Near-relativistic electron events. Monte Carlo simulations of solar injection and interplanetary transport

Abstract

We have developed a Monte Carlo model to simulate the transport of solar near-relativistic (NR; 30-300 keV) electrons along the interplanetary magnetic field (IMF), including adiabatic focusing, pitch-angle dependent scattering, and solar wind effects. By taking into account the angular response of the LEFS60 telescope of the EPAM experiment on board the "Advanced Composition Explorer" spacecraft, we have been able to transform simulated pitch-angle distributions into sectored intensities measured by the telescope. We have developed an algorithm that allows us, for the first time, to infer the best-fit transport conditions and the underlying solar injection profile of NR electrons from the deconvolution of the effects of interplanetary transport on observational sectored intensities. We have studied seven NR electron events observed by the LEFS60 telescope between 1998 and 2004 with the aim of estimating the roles that solar flares and CME-driven shocks play in the acceleration and injection of NR electrons, as well as the conditions of the electron transport along the IMF.<br/>In this set of seven NR electron events, we have identified two types of injection episodes in the derived injection profiles: short (< 20 min) and time-extended (> 1 h). The injection profile of three events shows both components; an initial injection episode of short duration, followed by a second much longer lasting episode; two events only show a time-extended injection episode; while the others show an injection profile composed by several short injection episodes. <br/>We have found that the timing of the prompt short injection episodes agrees with the timing of the hard X-rays and radio type III bursts. On the other hand, time-extended injection episodes seem to be related to intermittent radio emissions at the height of the CME leading edge or below, and sometimes to type II radio bursts. Thus, we conclude that short injection episodes are preferentially associated with the injection of flare-accelerated particles, while longer lasting episodes are provided by CME-driven shocks or post-eruptive reconnection phenomena at coronal heights lower than those of the CME-driven shocks.<br/>From the fit of the events, we have derived the transport conditions of the electrons. We have found that the electron propagation was almost scatter-free (the radial mean free path of the electrons was ~0.9 AU) during two of the events, whereas during five of the events the propagation occurred under strong scattering conditions (the radial mean free path of the electrons was smaller than 0.2 AU). Those events showing a long radial mean free path reached the maximum intensity shortly (< 15 min) after the onset of the event; whereas those events showing a small radial mean free path reached the maximum intensity more than one hour after the onset. <br/>The overall conclusion from this study is that there is a continuous spectrum of scenarios that allow for either flare or CME-driven shock NR electron injection, or for both, and that this can occur both under strong scattering and under almost "scatter-free" propagation conditions.<br/><br/>SUBJECT HEADINGS: Sun: coronal mass ejections (CMEs) Sun: flares Sun: particle emission

<I>Hemos desarrollado un modelo Monte Carlo para simular el transporte de electrones solares casi-relativistas (30-300 keV) en el medio interplanetario que tiene en cuenta los efectos de la focalización adiabática, la dispersión en ángulo de batida y los efectos del viento solar. Teniendo en cuenta la respuesta angular del telescopio, hemos desarrollado un método que permite transformar las distribuciones angulares de partículas simuladas en intensidades sectoritzadas observadas por el telescopio LEFS60 a bordo de la sonda interplanetaria ACE. Esto nos ha permitido desarrollar un algoritmo que permite, por primera vez, deconvolucionar los efectos del transporte interplanetario en las intensidades sectoritzadas observadas, con el objetivo de determinar el perfil de inyección solar de electrones y las características del transporte. Hemos aplicado el modelo al estudio de siete sucesos de electrones observados por la sonda ACE. Los resultados ponen de manifiesto que la inyección de electrones casi-relativistas está asociada con procesos fulgurativos, choques conducidos por eyecciones de masa coronal o con ambos, y que el transporte se puede producir tanto en condiciones muy dispersivas como en condiciones muy poco dispersivas. </I>

RESUM:<br/><br/>Hem desenvolupat un model Monte Carlo per simular el transport d'electrons solars quasi-relativistes (30-300 keV) en el medi interplanetari que té en compte els efectes de la focalització adiabàtica, la dispersió en angle de batuda i els efectes del vent solar. Hem desenvolupat un mètode per transformar les distribucions angulars de partícules simulades en intensitats sectoritzades observades pel telescopi LEFS60 a bord de la sonda interplanetària ACE, tenint en compte la resposta angular del telescopi. Això ens ha permès desenvolupar un algoritme que permet, per primera vegada, deconvolucionar els efectes del transport interplanetari en les intensitats sectoritzades observades, amb l'objectiu de determinar el perfil d'injecció solar d'electrons observats per la sonda ACE. Els resultats posen de manifest que la injecció d'electrons quasi-relativistes pot produir-se en processos fulguratius, en xocs conduïts per ejeccions de massa coronal o en ambdós, i que el transport es pot produir tant en condicions molt dispersives com en condicions molt poc dispersives.