title: Magnetic flux braiding: Force-free equilibria and current sheets

authors: Longbottom, A.W.

subjects: MHD-plasmas-sun

abstract: We use a numerical, nonlinear, multigrid magnetic relaxation technique to investigate the generation of current sheets in three-dimensional magnetic flux braiding experiments. We are able to catalog the relaxed nonlinear force-free equilibria resulting from the application of deformations to an initially undisturbed region of plasma containing a uniform vertical magnetic field. The deformations are manifested by imposing motions on the bounding planes to which the magnetic field is anchored. Once imposed, the new distribution of magnetic footpoints are then taken to be fixed, so that the rest of the plasma must then relax to a new equilibrium configuration. For the class of footpoint motions that we have examined, we find that singular and nonsingular equilibria can be generated. By singular we mean that within the limits imposed by numerical resolution, we find that there is no convergence to a well-defined equilibrium as the number of grid points in the numerical domain is increased. These singular equilibria contain current “sheets” of ever increasing current intensity and decreasing width; they occur when the footpoint motions exceed a certain threshold and must include both twist and shear in order to be effective. On the basis of these results, we contend that flux braiding will indeed result in significant current generation. We discuss the implications of our results for coronal heating.

text: Magnetic flux braiding: Force-free equilibria and current sheets We use a numerical, nonlinear, multigrid magnetic relaxation technique to investigate the generation of current sheets in three-dimensional magnetic flux braiding experiments. We are able to catalog the relaxed nonlinear force-free equilibria resulting from the application of deformations to an initially undisturbed region of plasma containing a uniform vertical magnetic field. The deformations are manifested by imposing motions on the bounding planes to which the magnetic field is anchored. Once imposed, the new distribution of magnetic footpoints are then taken to be fixed, so that the rest of the plasma must then relax to a new equilibrium configuration. For the class of footpoint motions that we have examined, we find that singular and nonsingular equilibria can be generated. By singular we mean that within the limits imposed by numerical resolution, we find that there is no convergence to a well-defined equilibrium as the number of grid points in the numerical domain is increased. These singular equilibria contain current “sheets” of ever increasing current intensity and decreasing width; they occur when the footpoint motions exceed a certain threshold and must include both twist and shear in order to be effective. On the basis of these results, we contend that flux braiding will indeed result in significant current generation. We discuss the implications of our results for coronal heating.

year: 1998-06

title: Tetramethyl-p-phenylenediamine (TMPD) oxidase activity and cytochrome distribution in the genus

authors: West, P.A.

subjects: biology

abstract: Kovacs' [1] qualitative test for N,N,N',N',-tetramethyl- p-phenylenediamine (TMPD) oxidase activity is a fundamental test in the identification of bacteria [2,3]. TMPD (E~ = +0.26 V at pH 7.0) acts as an artificial electron donor to the terminal portion of aerobic respiratory systems [4], and the ability to oxidise TMPD, reduced by ascorbate, has been widely used as a quantitative assay of terminal oxidase activity [5]. In a wide range of heterotrophic bacteria there is a close correlation between their response to the Kovacs test and their ascorbate-TMPD oxidase activity measured manometrically [6]. The inability to oxidise ascorbate-TMPD or a negative response to the Kovacs test, however, does not necessarily indicate that an organism lacks an aerobic respiratory system and a terminal oxidase. For example, species belonging to the family Enterobacteriaceae are oxidase- negative yet form aerobic respiratory systems[5,7].

text: Tetramethyl-p-phenylenediamine (TMPD) oxidase activity and cytochrome distribution in the genus Kovacs' [1] qualitative test for N,N,N',N',-tetramethyl- p-phenylenediamine (TMPD) oxidase activity is a fundamental test in the identification of bacteria [2,3]. TMPD (E~ = +0.26 V at pH 7.0) acts as an artificial electron donor to the terminal portion of aerobic respiratory systems [4], and the ability to oxidise TMPD, reduced by ascorbate, has been widely used as a quantitative assay of terminal oxidase activity [5]. In a wide range of heterotrophic bacteria there is a close correlation between their response to the Kovacs test and their ascorbate-TMPD oxidase activity measured manometrically [6]. The inability to oxidise ascorbate-TMPD or a negative response to the Kovacs test, however, does not necessarily indicate that an organism lacks an aerobic respiratory system and a terminal oxidase. For example, species belonging to the family Enterobacteriaceae are oxidase- negative yet form aerobic respiratory systems[5,7].

