Cross-field gradients: general concept, importance of multi-spacecraft measurements and study at 1 AU of the source intensity gradient for E 30 keV solar event electrons

摘要

Three main physical processes (and associatedproperties) are currently used to describe the flux and anisotropy time profilesof solar energetic par- ticle events, called SEP profiles. They are (1) theparticle scattering (due to magnetic waves), (2) the particle focusing (due tothe decrease of the amplitude of the interplanetary magnetic field (IMF) withthe radial distance to the Sun) and (3) the finite injection profile at thesource. If their features change from one field line to another, i.e. if thereis a cross IMF gradient (CFG), then the shape of the SEP profiles will depend,at onset time, on the relative position of the spacecraft to the IMF and mightvary significantly on small distance scale (e.g. 106 km). One type ofCFG is studied here. It is called intensity CFG and considers variations, at thesolar surface, only of the intensity of the event. It is shown here that dropsof about two orders of magnitude over distances of ~104 km at theSun (1° of angular distance) can influence dramaticallythe SEP profiles at 1 AU. This CFG can lead to either an under or overestimationof both the parallel mean free path and of the injection parameters by factor upto, at least, ~2-3 and 18, respectively. Multi-spacecraft analysis can beused to identify CFG. Three basic requirements are proposed to identify, fromthe observation, the type of the CFG being measured.Key words: Solar physics, astrophysics, and astronomy(energetic particles; flares and mass ejections) - Space plasma physics(transport processes)