We demonstrate that during the expansion phase of each of the three substorms detected by Ogo 5 on August 15, 1968, the plasma wave experiment detected significantly enhanced wave levels. For the two substorms encountered when Ogo was far downstream from earth, the peak turbulence levels were moderate. However, extremely intense signals were detected during the third substorm, when Ogo was closer to earth on auroral L shells. At this time the dominant electric field emission was one with wave frequency near 1.5 times the local electron gyrofrequency, although significant chorus signals were also present later in the expansion phase. We demonstrate that the maximum electric field itensities were sufficiently high to produce strong diffusion for energetic electrons, and we present evidence suggesting that this wave-particle interaction was operative, so that the waves controlled the local pitch angle distributions. (auth)