Relativistic electron precipitation (REP) has been monitored by observing anomalous changes in phase or amplitude of ionospheric radio waves which propagate through the ionospheric D region. Magnetospheric substorm activity appears to be a necessary though by no means sufficient condition for producing intense REP events. A general correlation is found between the onset of substorm activity and nocturnal REP events. A general correlation is found between the onset of substorm activity and nocturnal REP events. In contrast, dayside REP events occur less frequently, are more restricted in local time, and are often delayed by up to several hours following the onset of substorm activity. The observational evidence supports the concept of parasitic strong diffusion scattering of trapped relativistic electrons by intense ion cyclotron turbulence which is generated following the injection of ring current ions into the outer radiation zone during the substorm of expansive phase.