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N Nitta (Lockheed Martin Solar \& Astrophysics Laboratory)
Data from Yohkoh/SXT have shown that a single loop dominates in the main phase of a flare, no matter how complicated its initial morphology may be. We try to understand the geometry of this central loop. Because of the dynamic nature of the loop, we cannot use a stereoscopic technique that is based on different projections made by solar rotation. Instead, we have to rely on a single image, which gives only minor constraints on the geometry because one cannot determine the intrinsic shape and the inclination angle at the same time. However, we can at least show that the loop as seen in an image is often inconsistent with the commonly assumed geometry, i.e., a symmetric loop standing vertically. Moreover, the motion of the soft X-ray loop top source, and the offset of the hard X-ray loop top source from it, appear to be not consistent with the expectations from 2-d reconnection models. We discuss how soft X-ray spectra from Yohkoh/BCS and radio polarization maps can be used to put further constraints on the loop geometry.
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