A loop-top hard X-ray source in a compact solar flare as evidence for magnetic reconnection

SOLAR flares are thought to be the result of magnetic reconnection — the merging of antiparallel magnetic fields and the consequent release of magnetic energy. Flares are classified into two types¹: compact and two-ribbon. The two-ribbon flares, which appear as slowly-developing, long-lived large loops, are understood theoretically^2-6 as arising from an eruption of a solar prominence that pulls magnetic field lines upward into the corona. As the field lines form an inverted Y-shaped structure and relax, the reconnection of the field lines takes place. This view has been supported by recent observations^7-10. A different mechanism seemed to be required, however, to produce the short-lived, impulsive compact flares. Here we report observations made with the Yohkoh¹¹ Hard X-ray Telescope¹² and Soft X-ray Telescope¹³, which show a compact flare with a geometry similar to that of a two-ribbon flare. We identify the reconnection region as the site of particle acceleration, suggesting that the basic physics of the reconnection process (which remains uncertain) may be common to both types of flare.