Generation of highly energetic electrons at reconnection outflow shocks during solar flares

摘要

Context. During solar flares a large amount of energy is suddenly released and partly transferred into energetic electrons. They are of special interest since a substantial part of the energy released during a flare is deposited into the energetic electrons. RHESSI observations, e.g. of the 2003 October 28 solar event, show that 1036 electrons with energies >20keV are typically produced per second during large flares. They are related to a power of about 1022W. It is still an open question in which way so many electrons are accelerated up to high energies during a fraction of a second. Aims. Within the framework of the magnetic reconnection scenario, jets appear in the outflow region and can establish standing fast-mode shocks if they penetrate the surrounding plasma with super-Alfvénic speed. It is our aim to show that this shock can be the source of the energetic electrons produced during flares. Methods. The electrons are regarded as energized by shock drift acceleration. The process is necessarily treated in a fully relativistic manner. The resulting distribution function of accelerated electrons is a loss-cone one and it allows to calculate the differential electron flux, which can be compared with RHESSI. Results. The theoretically obtained fluxes of energetic electrons agree with the observed ones as demonstrated for the 2003 October 28 solar event. Key words: acceleration of particles - shock waves - Sun: flares - Sun: X-rays, gamma rays - Sun: radio radiation