Neutral lines and azimuthal “transport” of solar energetic particles

We discuss properties of solar energetic particle (SEP) events observed by both Helios spacecraft in the time period March 1976 to March 1980, in particular variations of the intensity time profiles with angular distance between the flare and the observer's magnetic footpoint. Special attention is paid to the neutral lines of the large‐scale coronal magnetic field. For individual events we will show that sector boundaries can have a strong influence on intensities and time‐scales of SEP events. This can be seen best in events in which both spacecraft are connected more or less symmetrically around the flare site with a sector boundary between them. In a statistical approach we discuss variations of onset and maximum times as well as maximum intensities with angular distance between the flare and the observer's magnetic footpoint for 39 SEP events. The use of two spaceprobes has the advantage that unknown flare parameters, such as the number of injected particles and the time of acceleration, can be eliminated by studying only relative variations of time and intensity with angular distance. These variations were analyzed under consideration of the sector boundaries of the large‐scale coronal magnetic field. We find (1) particles can be observed on both sides of sector boundaries during both, impulsive and gradual, events, (2) the onset times of ∼0.5‐MeV electrons can be ordered by the occurrence of sector boundaries, inside the flare sector we can define some kind of “azimuthal propagation velocity” Δϕ/Δtons with ≥4°/min, outside the flare sector this velocity is ≤1°/min, (3) the azimuthal propagation of ∼20‐MeV protons seems to be systematically slower, (4) there seems to be no difference in the variations of onset times or maximum intensities with angular distance between impulsive and gradual events, and (5) within the broad scatter the variation of the maximum intensity with angular distance is not systematically influenced by the crossing of sector boundaries, despite the obvious influence of sector boundary crossings in individual events. These observations support the association of the fast propagation region (FPR) with the large‐scale polarity cells of the coronal magnetic field. The “transport” inside the FPR can be understood in terms of an open cone of propagation. The “transport” outside the FPR can be interpreted in terms of a propagation mechanism or acceleration at a shock.