Implications on particle storage at the sun from observations of solar-flare proton spectrums

Observations of the differential energy spectrum of solar protons in the range 0.3 to 25 Mev obtained with the Injun 5 polar-orbiting satellite for the solar particle event that began on January 24, 1969, are presented. The data show that (a) the differential energy spectrum exhibits a peak for the first few hours after the event, with the maximum intensity moving from ∼1.1 Mev at ∼6 hours after the flare onset to 0.54 Mev at ∼12 hours after the flare, and (b) the differential intensity is a monotonically decreasing function of energy ∼30 hours and ∼56 hours after the flare. Spectrums observed in (a) are consistent with diffusive velocity dispersion during propagation in the interplanetary medium. The spectrums observed in (b) can be described by a power law in kinetic energy E of the form dj/dE=KE−γ. It has been suggested in the past that low-energy solar-flare particles are stored for periods of a few days in the immediate vicinity of the sun. By using the shape of the spectrums in (b) we derive upper limits to the electron (or ion) density in the (presumed) storage region in the vicinity of the sun of 4×105 to 3×104 cm−3. These electron densities correspond to solar radial distances of 3 to 6 Rs (solar radii). If adiabatic deceleration is taken into account, the densities are ∼105 cm−3 corresponding to ∼4 Rs. It is concluded that for the event of January 24, 1969, storage of low-energy protons did not take place in the immediate (3 to 6Rs) vicinity of the sun. The implications of the results are discussed in the context of current models of low-energy solar-particle propagation.