| Solar filaments are fascinating and unique magnetic structures in our solar system. The intricate structure, dynamics and plasma parameters of filaments are closely associated with the magnetic fields on the Sun. The filament formation involves the reconfiguration and then conversion of the magnetic fields on the Sun to filament magnetic fields. However, how these fields are reconfigured and converted to filament magnetic fields are not fully understood, and the detailed information on the magnetic structures of filaments is still unknown. The study of the filament formation could help not only in understanding the reconfiguration and then conversion of the magnetic fields on the Sun to filament magnetic fields,but also in understanding the supporting and the detailed magnetic structures of filaments. In addition, as good tracers of magnetic flux ropes in the corona,the study of the filament formation could shed new light on the formation and evolution of magnetic flux ropes. In this thesis, by means of excellent observations from SDO, SMART, and GONG, three filaments, which involve their complete formation process, are studied in detail. The main results are included as below:By analyzing the excellent observations derived from the SDO on 2012 August 5-6, we present the first observation of the formation and eruption of a small circular filament driven by a rotating network magnetic field(RNF) in the quiet Sun. In the negative footpoint region of an inverse J-shaped dextral filament,the RNF was formed by the convergence to supergranular junctions of several magnetic flux patches of the same polarity, and it then rotated counterclockwise(CCW) for approximately 11 hr and showed up as a CCW rotating EUV cyclone, during which time the filament gradually evolved into a circular filament that surrounded the cyclone. When the calculated convergence and vortex flows appeared around the RNF during its formation and rotation phases, the injected magnetic helicity calculation also showed negative helicity accumulation during the RNF rotation that was consistent with the dextral chirality of the filament.Finally, the RNF rotation stopped and the cyclone disappeared, and, probably due to an emerging bipole and its forced cancellation with the RNF, the closure filament underwent an eruption along its axis in the(clockwise) direction opposite to the rotation directions of the RNF and cyclone. These observations suggest that the RNFs might play an important role in the formation of nearby small-scale circular filaments as they transport and inject magnetic energy and helicity, and the formation of the EUV cyclones may be a further manifestation of the helicity injected into the corona by the rotation of the RNFs in the photosphere. In addition, the new emerging bipole observed before the filament eruption might be responsible for destabilizing the system and triggering the magnetic reconnection which proves useful for the filament eruption.From the observations of the formation of filament occurred on 2013 May 29,we present rare observations that a filament is formed rapidly within 20 minutes by magnetic reconnection between two sets of dark threadlike structures. The two sets of dark threadlike structures belong to distinct flux systems with their adjacent ends anchored in an opposite-polarity magnetic field region, where the calculated photospheric velocity field shows that converging flows dominate there.Due to the converging flows, opposite-polarity magnetic flux converged and then canceled, leading to the formation of extreme ultraviolet(EUV) brightening that spread in opposite directions along the spine of the dark threadlike structures.Meanwhile, very weak remote brightening in the other terminals of the dark threadlike structures, as well as EUV loops, which rooted in the opposite-polarity magnetic field region, appeared. In addition, all of the AIA Fe line observations reveal that a flux rope was formed and underwent a rolling motion during the fade away of the EUV brightening. Soon after, as the EUV brightening disappeared,a filament that is very likely composed of two sets of intertwined dark threadlike structures was formed. Via differential emission measure(EM) analysis, it is found that both the EM and temperature of the plasma around the flux-canceling site increased during the brightening, implying that there, magnetic reconnection may occur to heat the plasma. These observations provide evidence that the filament is formed by magnetic reconnection associated with flux convergence and cancellation, and the magnetic structure of the filament is most likely a flux rope.In addition, through detailed analysis of the growth of a filament that occurred on 2013 February 9, we present detailed observations that the growth of the filament caused by magnetic interaction among an active region filament,a superpenumbral filament, and a set of dark threadlike structure occurring at the periphery of AR 11669. Multistep reconnections are identified during the whole growing process. Magnetic flux convergence and cancellation occurring at the positive footpoint region of the filament is the first step reconnection, which resulted in the filament bifurcated into two sets of intertwined threads. One set of them anchored in situ, while the other set moved toward and interacted with SF and part of T. This indicates the second step reconnection, which gave rise to the disappearance of SF and the formation of a long threadlike structure that connecting the far ends of the filament and T. The long threadlike structure further interacted with T and then separated into two parts representing the third step reconnection. Finally, another similar long threadlike structure, which intertwining with the fixed filament threads, appeared. Hαobservations show that this twisted structure is a longer sinistral filament. Based on the observed photospheric vector magnetograms, we performed a nonlinear force-free field extrapolation to reconstruct the magnetic fields above the photosphere and found that the coronal magnetic field lines associated with the filament consists of two twisted flux ropes winding around each other. These results suggest that magnetic interactions among filaments and its adjacent superpenumbral filaments and dark threadlike structures could lead to the growth of the filaments, and the filament is probably supported in a flux rope. |
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