25 research outputs found

    Flux rope, hyperbolic flux tube, and late EUV phases in a non-eruptive circular-ribbon flare

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    We present a detailed study of a confined circular flare dynamics associated with 3 UV late phases in order to understand more precisely which topological elements are present and how they constrain the dynamics of the flare. We perform a non-linear force free field extrapolation of the confined flare observed with the HMI and AIA instruments onboard SDO. From the 3D magnetic field we compute the squashing factor and we analyse its distribution. Conjointly, we analyse the AIA EUV light curves and images in order to identify the post-flare loops, their temporal and thermal evolution. By combining both analysis we are able to propose a detailed scenario that explains the dynamics of the flare. Our topological analysis shows that in addition to a null-point topology with the fan separatrix, the spine lines and its surrounding Quasi-Separatix Layers halo (typical for a circular flare), a flux rope and its hyperbolic flux tube (HFT) are enclosed below the null. By comparing the magnetic field topology and the EUV post-flare loops we obtain an almost perfect match 1) between the footpoints of the separatrices and the EUV 1600~\AA{} ribbons and 2) between the HFT's field line footpoints and bright spots observed inside the circular ribbons. We showed, for the first time in a confined flare, that magnetic reconnection occured initially at the HFT, below the flux rope. Reconnection at the null point between the flux rope and the overlying field is only initiated in a second phase. In addition, we showed that the EUV late phase observed after the main flare episode are caused by the cooling loops of different length which have all reconnected at the null point during the impulsive phase.Comment: Astronomy & Astrophysics, in pres

    Three-dimensional magnetic reconnection in astrophysical plasmas

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    This research is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB41000000), the National Natural Science Foundations of China (11773039, 11903050, 11790304, 11790300 and 41704169), the National Key R&D Program of China (2019YFA0405000), Key Programs of the Chinese Academy of Sciences (QYZDJ-SSW-SLH050), the Youth Innovation Promotion Association of CAS (2017078) and NAOC Nebula Talents Program. R.G. is supported by the Incoming Post-Docs in Sciences, Technology, Engineering, Materials and Agrobiotechnology (IPD-STEMA) project from Université de Liège.Magnetic reconnection is a fundamental process in laboratory, magnetospheric, solar and astrophysical plasmas, whereby magnetic energy is converted into heat, bulk kinetic energy and fast particle energy. Its nature in two dimensions is much better understood than that in three dimensions, where its character is completely different and has many diverse aspects that are currently being explored. Here, we focus on the magnetohydrodynamics of three-dimensional reconnection in the plasma environment of the Solar System, especially solar flares. The theory of reconnection at null points, separators and quasi-separators is described, together with accounts of numerical simulations and observations of these three types of reconnection. The distinction between separator and quasi-separator reconnection is a theoretical one that is unimportant for the observations of energy release. A new paradigm for solar flares, in which three-dimensional reconnection plays a central role, is proposed.PostprintPeer reviewe

    Coronal magnetometry and energy release in solar flares

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    As the most energetic explosive events in the solar system and a major driver for space weather, solar flares need to be thoroughly understood. However, where and how the free magnetic energy stored in the corona is released to power the solar flares remains not well understood. This lack of understanding is, in part, due to the paucity of coronal magnetic field measurements and the lack of comprehensive understanding of nonthermal particles produced by solar flares. This dissertation focuses on studies that utilize microwave imaging spectroscopy observations made by the Expanded Owens Valley Solar Array (EOVSA) to diagnose the nonthermal electrons and coronal magnetic field in solar flares. In the first study, a partial eruption of a twisted solar filament is observed in Ha and extreme ultraviolet (EUV) wavelengths during an M1.4-class solar flare on September 6, 2017. The microwave counterpart of the filament is observed by EOVSA. The spectral properties of the microwave source are consistent with nonthermal gyrosynchrotron radiation. Using spatially resolved microwave spectral analysis, the magnetic field strength along the filament spine is derived, which ranges from 600-1400 Gauss from its apex to the legs. The results agree well with the non-linear force-free magnetic model extrapolated from the pre-flare photospheric magnetogram. The existence of the microwave counterpart also suggests that the newly reconnected magnetic field lines have the flare-accelerated electrons injected into the filament-hosting magnetic flux rope cavity. The second study focuses on another eruptive solar flare event that features three post-impulsive X-ray and microwave bursts immediately following its main impulsive phase. A tight positive correlation between the flux rope acceleration and electron energization is found during the post-impulsive phase bursts, conforming to the standard flare?coronal-mass-ejection scenario, in which positive feedback between flare reconnection and flux rope acceleration is expected. In contrast, such a correlation does not seem to hold during its main impulsive phase. The lack of flux rope acceleration during the main impulsive phase, as interpreted in this dissertation, is mainly attributed to the tether-cutting reconnection scenario when the flux rope eruption has not been fully underway. In addition, observations suggest a weakening guide field may contribute to the hardening of the nonthermal electron spectrum throughout the main- and post-impulsive phase of the event, shedding new light on understanding the electron acceleration mechanisms in solar flares

    Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections

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    A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation) . Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images. Our key conclusion is that the differentiation of pre-eruptive configurations in terms of SMAs and MFRs seems artificial. Both observations and modeling can be made consistent if the pre-eruptive configuration exists in a hybrid state that is continuously evolving from an SMA to an MFR. Thus, the 'dominant' nature of a given configuration will largely depend on its evolutionary stage (SMA-like early-on, MFR-like near the eruption).Comment: Space Science Reviews, accepted for publicatio

    Dynamics of filaments, flares and coronal mass ejections (CMEs)

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    The objective of this dissertation is to investigate the connection between the dynamics of solar surface phenomena such as filament eruptions, flares, the coronal mass ejections (CMEs), the core of so-called solar activity, and the properties of the associated magnetic field for the development of forecasts of solar activity and space weather. Both statistical and case studies have been carried out. The topics covered in this dissertation are: the statistical relationship among phenomena of solar activity, in particular, filament eruptions, flares and coronal mass ejections (CMEs); the correlation between magnetic reconnection rate and flux rope acceleration of two-ribbon flares; the correlation between magnetic twist of linear-force-free active region and solar eruptions; a case analysis of a quiet-sun flare associated with an erupting filament and a fast CME; a case analysis of the periodic motion along a filament initiated by a subflare; and a case analysis of the evolution of the twist of an eruptive filament. The findings and results confirm some of the theories and conjectures previously proposed and put forth some new insights into the physics of phenomena of solar activity, briefly summarized as follows: (1) a statistical relationship is found among filament eruptions, flares and CMEs; (2) the majority of filament eruptions is found to be associated with new magnetic flux emergence within or adjacent to the eruptive filament; (3) a rapid increase in pitch angle of the twisted structure of an eruptive filament appears to be a trigger of the solar eruption; (4) the hemispheric chirality preferences of quiescent filaments is confirmed; (5) the geoeffectiveness of halo CMEs is found to be associated with the orientation and the chirality of the magnetic fields associated with the eruptions; (6) the temporal correlation between the magnetic reconnection rate and the flare nonthermal emission is verified; (7) the coronal magnetic reconnection is found to be inhomogeneous along the flare ribbons; (8) a positive and strong correlation is found between magnetic reconnection rate and the acceleration of eruptive filaments which represents the early stages of flux rope eruptions in the low corona; and (9) a special type of periodic mass motion in a filaments is reported that remains a challenge to the classical and recent filament models and may provide information on the existence of the filaments

    The evolution of solar sigmoidal active regions

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    Thesis (Ph.D.)--Boston UniversityThe formation, evolution and eruption of solar active regions is a main theme in solar physics. Ultimately the goal is predicting when, where and how an eruption will occur, which will greatly aid space weather forecasting. Special kinds of S-shaped active regions (sigmoids) facilitate this line of research, since they provide conditions that are easier to disentangle and have a high probability for erupting as flares and/or coronal mass ejections (CME). Several theories have been proposed for the formation, evolution, and eruption of solar active regions. Testing these against detailed models of sigmoidal regions can provide insight into the dominant mechanisms and conditions required for eruption. This thesis explores the behavior of solar sigmoids via both observational and magnetic modeling studies. Data from the most modern space-based solar observatories are utilized in addition to state-of-the-art three-dimensional data-driven magnetic field modeling to gain insight into the physical processes controlling the evolution and eruption of solar sigmoids. We use X-ray observations and the magnetic field models to introduce the reader to the underlying magnetic and plasma structure defining these regions. By means of a large comprehensive observational study we investigate the formation and evolution mechanism. Specifically, we show that flux cancellation is a major mechanism for building the underlying magnetic structure associated with sigmoids, namely magnetic flux ropes. We make use of topological analysis to describe the complicated magnetic field structure of the sigmoids. We show that when data-driven models are used in sync with MHD simulations and observations we can arrive at a consistent picture of the scenario for CME onset, namely the positive feedback between reconnection at a generalized X-line and the torus instability. In addition we show that topological analysis is of great use in analyzing the post-eruption flare- and CME-associated observational features. Such analysis is used to extend the standard 2D flare/CME models to 3D and to find potentially large implications of topology to understanding 3D reconnection and the seed populations of energetic particles in CMEs
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