Interplanetary transport of decay protons from solar flare neutrons

We have developed models for the propagation of protons produced by the decay of solar flare neutrons. These models are applied to the observation of such protons on 1982 June 3 and 1984 April 25. Since the decay protons are produced with the same direction and velocity as the parent neutrons, these events provide a unique opportunity to measure the spectrum of emitted neutrons and to study the pitch-angle scattering of energetic protons as it transforms a monodirectional beam into an isotropic distribution. We have performed numerical simulations treating the deposition, pitch-angle scattering, and adiabatic focusing of neutron-decay protons. Fitting these simulations to the measurements of the time evolution of the decay proton flux strongly constrains the interplanetary scattering mean free path, λ, to be 0.26±0.05 AU on 1982 June 3 and 0.37±0.06 AU on 1984 April 25. We derive power-law exponents for the energy spectra of parent neutrons of differential 26.5-147 MeV that range from -1.7 to - 1.9 for the 1982 flare and from -1.2 to - 1.4 for the 1984 flare. In addition, the evolution of the neutron-decay proton flux seems to imply that the neutron emission was either nonisotropic or occurred over a time of several minutes.