During the early November 1993 study interval selected for the international Space Weather Initiative a remarkably intense early evening substorm event was observed at around 2200 UT on November 4, 1993, in high-latitude magnetometer and riometer recordings. The event, the most intense absorption event recorded by the imaging riometer in Sdr. Str¿mfjord (STF), Greenland, during 1993, occurred at the center of strong upward region 1 field-aligned currents during a major reconfiguration of the polar ionospheric plasma convection system. The event was preceded by some substorm activity at lower latitudes. At STF, however, it appeared with nicely isolated and exceptionally clear signatures in the geophysical observations, primarily in the imaging riometer measurements of ionospheric radio wave absorption at 38 MHz. The event was characterized by multiple intense and sudden absorption pulsations with increasing amplitudes of up to 11 dB separated by about 20--30 s and with duration of around 10--15 s superimposed on a general level of a few decibels of absorption. The event was selected for a modeling study to investigate the possible generation mechanisms for the ionized layers causing the absorption. We conclude that the event was most probably caused by precipitation of magnetospheric electrons accelerated repeatedly through transient field-aligned potential structures of 10--30 kV associated with the intense upward currents. We find that these structures were located at a few Earth radii's distance and that the field-aligned potential structures have positive feed-back to the ionospheric potentials affected by the conductivity changes caused by accelerated electrons. We suggest that such coupled potential systems may provide a general mechanism for acceleration of electrons of a thermal magnetospheric plasma to the much higher energies found in auroral precipitation and in the radiation belts. ¿ 1998 American Geophysical Union