Study of magnetic channel structure in active region 10930

The concept of "magnetic channel" was first introduced by Zirin & Wang. They were defined as a series of oppositely directed vertical-field inversions separated by extremely narrow elongated transverse fields. In this paper, we utilized unprecedented filtergraph and spectropolarimetry observations from Hinode, and studied the evolution and physical properties of channel structure of AR 10930 in detail. We found the following: (1) Channels are associated with new flux emergence in the middle of existing penumbra connecting the δ sunspot. (2) The width of each channel is in the order of 1″ or less. (3) The line-of-sight magnetic gradient is highest in the channel, 2.4-4.9 G km^-1. (4) The fields are highly sheared and inclined with a median shear angle around 64° and inclination angle around 25°. (5) Using nonlinear force-free field (NLFF) extrapolation, we derive a near surface current system carrying electric current in the order of 5 × 10 ¹¹ A. (6) The X3.4 flare on 2006 December 13 occurred during the period that the channels rapidly formed, but a few hours before the maximum phase of channel structure development. Based on the observational evidence, we propose that the channels are formed during the emergence of a sequence of magnetic bipoles that are squeezed in the compact penumbra of the δ sunspot and they are highly nonpotential. Formation of channels might be a precursor of major flares.