1,367 research outputs found

    Generalised models for torsional spine and fan magnetic reconnection

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    Three-dimensional null points are present in abundance in the solar corona, and the same is likely to be true in other astrophysical environments. Recent studies suggest that reconnection at such 3D nulls may play an important role in the coronal dynamics. In this paper the properties of the torsional spine and torsional fan modes of magnetic reconnection at 3D nulls are investigated. New analytical models are developed, which for the first time include a current layer that is spatially localised around the null, extending along either the spine or the fan of the null. These are complemented with numerical simulations. The principal aim is to investigate the effect of varying the degree of asymmetry of the null point magnetic field on the resulting reconnection process - where previous studies always considered a non-generic radially symmetric null. The geometry of the current layers within which torsional spine and torsional fan reconnection occur is found to be strongly dependent on the symmetry of the magnetic field. Torsional spine reconnection still occurs in a narrow tube around the spine, but with elliptical cross-section when the fan eigenvalues are different, and with the short axis of the ellipse being along the strong field direction. The spatiotemporal peak current, and the peak reconnection rate attained, are found not to depend strongly on the degree of asymmetry. For torsional fan reconnection, the reconnection occurs in a planar disk in the fan surface, which is again elliptical when the symmetry of the magnetic field is broken. The short axis of the ellipse is along the weak field direction, with the current being peaked in these weak field regions. The peak current and peak reconnection rate in this case are clearly dependent on the asymmetry, with the peak current increasing but the reconnection rate decreasing as the degree of asymmetry is increased

    On the formation of current sheets in response to the compression or expansion of a potential magnetic field

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    The compression or expansion of a magnetic field that is initially potential is considered. It was recently suggested by Janse & Low [2009, ApJ, 690, 1089] that, following the volumetric deformation, the relevant lowest energy state for the magnetic field is another potential magnetic field that in general contains tangential discontinuities (current sheets). Here we examine this scenario directly using a numerical relaxation method that exactly preserves the topology of the magnetic field. It is found that of the magnetic fields discussed by Janse & Low, only those containing magnetic null points develop current singularities during an ideal relaxation, while the magnetic fields without null points relax toward smooth force-free equilibria with finite non-zero current.Comment: Accepted for publication in Ap

    The structure of current layers and degree of field line braiding in coronal loops

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    One proposed resolution to the long-standing problem of solar coronal heating involves the buildup of magnetic energy in the corona due to turbulent motions at the photosphere that braid the coronal field, and the subsequent release of this energy via magnetic reconnection. In this paper the ideal relaxation of braided magnetic fields modelling solar coronal loops is followed. A sequence of loops with increasing braid complexity is considered, with the aim of understanding how this complexity influences the development of small scales in the magnetic field, and thus the energy available for heating. It is demonstrated that the ideally accessible force-free equilibrium for these braided fields contains current layers of finite thickness. It is further shown that for any such braided field, if a force-free equilibrium exists then it should contain current layers whose thickness is determined by length scales in the field line mapping. The thickness and intensity of the current layers follow scaling laws, and this allows us to extrapolate beyond the numerically accessible parameter regime and to place an upper bound on the braid complexity possible at coronal plasma parameters. At this threshold level the braided loop contains 102610^{26}--1028ergs10^{28}{\rm ergs} of available free magnetic energy, more than sufficient for a large nanoflare.Comment: To appear in ApJ. 20 pages, 10 figure

    Current sheet formation and nonideal behavior at three-dimensional magnetic null points

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    The nature of the evolution of the magnetic field, and of current sheet formation, at three-dimensional (3D) magnetic null points is investigated. A kinematic example is presented which demonstrates that for certain evolutions of a 3D null (specifically those for which the ratios of the null point eigenvalues are time-dependent) there is no possible choice of boundary conditions which renders the evolution of the field at the null ideal. Resistive MHD simulations are described which demonstrate that such evolutions are generic. A 3D null is subjected to boundary driving by shearing motions, and it is shown that a current sheet localised at the null is formed. The qualitative and quantitative properties of the current sheet are discussed. Accompanying the sheet development is the growth of a localised parallel electric field, one of the signatures of magnetic reconnection. Finally, the relevance of the results to a recent theory of turbulent reconnection is discussed.Comment: to appear in Phys. Plasmas. A version with higher quality figures can be found at http://www.maths.dundee.ac.uk/~dpontin/ In this replacement version, typos have been corrected, and in addition references and some further discussion adde

    On the nature of reconnection at a solar coronal null point above a separatrix dome

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    Three-dimensional magnetic null points are ubiquitous in the solar corona, and in any generic mixed-polarity magnetic field. We consider magnetic reconnection at an isolated coronal null point, whose fan field lines form a dome structure. We demonstrate using analytical and computational models several features of spine-fan reconnection at such a null, including the fact that substantial magnetic flux transfer from one region of field line connectivity to another can occur. The flux transfer occurs across the current sheet that forms around the null point during spine-fan reconnection, and there is no separator present. Also, flipping of magnetic field lines takes place in a manner similar to that observed in quasi-separatrix layer or slip-running reconnection.Comment: Accepted for publication in the Astrophysical Journa

