The temperature anisotropy of the hot (keV) protons of the outer magnetosphere can excite the electromagnetic proton cyclotron instability. Wave-particle scattering by this instability not only maintains the hot proton anisotropy at or below an upper bound but also imparts energy to the cool (eV) proton component. This instability and its consequences are examined through the use of linear and second-order Vlasov theory and one-dimensional hybrid simulations in a homogeneous plasma model which represents these two protonic constituents. A different form for the upper bound on the hot proton anisotropy is derived from linear theory and the simulations; comparison against plasma observations from Los Alamos magnetospheric plasma analyzers in geosynchronous orbit shows very good agreement. Second-order theory and simulations are used to obtain a different scaling for the late-time apparent temperature of the cool proton component Tc which indicates that it is inversely correlated with the cool proton density nc. Observations in both the plasmasphere and the outer magnetosphere sometimes show a similar inverse correlation, suggesting that the electromagnetic proton cyclotron instability is an important contributor to cool proton heating in these regions. ¿ American Geophysical Union 1995