Transient electric field events have been observed with the long double-probe instrumentation carried by the Imp 6 satellite. Nine clearly defined exceptionally large amplitude events are presented here. The events are observed in the midnight sector at geocentric distances of 3.5–5.5 RE at middle latitudes within a magnetic L shell range of 4.8–7.5. They usually have a total duration of 1 to several minutes with peak power spectra amplitudes occurring at a frequency of about 0.3 Hz. The events occur under magnetically disturbed conditions, and in most cases they can be associated with negative dH/dt excursions at magnetic observatories located near the foot of the magnetic field line intersecting Imp 6. These electric field events should cause violation of the second and third magnetic adiabatic invariants for the proton population and should violate all three invariants for the heavier ion populations. The magnetospheric motions calculated for these electric fields indicate a quasi-stochastical diffusive process rather than the general inward magnetospheric collapsing motion expected during the expansive phases of auroral substorm activity. The transient electric fields provide an obvious mechanism for the impulsive acceleration and injection of plasma to populate the outer radiation belt.