Optimal Energy Growth in Current Sheets 

David MacTaggart Submitted: 20170914 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 nonnormal, 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 normalmode analysis. We determine the transient growth for tearingstable and tearingunstable 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

Publication Status: Accepted for Solar Physics
Last Modified: 20170915 10:13



The prepenumbral magnetic canopy in the solar atmosphere 

David MacTaggart Submitted: 20161019 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

Publication Status: Accepted by ApJL
Last Modified: 20161019 13:08



The emergence of braided magnetic fields 

David MacTaggart Submitted: 20160808 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

Publication Status: accepted by GAFD
Last Modified: 20160810 16:04



The magnetic structure of surges in smallscale emerging flux regions 

David MacTaggart Submitted: 20150205 09:03
Aims. To investigate the relationship between surges and magnetic reconnection during the emergence of smallscale active regions. In particular, to examine how the largescale 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 nonuniform 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 largescale (CMEtype) eruption.
Authors: MacTaggart, D., Guglielmino, S.L., Haynes, A.L., Simitev, R., and Zuccarello, F.
Projects:

Publication Status: Accepted for A&A
Last Modified: 20150228 08:07



The magnetic structure of surges in smallscale emerging flux regions 

David MacTaggart Submitted: 20150205 09:03
Aims. To investigate the relationship between surges and magnetic reconnection during the emergence of smallscale active regions. In particular, to examine how the largescale 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 nonuniform 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 largescale (CMEtype) eruption.
Authors: The magnetic structure of surges in smallscale emerging flux regions MacTaggart, D., Guglielmino, S.L., Haynes, A.L., Simitev, R., and Zuccarello, F.
Projects: None

Publication Status: Accepted for A&A
Last Modified: 20150206 11:50



On the emergence of toroidal flux tubes: general dynamics and comparisons with the cylinder model 

David MacTaggart Submitted: 20090911 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

Publication Status: accepted (A&A)
Last Modified: 20090914 09:37




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