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 filament proceeded. From a set of full-disk line-of-sight magnetograms taken by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO), we have found a large canceling magnetic feature that was closely associated with the formation of the filament. The positive flux of the magnetic feature was initially 1.5 × 10²¹ Mx and exponentially decreased with an e-folding time of 28 hr throughout the period of observations. We also have determined the transverse velocities of the magnetic flux concentrations in the active region by applying local correlation tracking. As a result, a persistent pattern of shear motion was identified in the neighborhood of the filament. The shear motion had a speed of 0.2-0.5 km s^-1 and fed negative magnetic helicity of -3 × 10⁴² Mx² into the coronal volume during an observing run of 50 hr at an average rate of -6 × 10⁴⁰ Mx² hr^-1. This rate is an order of magnitude higher than the rate of helicity change due to the solar differential rotation. The magnetic flux of the field lines created by magnetic reconnection and the magnetic helicity generated by the photospheric shear motion are much more than enough for the formation of the filament. Based on this result, we conjecture that the filament formation may be the visible manifestation of the creation of a much bigger magnetic structure that may consist of a flux rope and an overlying sheared arcade.