year: 1978

title: Formulaic Discourse Patterning in Mōteatea

authors: Roa, Te Raukura o Te Rangimarie

subjects: mōteatea

abstract: Oral formulaic composition, which involves the use of communally owned formulae of various kinds, is a common feature of verbal arts produced in many different languages. It is particularly associated with pre-literate cultures and tends to be gradually replaced by more individualistic verbal art forms when societies become literate. There are very few publications in which the analysis of mōteatea (Māori laments) is linked explicitly to oral formulaic theory. Nevertheless, there is, I believe, sufficient evidence in published sources to indicate that traditional mōteatea (defined here as mōteatea that are not fundamentally influenced by European cultural beliefs and practices) exhibit evidence of regularly recurring, conventional themes (such as death, separation, loss and travel) and motifs (such as the setting sun, the presence of rain or mist and sleeplessness).  There is, however, considerably less evidence that traditional mōteatea make extensive use of verbal formulae, that is, of the same or very similar groups of words derived from a common store of poetic resources. Although there are references in the literature to the conventionalized use of language in mōteatea, and although it has been claimed that there are at least some commonly recurring verbal expressions, this is by no means evidence of reliance on a common stock of verbal formulae. Indeed, no attempt has hitherto been made to define precisely what might be meant by the term 'oral formula' in the context of compositions in te reo Māori, something that is clearly required in view, in particular, of the fact that the phrasal structure of the language means that certain groupings (e.g. i te X; ki te X) are very likely to occur in certain contexts irrespective of genre. Furthermore, in the literature on oral formulaic composition, oral formulae are generally linked to particular metrical conditions and there is, it is argued here, no convincing empirically-based evidence that mōteatea conform to any common metrical structure or structures (see Chapters 2 and 4).

In the context of a critical review of selected literature on oral formulaic theory (Chapter 2) and mōteatea (Chapter 3), this thesis sets out to test the hypothesis that traditional mōteatea were composed, in whole or in large part, from a common stock of oral formulae. The conclusion, based on analysis of a selection of mōteatea included in Ngā mōteatea, a collection initially established by Sir Apirana Ngata, is that this hypothesis must be rejected (Chapter 4). However, a model derived from a review of selected literature on discourse structure (Chapter 5) was also applied to the analysis of a sample of these mōteatea (Chapter 6). On the basis of this analysis, it is concluded that they exhibit one of two very similar discourse prototypes, each of which involves a particular combination of discourse elements. What distinguishes the two prototypes is the fact that only one of them is characterized by the inclusion of hortatory sections in which the poet urges one or more of the protagonists to undergo one or more steps or stages required to achieve a particular outcome, such as entry into the spirit world or vengeance for death and/ or humiliation.

text: Formulaic Discourse Patterning in Mōteatea Oral formulaic composition, which involves the use of communally owned formulae of various kinds, is a common feature of verbal arts produced in many different languages. It is particularly associated with pre-literate cultures and tends to be gradually replaced by more individualistic verbal art forms when societies become literate. There are very few publications in which the analysis of mōteatea (Māori laments) is linked explicitly to oral formulaic theory. Nevertheless, there is, I believe, sufficient evidence in published sources to indicate that traditional mōteatea (defined here as mōteatea that are not fundamentally influenced by European cultural beliefs and practices) exhibit evidence of regularly recurring, conventional themes (such as death, separation, loss and travel) and motifs (such as the setting sun, the presence of rain or mist and sleeplessness).  There is, however, considerably less evidence that traditional mōteatea make extensive use of verbal formulae, that is, of the same or very similar groups of words derived from a common store of poetic resources. Although there are references in the literature to the conventionalized use of language in mōteatea, and although it has been claimed that there are at least some commonly recurring verbal expressions, this is by no means evidence of reliance on a common stock of verbal formulae. Indeed, no attempt has hitherto been made to define precisely what might be meant by the term 'oral formula' in the context of compositions in te reo Māori, something that is clearly required in view, in particular, of the fact that the phrasal structure of the language means that certain groupings (e.g. i te X; ki te X) are very likely to occur in certain contexts irrespective of genre. Furthermore, in the literature on oral formulaic composition, oral formulae are generally linked to particular metrical conditions and there is, it is argued here, no convincing empirically-based evidence that mōteatea conform to any common metrical structure or structures (see Chapters 2 and 4).

In the context of a critical review of selected literature on oral formulaic theory (Chapter 2) and mōteatea (Chapter 3), this thesis sets out to test the hypothesis that traditional mōteatea were composed, in whole or in large part, from a common stock of oral formulae. The conclusion, based on analysis of a selection of mōteatea included in Ngā mōteatea, a collection initially established by Sir Apirana Ngata, is that this hypothesis must be rejected (Chapter 4). However, a model derived from a review of selected literature on discourse structure (Chapter 5) was also applied to the analysis of a sample of these mōteatea (Chapter 6). On the basis of this analysis, it is concluded that they exhibit one of two very similar discourse prototypes, each of which involves a particular combination of discourse elements. What distinguishes the two prototypes is the fact that only one of them is characterized by the inclusion of hortatory sections in which the poet urges one or more of the protagonists to undergo one or more steps or stages required to achieve a particular outcome, such as entry into the spirit world or vengeance for death and/ or humiliation.

year: 2008