Several observations of low-energy solar-proton spectra and possible interpretations

Abstract

The differential energy spectrum of solar protons in the range 0.3-25 Mev obtained with the Injun 5 polar-orbiting satellite is examined for several solar-particle events. Of these, the events of February 25, 1969, April 26, 1969, and March 6, 1970, are discussed in detail. Except for the spectra observed early in the event, which are consistent with diffusive velocity dispersion effects, it is found that the majority of the events exhibit undistorted monotonically decreasing (∼E−γ) spectra within the energy range measured. The solar-particle events of April 26, 1969, and March 6, 1970, produced spectra late in the event, implying energy degradation of the very-low-energy solar protons. By using one of the degraded spectra (March 8, 1970) it was found that the energy degradation could be explained by (1) postulating a temporary storage region ≳3–7 RS above the photosphere or (2) requiring continuous acceleration and emission of particles at the sun in the manner discussed by Parker (1965). Present theoretical and observational evidence indicates that the magnetic trapping is effective only to an altitude of about 1 RS above the photosphere, whereas our data suggest higher coronal trapping regions for the low-energy protons. If it is assumed that closed-field structures are necessary for particle confinement, one must conclude that alternative 2 is more plausible in explaining the observations. The effects of curvature and gradient drifts on the stored particles are computed by assuming a dipolelike trapping region extending to 1 RS above the photosphere. It is found that these drifts are ineffective in depleting the trapping region of low-energy protons (1–10 Mev) and that for typical confinement times of ∼105 sec most of this low-energy component of flare-generated particles should be lost to the photosphere owing to ionization losses.