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High Angular Resolution Measurements of Geomagnetically Trapped Protons The Source/Loss-Cone Energetic Particle Spectrometer (SEPS) aboard the POLAR spacecraft records ion angular distributions between 150 and 430 keV. Each of six detector units consists of a focal plane of solid-state pixels mounted behind a pinhole aperture. Each pixel on the focal plane records the particle flux at a single pitch angle with angular resolution of 1.3 degrees. The field of view is about ±10 degrees about the central axis. SEPS is mounted on the despun platform of the satellite and when inside the magnetosphere the field of view usually includes the local magnetic field direction and the trapped particle loss cone. During a magnetic storm the ions can be examined to study the processes which influence the angular distributions at small pitch angles These processes include pitch angle scattering by waves, transport in L by radial diffusion, and the removal of protons by charge exchange with the hydrogen geocorona. Data will be shown for a magnetic storm in which ring current ions are accelerated and redistributed into a non-equilibrium profile. |