The average quiet time structure of energetic radiation belt protons can be explained as an equilibrium balance among radial diffusive transport from a proton source located just within the first closed field lines, losses due to Coulomb collisions, and charge exchange with the ambient neutral hydrogen geocorona. The mode of transport is diffusion due to substorm-associated fluctuations in the large-scale electric and magnetic field. Attention is restricted to equatorially mirroring protons, and comparison is made between theoretical predictions of proton energy spectra at L values between 2 and 6.6 and in situ radiation belt ion observation on board the satellites Explorer 45 and ATS 6, both orbiting close to the equatorial plane. Good agreement between theory and observation suggests that the dominant ion population in the energy range 100--1000 keV in the inner magnetosphere is indeed a proton population. Beyond this interval the major ions may be different from protons.