Up to this time all explanations of the radiation belt linked it to the aurora, but in the second half of 1958 at least three investigators independently conceived a different explanation, related to cosmic rays: Singer , Kellogg  and Vernov . Cosmic rays are rapidly moving atomic nuclei (mostly protons) whose energies start in the GeV range and extend in diminishing numbers much higher. They arrive at the solar system from distant space and when they colide with nuclei of the atmosphere they produce a spray of secondary fragments.
Most secondary particles from such collisions move earthward and are lost, but a small fraction ("albedo") is splashed away from the Earth. If an electrically charged albedo particle has high energy it either escapes or else is guided by magnetic field lines to the opposite hemisphere, where it is usually absorbed by the atmosphere. An albedo neutron however moves unimpeded until it decays into an electron, a neutrino and a proton, with the latter receiving almost all the kinetic energy.
The mean decay time of a free neutron is about 10 minutes, but actual times for individual neutrons are distributed statistically and for a typical fragment energy of 20-50 MeV, a few neutrons may already decay within the first few hundredths of a second. The decay protons may then materialize deep enough in the magnetic field to remain trapped there, and their lifetime in such trapped orbits is long enough to allow an appreciable density of energetic protons to accumulate. This was what Singer, Kellogg and Vernov proposed, and the existence of such protons was confirmed in 1959 by nuclear emulsions flown aboard rockets into the near-earth radiation belt and later recovered [Freden and White, 1959; Hess, 1962; White, 1966, 1973].
In hindsight, it was realized much later that powerful shocks, created inside the magnetosphere by interplanetary shocks of solar origin, could on rare occasions accelerate protons near Earth to energies of many MeV. Thus the event of March 24, 1991 was observed to create a belt of 20 MeV protons just outside the inner belt, of comparative intensity [Blake et al., 1992]. It is possible that a similar event produced the double-peaked structure of the inner belt observed by Explorer 15 [McIlwain, 1963].