During the main phase of the 6 April 2000 magnetic storm with Dst $cong$ -300 nT, four Defense Meteorological Satellite Program (DMSP) satellites encountered intense sheets of field-aligned currents (FACs). Their magnetic perturbations were >1300 nT, corresponding to integrated currents ∣J$parallel$∣ > 1 A/m. The FACs appeared in both the evening and dawn magnetic local time sectors. They had relatively fast rise times (~5 min), lasted for ~0.5 hours, and were associated with widespread reconfigurations of plasma in the near-Earth magnetotail. The largest magnetic and related electric field perturbations occurred at magnetic latitudes <60¿. Magnetometer measurements from DMSP satellites show repeated episodes of similarly large FACs late in the main phase of this and other superstorms. Poynting flux calculations indicate that a few percent of the total ring current energy is dissipated as Joule heat in the midlatitude ionosphere during each of these events. A survey of ground magnetometers at auroral and middle latitudes found perturbations typically <200 nT, incommensurate with magnetic observations at the altitude of the DMSP satellites. The small ground responses reflect weak ionospheric Hall currents and provide an empirical validation of the theorem derived by Fukushima <1976>. Height-integrated Pedersen conductances (ΣP) calculated with observed precipitating electron fluxes have significantly lower values than those estimated from Ohm's law using simultaneously measured electric and magnetic field variations. This discrepancy suggests that under some highly stressed conditions the neglect of contributions from low energy electrons and precipitating ions can lead to serious underestimates of ΣP and a consequent misunderstanding of the magnetosphere-ionosphere circuit.