Interaction between a fast rotating sunspot and ephemeral regions as the origin of the major solar event on 2006 December 13

The major solar event on 2006 December 13 is characterized by the approximately simultaneous occurrence of a heap of hot ejecta, a great two- ribbon flare, and an extended Earth- directed coronal mass ejection. We examine the magnetic field and sunspot evolution in NOAA AR 10930, the source region of the event, while it transited the solar disk center from December 10 to 13. We find that the obvious changes in the active region associated with the event are the development of magnetic shear, the appearance of ephemeral regions, and fast rotation of a smaller sunspot. Around the area of the magnetic neutral line of the active region, interaction between the fast rotating sunspot and the ephemeral regions triggers continual brightening and finally the major flare. This indicates that only after the sunspot rotates up to 200 degrees does the major event take place. The sunspot rotates at least 240 degrees about its center, the largest sunspot rotation angle that has been reported.