Twisted Flux Rope Model for a Flare-productive Sunspot Group NOAA 10486

What is the common magnetic field configuration among flare-productive active regions? In our previous studies, we have found that the magnetic neutral line shows a rotational motion in a δ-type active region NOAA 9026, where three X-class flares have occurred successively. During the current solar maximum (cycle 23), we studied the evolution of all active regions that have produced at least one X-class flare and have been observed by SOHO/MDI magnetograms. Active region NOAA 10486 is the most flare-productive region during this solar cycle, which have produced seven X-class flares including X28 flare on Nov. 4, 2003. We studied the formation process of δ-type magnetic configuration using SOHO/MDI magnetograms. We also studied the evolution of magnetic shear and Hα filaments using vector magnetograms obtained at Huairou Solar Observing Station and Hα images obtained with the Sartorius telescope at Kwasan Observatory, Kyoto University. In this active region, we found that a development of a strong magnetic shear with a new magnetic flux emergence along a magnetic neutral line plays an important role as the trigger of the X-class flare on Oct. 28. Based on these observational characteristics, we discuss the model of emerging bundles of a magnetic flux rope and its causal relation to the trigger of strong flares.