Comparison of auroral zone riometer data with high-resolution energetic electron data at the geosynchronous orbit confirms previous observations that significant electron (E>30 keV) precipitation begins remarkably near the flux level predicted in the Kennel-Petschek theory. The present new observations, however, also provide adequate pitch angle information to study the detailed evolution of energetic electron anisotropies from the weak pitch angle diffusion limit to the strong diffusion limit predicted in the theory of wave-particle interactions. Since all substorm-accelerated electrons which are injected after fluxes have reached the strong diffusion limit are rapidly scattered and lost from the magnetosphere, the present measurements provide a new experimental electron upper flux limit of 5¿107 electrons (cm2-sec-sr)-1 at L=6.6.