About these netCDF files:

These files contain results for the joint NOAA-NRCAN US-Canada 1D geoelectric field model, developed in collaboration with USGS and the Electric Power Research Institute. The model uses 1D physiographic conductivity models, with the U.S. portion developed by the Electric Power Research Institude, EPRI (2020), and the Canadian portion developed by Trichtchenko et al. (2019). 

Users please note that there is also a 3D empirical version of the Geoelectric Field Maps (for the continental US only) running at SWPC (deployed to operations in FY2020); The 3D empirical model uses Magnetotelluric Transfer Functions (EMTF's) (see Kelbert et al., 2011 for details), which provide an Earth Conductivity description that incorporates the full 3D effects of Earth conductivity structures. The coverage area of the 3D empirical model is limited to locations where MT surveys have been published. In general we recommend that users located in the 3D empirical model coverage area use that model instead of the 1D model. The US-Canada-1D map, however, covers a larger area, using available information.

The local geoelectric field is specified in millivolts per kilometer (mV/km) and is based on convolving a geomagnetic time-series signature with an Earth impulse response function, where the impulse response function depends on the local Earth conductivity (Boteler and Pirjola, 2022). In the US-Canada-1D version, geomagnetic time-series are interpolated onto a 0.5 degree by 0.5 degree grid using the method of Spherical Elementary Current Systems (SECS - see Amm & Viljanen, 1999; Pulkkinen et al. 2003 for more information about the method). The Earth conductivity is determined, based on the physiographic region that the grid point lies in and the associated, one-dimensional conductivity profile.

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 netcdf files have the following attributes which can be read from the file:
FILENAME, PRODUCT_VERSION, CADENCE, NGRIDPTS, LAST_TTAG

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 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)

Updated 1D models for the U.S. were provided courtesy of the Electric Power Research Institute (EPRI 2020). 

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.

Boteler, D.H. and R.J. Pirjola, "Electric Field Calculations for Real-Time Space Weather Alerting Systems". Geophys. J. Int., https://doi.org/10.1093/gji/ggac104, 2022.

Electric Power Research Institute (EPRI), "Use of Magnetotelluric Measurement Data to Validate/Improve Existing Earth Conductivity Models". EPRI Product ID 3002019425, June 08, 2020.

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

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.

Trichtchenko, L., P.A. Fernberg, D. Boteler, "One-dimensional Layered Earth Models of Canada for GIC Applications". Geological Survey of Canada Open Files 8594 & 8595, 2019.