Pitch-angle distributions of 100- to 300-kev protons measured by the Esro IB satellite

The pitch-angle distribution of 100- to 300-kev protons in the early morning/late afternoon local magnetic-time sector has been studied by using data from the early lifetime of the Esro IB (Boreas) satellite. The pitch-angle distribution is described as a function of invariant latitude for different geomagnetic activity levels. On the nightside, going poleward, the particle fluxes were found to peak at 90°, followed by a latitude range with isotropy. On the dayside, two precipitation zones were found during slight disturbances, the poleward zone showing an isotropic pitch-angle distribution and the equatorward zone showing an anisotropic distribution peaked at 90°. During disturbed conditions, the two zones merge and the region of isotropic pitch-angle distribution expands. The particle loss cones were determined and compared with theoretical loss cones in a realistic field model.