Magnetic Winding as an Indicator of Flare Activity in Solar Active Regions |
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David MacTaggart Submitted: 2022-03-16 03:19
Magnetic helicity is a measure of the entanglement of magnetic field lines used to characterize the complexity of solar active region (AR) magnetic fields. Previous attempts to use helicity-based indicators to predict solar eruptive/flaring events have shown promise but not been universally successful. Here we investigate the use of a quantity associated with the magnetic helicity, the magnetic winding, as a means to predict flaring activity. This quantity represents the fundamental entanglement of magnetic field lines and is independent of the magnetic field strength. We use vector magnetogram data derived from the Helioseismic Magnetic Imager (HMI) to calculate the evolution and distribution of the magnetic winding flux associated with five different ARs, three of them with little flaring activity/nonflaring (AR 11318, AR 12119, AR 12285) and two highly active with X-class flares (AR 11158, AR 12673). We decompose these quantities into "current-carrying" and "potential" parts. It is shown that the ARs that show flaring/eruptive activity have significant contributions to the winding input from the current-carrying part of the field. A significant and rapid input of current-carrying winding is found to be a precursor of flaring/eruptive activity, and, in conjunction with the helicity, sharp inputs of both quantities are found to precede individual flaring events by several hours. This suggests that the emergence/submergence of topologically complex current-carrying field is an important element for the ignition of AR flaring.
Authors: Raphaldini, B., Prior, C., MacTaggart, D.
Projects: None
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Publication Status: Published in Astrophysical Journal
Last Modified: 2022-03-16 13:08
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Direct evidence that twisted flux tube emergence creates solar active regions |
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David MacTaggart Submitted: 2021-11-16 06:37
The magnetic nature of the formation of solar active regions lies at the heart of understanding
solar activity and, in particular, solar eruptions. A widespread model, used in many theoretical
studies, simulations and the interpretation of observations, is that the basic structure of an
active region is created by the emergence of a large tube of pre-twisted magnetic field.
Despite plausible reasons and the availability of various proxies suggesting the accuracy of
this model, there has not yet been a methodology that can clearly and directly identify the
emergence of large pre-twisted magnetic flux tubes. Here, we present a clear signature of the
emergence of pre-twisted magnetic flux tubes by investigating a robust topological quantity,
called magnetic winding, in solar observations. This quantity detects the emerging magnetic
topology despite the significant deformation experienced by the emerging magnetic field.
Magnetic winding complements existing measures, such as magnetic helicity, by providing
distinct information about field line topology, thus allowing for the direct identification of
emerging twisted magnetic flux tubes.
Authors: MacTaggart, D, Prior, C, Raphaldini, B, Romano, P, Guglielmino, SL
Projects: None
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Publication Status: Published in Nature Communications
Last Modified: 2021-11-17 13:12
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Helicity and winding fluxes as indicators of twisted flux emergence |
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David MacTaggart Submitted: 2020-03-10 02:53
Evidence for the emergence of twisted flux tubes into the solar atmosphere has, so far, come from indirect signatures. In this work, we investigate the topological input of twisted flux tube emergence directly by studying helicity and winding fluxes. In magnetohydrodynamic simulations with domains spanning from the top of the convection zone to the lower corona, we simulate the emergence of twisted flux tubes with a range of different initial field strengths. One important feature of this work is the inclusion of a convectively-unstable layer beneath the photosphere. We find approximately self-similar behaviour in the helicity input for the different field strengths considered. As the tubes rise and reach the photosphere, there is a strong input of negative helicity since we consider left-handed twisted tubes. This phase is then followed by a reduction of the negative input and, for low initial field strengths, a net positive helicity input. This phase corresponds to the growing influence of convection on the field and the development of serpentine field structures during emergence. The winding flux can be used to detect when the twisted cores of the tubes reach the photosphere, giving clear information about the input of topologically complex magnetic field into the solar atmosphere. In short, the helicity and winding fluxes can provide much information about how a magnetic field emerges that is not directly available from other sources, such as magnetograms. In evaulating the helicity content of these simualtions we test numerous means for creating synthetic magnetograms, including methods which acount for both the evolving geometry and the finite extent of the photosphere. Whilst the general qualitative behaviours are same in each case, the different forms of averaging do affect the helicity and winding inputs quantitatively.
Authors: MacTaggart, D., Prior, C.
