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

Abstract

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 diffusion rates during a period of low magnetic activity. To improve the sensitivity to pitch angle diffusion, the measurements were made near the loss cone where angular gradients in the distribution function are large. In this region the phase space density is such that both radial diffusion and pitch angle diffusion lead to flux increases while charge exchange leads to proton loss. However, the conventional prescriptions for the diffusion coefficients predict too much diffusion at L=4, implying growth in the proton fluxes while observations show loss. At L=5, theory and observation both show slight 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 days preceding the measurements, the proton belt had not reached a steady state with diffusion sources matching charge‐exchange losses.