A hard X-ray sigmoidal structure during the initial phase of the 2003 October 29 X10 flare

We find a hard X-ray (HXR) sigmoidal (S-shaped) structure observed by RHESSI between 6 and 150 keV during the initial phase of the X10 flare of 2003 October 29. Its counterparts are seen with the Solar X-Ray Imager and TRACE. The flare evolves from a sigmoid to an arcade phase as observed in TRACE 195 Å images. According to the spatial structure of HXR emission, the flare process can be divided into two different phases. During the first phase, HXR emission in different energy ranges shares a similar sigmoidal evolving structure. The structure appears to contract initially as shown by the time profile of the separation between the two footpoints (FPs) at the ends of the HXR sigmoid. During the second phase, HXR emission in the lower energy range (≲ 30 keV) evolves into two sources located along the neutral line. Meanwhile, the FPs in the higher energies (≳ 30 keV) move apart as usual. During the whole flaring process, the value of the flare shear, defined as the angle between the line connecting two FPs and the line perpendicular to the neutral line, decreases steadily. We conclude that the flare was triggered by magnetic reconnection near the center of the sigmoid. The converging and unshearing motion during the sigmoid period is explained by continuing magnetic reconnection driven by the erupting sigmoidal flux rope. The reconnection progresses from highly sheared magnetic field lines to less sheared field lines surrounding the flux rope.