Projects: None
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Publication Status: Accepted for Geophysical and Astrophysical Fluid Dynamics
Last Modified: 2020-03-11 13:14
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The Effect of Anisotropic Viscosity on the Nonlinear Kink Instability |
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David MacTaggart Submitted: 2019-12-04 02:29
The kink instability of magnetohydrodynamics is believed to be fundamental to many aspects of the dynamic activity of the solar atmosphere, such as the initiation of flares and the heating of the solar corona. In this work, we investigate the importance of viscosity on the kink instability. In particular, we focus on two forms of viscosity; isotropic viscosity (independent of the magnetic field) and anisotropic viscosity (with a preferred direction following the magnetic field). Through the detailed analysis of magnetohydrodynamic simulations of the kink instability with both types of viscosity, we show that the form of viscosity has a significant effect on the nonlinear dynamics of the instability. The different viscosities allow for different flow and current structures to develop, thus affecting the behaviour of magnetic relaxation, the formation of secondary instabilities and the Ohmic and viscous heating produced. Our results have important consequences for the interpretation of solar observations of the kink instability.
Authors: Quinn, J., MacTaggart, D., Simitev, R.
Projects: None
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Publication Status: Accepted for Communications in Nonlinear Science and Numerical Simulation
Last Modified: 2019-12-04 11:35
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The plasmoid instability in a confined solar flare |
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David MacTaggart Submitted: 2019-05-07 06:31
Eruptive flares (EFs) are associated with erupting filaments and, in some models, filament eruption drives flare reconnection. Recently, however, observations of a confined flare (CF) have revealed all the hallmarks of an EF (impulsive phase, flare ribbons, etc.) without the filament eruption itself. Therefore, if the filament is not primarily responsible for impulsive flare reconnection, what is? In this Letter, we argue, based on mimimal requirements, that the plasmoid instability is a strong candidate for explaining the impulsive phase in the observed CF. We present magnetohydrodynamic simulation results of the nonlinear development of the plasmoid instability, in a model active region magnetic field geometry, to strengthen our claim. We also discuss how the ideas described in this Letter can be generalised to other situations, including EFs.
Authors: David MacTaggart, Lyndsay Fletcher
Projects: None
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Publication Status: accepted for MNRAS
Last Modified: 2019-05-08 09:51
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Optimal Energy Growth in Current Sheets |
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David MacTaggart Submitted: 2017-09-14 08:21
In this paper, we investigate the possibility of transient growth in the linear perturbation of current sheets. The resistive magnetohydrodynamic (MHD) operator for a background field consisting of a current sheet is non-normal, meaning that associated eigenvalues and eigenmodes can be very sensitive to perturbation. In a linear stability analysis of a tearing current sheet, we show that modes that are damped as t\rightarrow\infty can produce transient energy growth, contributing faster growth rates and higher energy attainment (within a fixed finite time) than the unstable tearing mode found from normal-mode analysis. We determine the transient growth for tearing-stable and tearing-unstable regimes and discuss the consequences of our results for processes in the solar atmosphere, such as flares and coronal heating. Our results have significant potential impact on how fast current sheets can be disrupted. In particular, transient energy growth due to (asymptotically) damped modes may lead to accelerated current sheet thinning and, hence, a faster onset of the plasmoid instability, compared to the rate determined by the tearing mode alone.
Authors: David MacTaggart, Peter Stewart
Projects: None
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Publication Status: Accepted for Solar Physics
Last Modified: 2017-09-15 10:13
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The pre-penumbral magnetic canopy in the solar atmosphere |
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David MacTaggart Submitted: 2016-10-19 02:51
Penumbrae are the manifestation of magnetoconvection in highly inclined (to the vertical direction) magnetic field. The penumbra of a sunspot tends to form, initially, along the arc of the umbra antipodal to the main region of flux emergence. The question of how highly inclined magnetic field can concentrate along the antipodal curves of umbrae, at least initially, remains to be answered. Previous observational studies have suggested the existence of some form of overlying magnetic canopy which acts as the progenitor for penumbrae. We propose that such overlying magnetic canopies are a consequence of how the magnetic field emerges into the atmosphere and are, therefore, part of the emerging region. We show, through simulations of twisted flux tube emergence, that canopies of highly inclined magnetic field form preferentially at the required locations above the photosphere.
Authors: MacTaggart, D., Guglielmino, S.L., Zuccarello, F.
Projects: None
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Publication Status: Accepted by ApJL
Last Modified: 2016-10-19 13:08
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The emergence of braided magnetic fields |
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David MacTaggart Submitted: 2016-08-08 02:57
We study the emergence of braided magnetic fields from the top of the solar interior through to the corona. It is widely believed that emerging regions smaller than active regions are formed in the upper convection zone near the photosphere. Here, bundles of braided, rather than twisted, magnetic field can be formed, which then rise upward to emerge into the atmosphere. To test this theory, we investigate the behaviour of braided magnetic fields as they emerge into the solar atmosphere. We compare and contrast our models to previous studies of twisted flux tube emergence and discuss results that can be tested observationally. Although this is just an initial study, our results suggest that the underlying magnetic field structure of small emerging regions need not be twisted and that braided field, formed in the convection zone, could suffice.
Authors: Prior, C., MacTaggart, D.
