Several recent observations of low-energy protons in the in• terplanetary medium following solar particle events have con• clusively demonstrated that their differential energy spectra can significantly deviate from a power law form (see, for exam• ple. l.anzerotti (1970, 1974), Krimigis and Verzariu [1971). and Verzariu and K rimigis (19721). Early in an event the differential energy spectrum shows a peak that progressively moves to lower energies as the event develops and eventually moves hevond the lowest-energy threshold of the detector. Spectra which show the movement of such a relative maximum are consistent with the velocity dispersion of solar protons that propagate in the interplanetary medium due to diffusion [Krimigis and Verzariu, 1971). The distortion remaining in the shape of the differential energy spectrum after velocity disper• sion effects disappear is seen to become less pronounced ith the passage of time; late in an event the spectrum assumes a near power law form. Several lines of interpretation can be ad• vanced to explain the change in the spectral shape at later times during an event. The simplest of these is perhaps the J,sumpti()n of continuous emission of solar particles wuh a hanging source spectruni that is the same as that measured at
I • l 1. An analysis of this hypothesis has not yet been possible. Several other effects, individually or collectively, could also produce the spectral modulation. Solar particles can be stored m the corona or the interplanetary medium and can slowly leak out of the trapping region. Depending on the location of :he storage region and the duration of storage, particles can o-e sufficient energy to account for the spectral modulation .inder the assumption of a power law energy spectrum at the .rne of injection). The spectra can also be altered due to in planetary deceleration and acceleration.

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