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

Abstract

Abstract. Three main physical processes (and associated properties) are currently used to describe the flux and anisotropy time profiles of solar energetic par- ticle events, called SEP profiles. They are (1) the particle scattering (due to magnetic waves), (2) the particle focusing (due to the decrease of the amplitude of the interplanetary magnetic field (IMF) with the radial distance to the Sun) and (3) the finite injection profile at the source. If their features change from one field line to another, i.e. if there is 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 might vary significantly on small distance scale (e.g. 106 km). One type of CFG is studied here. It is called intensity CFG and considers variations, at the solar surface, only of the intensity of the event. It is shown here that drops of about two orders of magnitude over distances of ~104 km at the Sun (1° of angular distance) can influence dramatically the SEP profiles at 1 AU. This CFG can lead to either an under or overestimation of both the parallel mean free path and of the injection parameters by factor up to, at least, ~2-3 and 18, respectively. Multi-spacecraft analysis can be used to identify CFG. Three basic requirements are proposed to identify, from the 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)