On May 4, 1977, a polar orbiting low-altitude DMSP satellite passed close to the Finnish riometer stations at the same time as an absorption bay preceding an onset of substorm was observed. The combined ground and satellite measurements show that the absorption bay was characterized by intense precipitation of energetic electrons at the inner border of the compressed plasma sheet. The precipitation occurred in a narrow band of latitude, perhaps less than 50 km wide. We suggest that the pre-bay precipitation was due to strong pitch angle diffusion and/or scatter of energetic electrons at the inner border of the plasma sheet, probably from the outer radiation zone.