Nature 371, 495 - 497 (06 October 1994); doi:10.1038/371495a0

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

S. Masuda^*†, T. Kosugi^*, H. Hara^*, S. Tsuneta^‡ & Y. Ogawara^§

^* National Astronomical Observatory, Mitaka, Tokyo 181, Japan
^‡ Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181, Japan
^§ Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229, Japan
^† Present address: Solar Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Aichi 442, Japan.

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.

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