Explorer 45 satellite observations of medium energy electrons and ELF waves (100‐1000 hz) have been analysed during a modest isolated substorm which occurred at about 0430 UT on Feb. 13, 1972. Approximately 50 minutes before the sharp rise in AE the electron flux (E ∼ 30 ‐ 240 KeV) measured within the plasmapause near dusk at L = 5 dropped below the normal quiet time level. Simultaneously the intensity of plasmaspheric hiss emissions fell to background levels. Following the substorm activity the electron flux recovered and plasmaspheric emissions first reappeared when the electron flux (E>30 KeV) attained a value comparable to the Kennel‐Petschek stably trapped level required for cyclotron resonant instability. The substorm associated electron flux reduction is explained as a modification in the drift orbits of geomagnetically trapped electrons caused by the onset of a large scale convection electric field. The simultaneous wave and particle observations are consistent with electron cyclotron resonant generation for plasmaspheric hiss emissions. During this relatively quiet period the source location appears to be restricted to the outer plasmasphere