Supporting Information for "The United States Magnetotelluric Array and the National Impedance Map"

The United States Magnetotelluric Array (USMTArray) data set, collected in the years 2006-2024, consists of more than 1,700 long-period magnetotelluric stations covering the entirety of the contiguous United States on a quasi-regular 70km grid. Funding across multiple federal agencies was critical to sustaining this effort to its completion. Important components of the project included active guidance and participation from the MT community, the open and timely availability of all data, and the application of consistent instrumentation and robust data processing. Together with parallel advancement in the development of publicly available three-dimensional (3D) inversion codes, the USMTArray has revitalized the U.S. magnetotelluric community and increased the visibility of magnetotellurics within the Earth-science community. Taken as a whole, these data are visualized as the National Impedance Map, which, together with a 3D synthesis conductivity model of the nation, reveals the electrical architecture of the contiguous U.S. USMTArray data are used by researchers worldwide for fundamental and applied studies, including investigations of continental architecture and evolution, estimation of hazards to critical infrastructure due to geomagnetic storms, and assessment of the nation's undiscovered geothermal and mineral resources. We here review the history and development of the project, discuss the challenges and successes in its execution, present the National Impedance Map and synthesis conductivity model, and highlight the breadth of research stemming from this rich data set. This record is a supplement to the Kelbert, Bedrosian et al, 2026 publication in the Reviews of Geophysics (doi:10.1029/2024RG000850) and includes a copy of the CONUS-MT-2026 3D continental electrical conductivity model for the contiguous United States, synthetic modeled data for the MT gridded array impedance map and all the intermediate data products that we visualize in that publication, as described in the Data Availability Statement.