Differential energy spectrum of geomagnetically trapped protons with the ESRO 2 satellite

The proton energy spectrum in the inner Van Allen zone was investigated in the range 30 to 300 Mev with a solid state dE/dx telescope aboard the Esro 2 satellite. The B-L domain covered is 1.3 ≤ L ≤ 2.4 and 0.18 ≤ B ≤ 0.24. It was found that an exponential spectrum f [E] = N0 exp (—E/E0) is a good fit to the data in the range L = 2 to L = 2.4. For L Ec, f[E] = N0 exp (—E/E0). From our data, the variations with L of the parameters Ec, E0, E1 were found to be (a) E1 = (148 ± 8) Mev; (b) E0 = (290 ± 20) L−3.3±0.3; and (c) Ec = (400 ± 20) L−3±0.2. We suggest that two injection mechanisms are operating for all of this energy range, and that they give rise to two populations. The first one is the main component at energies lower than 25 Mev. At energies higher than our threshold (30 Mev), the detected fluxes are the addition of a second population (main component) and the high-energy tail of the first population. The spectrum of this second population appears to have the characteristic shape of a spectrum altered by ionization losses and generated by a continuous source, the slope for energies greater than Ec being E0.