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Up to present, the largest data set of SEP events in the inner heliosphereare the observations by the two Helios spacecraft. We re-visit a sample of 15solar relativistic electron events measured by the Helios mission with the goalof better characterising the injection histories of solar energetic particlesand their interplanetary transport conditions at heliocentric distances <1 AU.The measurements provided by the E6 instrument on board Helios provide us withthe electron directional distributions in eight different sectors that we useto infer the detailed evolution of the electron pitch-angle distributions. Theresults of a Monte Carlo interplanetary transport model, combined with a fullinversion procedure, were used to fit the observed directional intensities inthe 300-800 keV nominal energy channel. Unlike previous studies, we haveconsidered both the energy and angular responses of the detector. This methodallowed us to infer the electron release time profile at the source anddetermine the electron interplanetary transport conditions. We discuss theduration of the release time profiles and the values of the radial mean freepath, and compare them with the values reported previously in the literatureusing earlier approaches. Five of the events show short injection histories(<30 min) at the Sun and ten events show long-lasting (>30 min) injections. Thevalues of mean free path range from 0.02 AU to 0.27 AU. The inferred injectionhistories match with the radio and soft x-ray emissions found in literature. Wefind no dependence of the radial mean free path on the radial distance. Inaddition, we find no apparent relation between the strength of interplanetaryscattering and the size of the solar particle release.

Full inversion of solar relativistic electron events measured by the Helios spacecraft