Particle propagation in the inner heliosphere

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Abstract
For 27 energetic particle events observed on one of the Helios spacecraft we present fits on the intensity and anisotropy time profiles performed with the model of focused transport. The events have local mean free paths λ∥ between 0.02 and ≥1 AU, for most events λ∥ lies within or above the consensus range between 0.08 and 0.3 AU. The propagation in general can be described as focused or diffusive transport, therefore in the majority of events interplanetary scattering plays an important role. Comparing with results from earlier studies we find that scatter‐free events can be observed inside 0.5 AU as well as close to 1 AU, therefore extreme scattering properties seem not to be related to radial changes in scattering conditions. In addition, any radial variation in the scattering mean free path seems to be small compared to the event‐to‐event variations. A comparison of fits on ∼18‐MeV protons and ∼1‐MeV electrons shows that the scattering mean free path λp of the protons is related to the electron mean free path λe, whereby the average ratio λp/λe is 1.6±0.9. This latter observation has important consequences for our understanding of the underlying scattering mechanism, in particular as it is in contradiction with conventional results from quasi‐linear theory.