Diffusion of 155 to 430 keV protons in the Earth's radiation belt

Pitch angle distributions and energy spectra of 155-430 keV trapped protons between L=4 and L=5.5 are examined to study pitch angle and radial diffusi on rates during a period of low magnetic activity. To improve the sensitivi ty to pitch angle diffusion, the measurements were made near the loss cone where angular gradients in the distribution function are large. In this reg ion the phase space density is such that both radial diffusion and pitch an gle diffusion lead to flux increases while charge exchange leads to proton loss. However, the conventional prescriptions for the diffusion coefficient s predict too much diffusion at L=4, implying growth in the proton fluxes w hile observations show loss. At L=5, theory and observation both show sligh t growth, and at low pitch angles diffusion in pitch angle is necessary to compensate for charge-exchange losses. Although Dst was low during the 13 d ays preceding the measurements, the proton belt had not reached a steady st ate with diffusion sources matching charge-exchange losses.