    Psychological Processes Mediate the Impact of Familial Risk, Social Circumstances and Life Events on Mental Health

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    <p>Results of a structural equation model testing the mediating effects of the psychological processes of response style and self-blame on the contribution of familial mental health history, relationship status, income and education, social inclusion and life events on mental health problems and well-being, with S-B χ<sup>2</sup> (3,199, N = 27,397) = 126,654·8, p<·001; RCFI = ·97; RMSEA = ·04 (·038–·039). The path diagram shows completely standardised robust parameter estimates which represent the relative contribution of each latent factor to the model. All coefficients are statistically significant, p<·001. Latent factors are represented by ovals. The double headed arrow between mental health problems and well-being represents the correlations between these latent constructs.</p

    "Neither One Nor Two": Aligning De Beauvoir and Irigaray's Use of Ambiguity

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    Despite their differences, Simone de Beauvoir’s existentialist critique of the gender binary and Luce Irigaray’s model of sexuate difference run parallel in their resistance to absolute, gendered notions of activity and passivity. Both streams of thought—while ontologically distinct—are interested in presenting "relative" alternatives to the negated feminine subject. With attention to De Beauvoir’s vision of ambiguity, this paper will argue that Irigaray’s “vulvic” and “phallic” subjects are compatible with existentialism’s beings “for-itself” and “in-itself." Further, by embracing the potential for both sexes hold “finitude and transcendence,” Irigaray’s strategic essentialism may be read as radically continuous with (as opposed to departing from) The Second Sex

    Super-resolution imaging of a low frequency levitated oscillator

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    We describe the measurement of the secular motion of a levitated nanoparticle in a Paul trap with a CMOS camera. This simple method enables us to reach signal-to-noise ratios as good as 106^{6} with a displacement sensitivity better than 1016m2^{-16}\,m^{2}/Hz. This method can be used to extract trap parameters as well as the properties of the levitated particles. We demonstrate continuous monitoring of the particle dynamics on timescales of the order of weeks. We show that by using the improvement given by super-resolution imaging, a significant reduction in the noise floor can be attained, with an increase in the bandwidth of the force sensitivity. This approach represents a competitive alternative to standard optical detection for a range of low frequency oscillators where low optical powers are require

    Political Economy and the roots of Rawls’ original position

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    O impacto e a relevância da teoria econômica para o desenvolvimento de A Theory of Justice são, frequentemente, deixados de lado na imensa literatura a respeito das obras de John Rawls. Ainda assim, entender os elementos de economia política na obra de Rawls é fundamental para a compreensão dos motivos pelos quais ele abandonará uma abordagem utilitarista para a filosofia política e, consequentemente , par a noss o entendimento das principais questões que dão origem à noção de justiça como equidade. Este artigo tem dois objetivos principais: primeiramente, descrever a influência de John Stuart Mill, Kenneth Arrow e Vilfredo Pareto na virada metodológica encontrada na ideia de posição original; segundo, descrever os elementos históricos pressupostos para a compreensibilidade dos ideais rawlsianos. Concluirei com algumas observações que sugerem um complemento historicista e fenomenológico para as descrições ideais encontradas no liberalismo político tal como é entendido por Rawls.The impact and relevance of economic theory to the development of “A Theory of Justice” are often left aside in the immense literature regarding the works of John Rawls. And yet, understanding the elements of political economy in Rawls’ work is fundamental for the comprehension of why Rawls’ will abandons a utilitarian approach to political philosophy and, consequentially, for our understanding of the main issues that motivate the notion of Justice as Fairness. This article has two mains goals: First, to describe the influence of John Stuart Mill, Kenneth Arrow and Vilfredo Pareto in the methodological turn found in the idea of the Original Position; and second, to describe the historical elements that are presupposed for the conceivability of Rawls’ ideals. I conclude the present paper with some remarks suggesting a historicist and phenomenological complement to the ideal descriptions found in political liberalism as understood by Rawls

    Environmental law-making public opinion in Victorian Britain: The cross-currents of Bentham's and Coleridge's ideas

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    It is increasingly clear that law and its enforcement in Victorian Britain were quite effective in tackling formative industrial problems concerning pollution and broader threats to nature. What is unclear is the political philosophy, if any, underlying this historic achievement. A prevalent view is that early 'environmental' law lacked any philosophical underpinning (being instead a piecemeal reaction to the various problems of industrialization as and when these presented themselves). The article revisits this issue with reference to Dicey's analysis of 19th century 'lawmaking public opinion'. Dicey identified three broad streams of seminal opinion that, he argued, shaped laws as the century unfolded. The early part of the century was dominated by 'Old Toryism', including the romantic conservatism associated with Samuel Taylor Coleridge. This then gave ground to 'Benthamism' (or 'individualism') which in turn ceded dominance to 'collectivism' (also influenced by Bentham's ideas). Whilst Dicey ignored laws relating to the environment, I argue that this is not because these presented a particular difficulty for his thesis. Indeed, all three seams of 'law-making opinion' converged around the legal protection of nature to offer a rich and diverse philosophical foundation for environmental law
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