Balch, "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.