During the late main phase of the April 6, 2000 storm with Dst approaching -300 nT, four Defense Meteorological Satellite Program (DMSP) satellites encountered repeated episodes of intense field-aligned currents whose magnetic perturbations exceeded 1300 nT, corresponding to |J∥| > 1 A/m. They had relatively fast rise times (∼5 min) and lasted for ∼20 min. The large magnetic perturbations occurred within the expanded auroral oval at magnetic latitudes below 60°. From Poynting-flux calculations we estimate that during each event several hundred tera-Joules of energy that dissipates in the mid-latitude ionosphere and thermosphere. Ground magnetometers at auroral and middle latitudes detected weak fluctuations that were incommensurate with magnetic perturbations observations at DMSP altitudes. Observed discrepancies between ground and satellite magnetometer measurements suggest that under storm conditions operational models systematically underestimate the level of electromagnetic energy available to the ionosphere thermosphere. We demonstrate a transmission-line model for M-I coupling that allows calculations of this electromagnetic energy input with no a priori knowledge of ionospheric conductances.