Mapping a Carrington Storm

Abstract A map is presented of median 1‐min‐resolution peak geoelectric‐field strength across the United States as would be induced by magnetic storms as intense as the 2 September 1859 Carrington storm. The map is constructed from two data sets: Magnetometer time series from 22 ground‐based observatories recording 40 magnetic storms, and surface impedance tensors derived from magnetotelluric measurements acquired at 1616 survey sites across the contiguous United States. Carrington‐class storm geoelectric fields are likely to be very strong in the United States East and Midwest; V/km at many places. In Virginia, strengths would likely range from 30.30 V/km, with a 68% confidence interval of [19.44,47.20] V/km, to as low as 0.05 [0.03,0.07] V/km. Comparison of model geopotentials with those measured on 30 long lines, indicates errors of about 18%. A Carrington‐class storm would likely induce geoelectric fields with strengths 55% greater than for the 13–14 March 1989 storm.