The formation of a prominence in active region NOAA 8668. I. SOHO/MDI observations of magnetic field evolution

We have studied the evolution of the photospheric magnetic field in active region NOAA 8668 for 3 days while the formation of a reverse S-shaped filam ent proceeded. From a set of full-disk line-of-sight magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Obse rvatory (SOHO), we have found a large canceling magnetic feature that was c losely associated with the formation of the filament. The positive flux of the magnetic feature was initially 1.5x10(21) Mx and exponentially decrease d with an e-folding time of 28 hr throughout the period of observations. We also have determined the transverse velocities of the magnetic flux concen trations in the active region by applying local correlation tracking. As a result, a persistent pattern of shear motion was identified in the neighbor hood of the filament. The shear motion had a speed of 0.2-0.5 km s(-1) and fed negative magnetic helicity of -3x10(42) Mx(2) into the coronal volume d uring an observing run of 50 hr at an average rate of -6x10(40) Mx(2) hr(-1 ). This rate is an order of magnitude higher than the rate of helicity chan ge due to the solar differential rotation. The magnetic flux of the field l ines created by magnetic reconnection and the magnetic helicity generated b y the photospheric shear motion are much more than enough for the formation of the filament. Based on this result, we conjecture that the filament for mation may be the visible manifestation of the creation of a much bigger ma gnetic structure that may consist of a flux rope and an overlying sheared a rcade.