Observations of low-energy (0.3- to 1.8-Mev) Differential spectrums of trapped protons

Measurements of differential energy spectrums of trapped protons obtained from several passes during the period January to November 1969 using the polar-orbiting (inclination, 80.7°), low-altitude (2525 km by 644 km) satellite Injun 5 of the University of Iowa are presented. The solid-state detector proton-electron telescope on Injun 5 consists of a 24-μ front element and two back elements of 862 and 905 μ, respectively, and provides several differential energy channels in the range 0.3 to ∼25 Mev. The study reveals the existence of a quasi-persistent peak in the differential energy spectrum in the L range 2 to 2.6 and the energy range ∼0.38 to 0.72 Mev. The average position of this peak is at L≃2.3 and E≃0.54 Mev, but both the position in L and the peak energy appear to vary with geomagnetic activity in an as yet undetermined manner. The fact that the shape of the spectrum is stable for several days or can change with time scales as small as 4 hours suggests an impulsive acceleration mechanism deep in the radiation belts like that suggested by other authors for energetic electrons. Other features of the spectrum show that the spectral parameter E0 in the form dj/dE=K exp (−E/E0) varies as ∼L−3 in the L range 2 to 4, in general agreement with previous observations. The last result adds to the general body of evidence favoring diffusion from the solar wind as the primary mechanism for populating the radiation belts.