Solar Physics Division Meeting 2000, June 19-22
Session 1. Helioseismology, Magnetic Fields, Chromosphere and Transition Region
Display, Chair: C. U. Keller, Monday-Thursday, June 19, 2000, 8:00am-6:00pm, Forum Ballroom

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[1.52] On Formation of Sigmoidal Structure in Active Region AR8100

Y. Liu (HEPL,Stanford Univ.), X.P. Zhao (HEPL,Stanford Univ.), J.T. Hoeksema (HEPL,Stanford Univ.), J. Wang (Beijing Astro. Obs.), Y. Yan (Beijing Astro. Obs.)

Using Soft X-ray images taken by the Soft X-ray telescope aboard Yohkoh, line-of-sight magnetograms taken by SOHO/MDI and the vector magnetograms taken at Beijing Astronomical Observatory, we have studied the formation of the sigmoidal structure in active region NOAA8100 in November 1997. The sigmoidal structure appeared after a C-class flare generated by interaction of emerging magnetic flux and the pre-existing magnetic field. This implies reconnection may cause this sigmoid structure.

Theoretical study shows that if the twist of a magnetic rope is high enough, the rope will be unstable and it will kink, resulting in a sigmoid structure (See Rust, 1998 and references therein). We suggest that reconnection increases the twist of magnetic flux tubes by transferring the mutual helicity to self helicity. The twist is roughly estimated from the calculated non-linear force lines and the vertical electric current derived from the observed vector magnetograms. A bundle of force lines that match the sigmoid structure very well twist -2.62\pi, while the newly emerged magnetic flux possesses just -1.65\pi twist, smaller than the critical twist for causing kink instability. The free energy in the sigmoidal magnetic ropes is 1.61 times that of the newly emerged magnetic flux. In this case energy can be transferred from one magnetic system to another via reconnection. This result provides a mechanism to energize and destabilize the large-scale magnetic loops by means of interaction and reconnection of small-scale magnetic field and large-scale magnetic field. Changes in large-scale loops may be associated with large-scale eruptions such as coronal mass ejections.

We conclude that (1) the emerging magnetic flux itself may not possess enough twist to induce kink instability, and (2) reconnection can build up free energy and set up instability.

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