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¯ux and anisotropy time pro®les of solar energetic particle events, called SEP pro®les. 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 ®eld (IMF) with the radial distance to the Sun) and (3) the ®nite injection pro®le at the source. If their features change from one ®eld line to another, i.e. if there is a cross IMF gradient (CFG), then the shape of the SEP pro®les will depend, at onset time, on the relative position of the spacecraft to the IMF and might vary signi®cantly on small distance scale (e.g. 10 6 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 $10 4 km at the Sun (1 of angular distance) can in¯uence dramatically the SEP pro®les 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.