Magnetic changes observed in the formation of two filaments in a complex active region: TRACE and MSDP observations

Abstract

This paper is focused on the formation of two filaments in a complex center of decaying active regions (AR 8329 and AR 8326), located in the northern hemisphere. The observations were obtained in Hα by the Multi-channel Subtractive Double Pass spectrograph (MSDP mounted on the German telescope VTT in Tenerife) and EUV lines with TRACE (Transition Region And Corona Explorer). High Doppler shifts are found to be related to the ends of filament segments where canceling magnetic fields are also located (as seen on magnetograms from Big Bear Solar Observatory). At these locations, velocities along the line of sight, derived by using a cloud model method reach −20 km s −1 , the segments of filaments merge and frequently a time-related sub-flare is observed by TRACE. The chirality of the filament segments has been determined by different methods: the segments of dextral chirality join together and form a long dextral filament, and a single filament of sinistral chirality forms end to end with the dextral filament but does not merge with it. Assuming a model of twisted flux tube for filament material, we suggest that the dextral filament has negative helicity and a relationship between its formation and the close by sunspot with the same sign of helicity.

Physics of Solar Prominences: II—Magnetic Structure and Dynamics

“…Furthermore, the models predict that filament channels with dextral orientation of the axial field have left-helical flux ropes and left-skewed coronal arcades, and sinistral channels have right-helical flux ropes and right-skewed arcades. This is consistent with the observation that dextral (sinistral) channels have left (right)-skewed arcades (Martin and McAllister, 1996;McAllister et al, 2002;Schmieder et al, 2004).…”

“…This multi-step merging mechanism couples photospheric shear, coronal reconnection, and relaxation to form a longer structure containing numerous dipped field lines where plasma can collect most easily . Furthermore, the case of identical chiralities and aligned axial fields successfully reproduces the observations of filament merging (e.g., Rust, 2001;Schmieder et al, 2004;van Ballegooijen, 2004). A second model configuration (Fig.…”

“…The loops can be right-bearing or left-bearing with respect to the direction along the PIL (as seen from above). The observations indicate that dextral (sinistral) channels have left (right)-skewed coronal arcades, opposite to the right (left)-bearing structure of the filament barbs (Martin and McAllister, 1996;Martin, 1998;McAllister et al, 2002;Schmieder et al, 2004). Taking into account the polarity of the dominant flux on either side of the filament, this implies that the axial component of magnetic field in the arcade is the same as that in the filament channel.…”

“…In each case channels and quiescent filaments form on the boundaries of these activity nests. Schmieder et al (2004) studied the formation of a filament in the complex center of a decaying active region formed out of smaller individual components, using multi-wavelength observations obtained during a "Joint Observing Programme" between ground-based instruments in the Canary Islands (the SVST and the MSDP on the VTT) and the TRACE satellite. They followed the evolution of three individual filament segments denoted F1, F2 and F3 over several days, and found that F1 and F2 gently merged into a single structure, as observed by a gradual filling in Hα of the gap between them.…”

“…Two days later segments F2 and F3 came into contact and produced a confined flare, as evidenced by EUV post-flare loops (Deng et al, 2002). To determine the directions of the axial fields in the three filament segments, Schmieder et al (2004) used the chirality rules for chromospheric fibrils and magnetic field polarity, the skew of the overlying coronal arcades, and the sense of twist in neighboring sunspots. It was then confirmed that when two filaments interact, magnetic reconnection takes place and leads to a merging when their chiralities are of the same sign, but leads to a flare when the chiralities are opposite (also see Malherbe, 1989;Martin, 1998;Rust, 2001;van Ballegooijen, 2004;DeVore et al, 2005).…”