Projects: None
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Publication Status: accepted by GAFD
Last Modified: 2016-08-10 16:04
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The magnetic structure of surges in small-scale emerging flux regions |
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David MacTaggart Submitted: 2015-02-05 09:03
Aims. To investigate the relationship between surges and magnetic reconnection during the emergence of small-scale active regions. In particular, to examine how the large-scale geometry of the magnetic field, shaped by different phases of reconnection, guides the flowing of surges.
Methods. We present three flux emergence models. The first model, and the simplest, consists of a region emerging into a horizontal ambient field that is initially parallel to the top of the emerging region. The second model is the same as the first but with an extra smaller emerging region which perturbs the main region. This is added to create a more complex magnetic topology and to test how this complicates the development of surges compared to the first model. The last model has a non-uniform ambient magnetic field to model the effects of emergence near a sunspot field and impose asymmetry on the system through the ambient magnetic field. At each stage, we trace the magnetic topology to identify the locations of reconnection. This allows for field lines to be plotted from different topological regions, highlighting how their geometry affects the development of surges.
Results. In the first model, we identify distinct phases of reconnection. Each phase is associated with a particular geometry for the magnetic field and this determines the paths of the surges. The second model follows a similar pattern to the first but with a more complex magnetic topology and extra eruptions. The third model highlights how an asymmetric ambient field can result in preferred locations for reconnection, subsequently guiding the direction of surges.
Conclusions. Each of the identified phases highlights the close connection between magnetic field geometry, reconnection and the flow of surges. These phases can now be detected observationally and may prove to be key signatures in determining whether or not an emerging region will produce a large-scale (CME-type) eruption.
Authors: MacTaggart, D., Guglielmino, S.L., Haynes, A.L., Simitev, R., and Zuccarello, F.
Projects:
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Publication Status: Accepted for A&A
Last Modified: 2015-02-28 08:07
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The magnetic structure of surges in small-scale emerging flux regions |
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David MacTaggart Submitted: 2015-02-05 09:03
Aims. To investigate the relationship between surges and magnetic reconnection during the emergence of small-scale active regions. In particular, to examine how the large-scale geometry of the magnetic field, shaped by different phases of reconnection, guides the flowing of surges.
Methods. We present three flux emergence models. The first model, and the simplest, consists of a region emerging into a horizontal ambient field that is initially parallel to the top of the emerging region. The second model is the same as the first but with an extra smaller emerging region which perturbs the main region. This is added to create a more complex magnetic topology and to test how this complicates the development of surges compared to the first model. The last model has a non-uniform ambient magnetic field to model the effects of emergence near a sunspot field and impose asymmetry on the system through the ambient magnetic field. At each stage, we trace the magnetic topology to identify the locations of reconnection. This allows for field lines to be plotted from different topological regions, highlighting how their geometry affects the development of surges.
Results. In the first model, we identify distinct phases of reconnection. Each phase is associated with a particular geometry for the magnetic field and this determines the paths of the surges. The second model follows a similar pattern to the first but with a more complex magnetic topology and extra eruptions. The third model highlights how an asymmetric ambient field can result in preferred locations for reconnection, subsequently guiding the direction of surges.
Conclusions. Each of the identified phases highlights the close connection between magnetic field geometry, reconnection and the flow of surges. These phases can now be detected observationally and may prove to be key signatures in determining whether or not an emerging region will produce a large-scale (CME-type) eruption.
Authors: The magnetic structure of surges in small-scale emerging flux regions MacTaggart, D., Guglielmino, S.L., Haynes, A.L., Simitev, R., and Zuccarello, F.
Projects: None
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Publication Status: Accepted for A&A
Last Modified: 2015-02-06 11:50
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On the emergence of toroidal flux tubes: general dynamics and comparisons with the cylinder model |
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David MacTaggart Submitted: 2009-09-11 12:50
In this paper we study the dynamics of toroidal flux tubes emerging from the solar interior, through the photosphere and into the corona. Many previous theoretical studies of flux emergence use a twisted cylindrical tube in the solar interior as the initial condition. Important insights can be gained from this model, however, it does have shortcomings. The axis of the tube never fully emerges as dense plasma becomes trapped in magnetic dips and restrains its ascent. Also, since the entire tube is buoyant, the main photospheric footpoints (sunspots) continually drift apart. These problems make it difficult to produce a convincing sunspot pair. We aim to address these problems by considering a different initial condition, namely a toroidal flux tube. We perform numerical experiments and solve the 3D MHD equations. The dynamics are investigated through a range of initial field strengths and twists. The experiments demonstrate that the emergence of toroidal flux tubes is highly dynamic and exhibits a rich variety of behaviour. In answer to the aims, however, if the initial field strength is strong enough, the axis of the tube can fully emerge. Also, the sunspot pair does not continually drift apart. Instead, its maximum separation is the diameter of the original toroidal tube.
Authors: David MacTaggart, Alan Hood
Projects: None
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Publication Status: accepted (A&A)
Last Modified: 2009-09-14 09:37
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