Studies On The Evolution Of Solar Filament Eruptions And Their Associated Magnetic Field

Solar filaments are plasma materials that are supported by magnetic field and exist in the corona.Their temperature reaches～10,000 degrees,about 1%of the temperature of the corona,while they are two orders of magnitude denser than the coronal density.On the solar disk,filament materials absorb the background emission and appear as dark strips.At the limb they show an extrusive bright structure,and are called prominences.Previous studies are dedicated to physical properties,magnetic structure,and eruption process of single filaments.Recently,high-resolution observations from the space-and ground-based instruments reveal that filaments exist in groups,and there are transfers or exchanges of mass and/or magnetic flux between them.Multi-filament systems are gathering concern,among which the simplest is a two-filament system.A special case is that the two filaments in the system lie over the same polarity inversion line but differ in height,which is termed a "double-decker structure".Such systems may consist of two flux ropes,or a flux rope atop a magnetic arcade.Recent observations show that the two filaments can erupt together or successively.In more cases,the upper branch erupts while the lower one is left behind.Combining the multi-wavelength observations from SDO/AIA,Ha data from GONG,and magnetic field data from SDO/HMI,we carried out a detailed study on the eruption event(SOL2014-07-05)of the C-shaped double-decker filament system occurred on July 5,2014,which is presented in Chapter 2.We found that the upper branch erupted but failed,and the lower branch showed no significant changes.In order to find out the mechanisms of trigger and confinement,we analyzed the observation from 4 hours before the eruption to the end of the whole process.We found that before the eruption onset,intermittent bright ejections from the northwestern footpoint of the lower branch injected to the system,one of which disturbed the upper branch and made it oscillate longitudinally.The bright ejections that were closely related to photospheric magnetic flux cancellation were able to add flux to the system.The tether-cutting reconnection at the HFT between the two filaments led to the precursor flare of GOES class C1.3,and then the upper filament erupted,associated with a C2.5 quasi-circular flare.We simulated the magnetic field of the eruption region using the flux rope insertion method,and found that the overlying field was a fan-spine structure without a null point and the spine was indeed replaced by a flat current sheet.The poloidal flux of the inserted flux bundles was zero at the beginning(i.e.,no twist),and increased during the magnetic-frictional relaxation,thus accumulated twist.The upper branch erupted when it reached the threshold height of the torus instability.At its maximum height,the upper branch had become a flux rope and reconnected with the magnetic fields at the fan QSL,causing the release of twist and the erosion of the flux rope,and the eruption failed.Therefore,the whole process would be a failed partial eruption of a double-decker filament system with a magnetic structure consisting of a flux rope atop a sheared arcade.We reproduced the magnetic structure before,during,and after the eruption through magnetic field modeling,which was well consistent with the observed filaments.Our research shows for the first time that the magnetic-frictional relaxation is able to simulate a confined eruption of double-decker filaments qualitatively,and reproduce the evolution of the twist.In Chapter 3,we studied another two-filament eruption event(SOL2014-03-29)with similar magnetic field layout as the event SOL2014-07-05.The eruption occurred on March 29,2014.The eastern filament erupted and was associated with a C2.4 flare and a CME,thus this event was an ejective eruption.The filament showed a highly twisted structure during its eruption,and presented a short-lived kinking or writhing structure on the top of the filament,and then the filament gradually untwisted.The kinking or writhing structure might be resulted from a pair of oppositely directed Lorentz forces that were exerted on the two legs of the magnetic flux rope from a sheared magnetic field component over the flux rope.The observations showed that before the eruption the unerupted filament was also more active,and increased the magnetic flux and instability of the erupted one through interactions between them.The filament began its fast rise after it rose up to the threshold height of the torus instability,in the meantime,the flare set off.The onset site of the flare was under the middle of the filament,and was relevant to the photospheric magnetic cancellations before the eruption.The topology analyses of the potential field model showed that the erupted filament lay near the separator between two arch-shaped QSL.The best-fit non-potential model built by the flux insertion method revealed that the twist number of the corresponding flux rope of the erupted filament reached above 1.75,and gradually weakened during the eruption,which was in consistent with the observation.Compared this ejective event with the confined eruption SOL2014-07-05,the main reasons that the two events had similar magnetic field layouts but different endings might be as following:On the one hand,the confined eruption of the flux rope occurred under the dome of the fan-spine structure,and released twist by reconnecting with the dome fieldlines during its rise,and suppressed the eruption;While in the ejective eruption,the filament located near the separator between the two arch-shaped QSL.On the other hand,the occurrence sites of the flare reconnections were different in those two events.The flare reconnection of the event SOL2014-07-05 occurred near the northern footpoint of the unerupted filament;While for the ejective event SOL2014-03-29,the flare reconnection of the erupted filament began under its middle part,which was benefitial to an ejective eruption.In Chapter 4,we investigated a transversal coronal loop oscillation event which occurred on December 7,2012.It was resulted from the non-radial eruption of a prominence-carrying flux rope.With the data offered by BBSO,SDO,SOHO/LASCO,STEREO,and Fermi,we found from the analyses that the non-radial eruption of the prominence and/or the flux rope escaped the constraint and erupted due to the weaker magnetic field strength along its trajectory.In the meantime,the eruption produced a region with lower magnetic pressure,led to a larger magnetic pressure gradient,which triggered the transversal oscillations of the coronal loops.We also applied the coronal seismology and derived the internal Alfvén velocity and the magnetic field strength of the oscillating coronal loops.