title: Fast dynamic reconnection at X-type neutral points

authors: Craig, Ian J.D.

abstract: We consider the linear and nonlinear evolution of disturbed magnetic X-type neutral points. The problem is formulated within a unified analytic and computational framework which highlights the essence of the magnetic annihilation process, namely, the coupling of a global convection region to a localized diffusion region surrounding the neutral point. An analytic treatment is given for the case of small disturbances of the equilibrium field in the absence of gas pressure. This problem admits well-defined azimuthal modes which allow a formally exact determination of the magnetic annihilation rate. It is shown that reconnection can only occur in the case of purely radial (m = 0) disturbances: the reconnection process is oscillatory and "fast," depending only logarithmically on the plasma resistivity (η). We show that the linear theory supports the notion of an initial implosive stage which rapidly releases the bulk of the energy associated with reconnective field disturbances. This phase is initiated by the advective focusing of the perturbation energy into the neutral point and culminates in the formation of a cylindrical diffusion region of area A ∼ η and current density J ∼ η-1. This scaling provides a signature for fast linear reconnection. Next we consider the breakdown of the linear theory. Although fast reconnection is maintained for low-amplitude disturbances in noncylindrical geometries, it is shown that finite gas pressure can stall the reconnection if sufficiently large. This effect, however, may not be critical in more complex X-point geometries. More seriously, for finite-amplitude displacements the cylindrical current structure close to the neutral point is distorted into a quasi-one-dimensional current sheet whose thickness is limited by resistive diffusion. In this case fast reconnection is consistent with a flux pileup solution in which the bulk of the energy is released as heat rather than as the kinetic energy of mass motion.

text: Fast dynamic reconnection at X-type neutral points We consider the linear and nonlinear evolution of disturbed magnetic X-type neutral points. The problem is formulated within a unified analytic and computational framework which highlights the essence of the magnetic annihilation process, namely, the coupling of a global convection region to a localized diffusion region surrounding the neutral point. An analytic treatment is given for the case of small disturbances of the equilibrium field in the absence of gas pressure. This problem admits well-defined azimuthal modes which allow a formally exact determination of the magnetic annihilation rate. It is shown that reconnection can only occur in the case of purely radial (m = 0) disturbances: the reconnection process is oscillatory and "fast," depending only logarithmically on the plasma resistivity (η). We show that the linear theory supports the notion of an initial implosive stage which rapidly releases the bulk of the energy associated with reconnective field disturbances. This phase is initiated by the advective focusing of the perturbation energy into the neutral point and culminates in the formation of a cylindrical diffusion region of area A ∼ η and current density J ∼ η-1. This scaling provides a signature for fast linear reconnection. Next we consider the breakdown of the linear theory. Although fast reconnection is maintained for low-amplitude disturbances in noncylindrical geometries, it is shown that finite gas pressure can stall the reconnection if sufficiently large. This effect, however, may not be critical in more complex X-point geometries. More seriously, for finite-amplitude displacements the cylindrical current structure close to the neutral point is distorted into a quasi-one-dimensional current sheet whose thickness is limited by resistive diffusion. In this case fast reconnection is consistent with a flux pileup solution in which the bulk of the energy is released as heat rather than as the kinetic energy of mass motion.

year: 1992

title: Utuhina Stream monitoring 2014: Effects of continuous alum dosing on fish and aquatic invertebrates

authors: Ling, Nicholas

abstract: This report presents the results of an ongoing assessment of the fish and aquatic macroinvertebrate communities of the Utuhina Stream from 2006 to 2014, and an assessment of the bioavailability of aluminium in fish and koura to satisfy annual resource consent conditions 9.6, 9.8 and 9.7, respectively, for the discharge of alum. Macroinvertebrates, fish and koura were sampled from one control and two treatment reaches of the Utuhina Stream in August 2014. Catch rates for common bully, juvenile trout and koura have fluctuated across all sites since monitoring began in 2006. Common bully, koura and juvenile trout were present at all sites. Differences in species abundance compared with previous years is most likely due to flood related disturbances to stream bank morphology and vegetative cover. No obvious effects of alum dosing on stream fish or macroinvertebrate communities were observed.

text: Utuhina Stream monitoring 2014: Effects of continuous alum dosing on fish and aquatic invertebrates This report presents the results of an ongoing assessment of the fish and aquatic macroinvertebrate communities of the Utuhina Stream from 2006 to 2014, and an assessment of the bioavailability of aluminium in fish and koura to satisfy annual resource consent conditions 9.6, 9.8 and 9.7, respectively, for the discharge of alum. Macroinvertebrates, fish and koura were sampled from one control and two treatment reaches of the Utuhina Stream in August 2014. Catch rates for common bully, juvenile trout and koura have fluctuated across all sites since monitoring began in 2006. Common bully, koura and juvenile trout were present at all sites. Differences in species abundance compared with previous years is most likely due to flood related disturbances to stream bank morphology and vegetative cover. No obvious effects of alum dosing on stream fish or macroinvertebrate communities were observed.

year: 2015

title: Context counts : leading educational reform in New Zealand secondary schools

authors: Morrison, Michele

subjects: context

abstract: Educational leadership and organisational literature is replete with syntheses of effective leader behaviours, change sequences and reform models, yet few studies explore the nuanced manner in which individual leaders make sense of and respond to the dynamic influences shaping their thinking and practice over time. In posing the question, how do principals experience leadership during periods of intentional educational change, this study gains insight into the lived experience of five New Zealand secondary school principals as they lead school wide implementation of Phase 5 Te Kotahitanga, a reform initiative designed to reduce disparities in the educational achievement of indigenous Māori students through the introduction of a culturally responsive pedagogy of relations. The case narratives arising from a three year dialogue with participants examine the genesis of each principal’s commitment to equity in educational access, opportunity and outcome; their evolving thinking and practice; and the manner in which people and structures within each school field enable and constrain their leadership endeavours. Findings demonstrate that leading counter-hegemonic change is a situated, complex and demanding undertaking, the intensity of which eludes much of the leadership and change literature. Hermeneutic reading of these rich qualitative accounts of lived experience suggests that, far from being detached observers, leaders both embody and are embedded in context. This exposes the shortcomings of literature that dichotomises the two and makes Bourdieu’s lens on practice as a temporal, embodied, discursive and social activity an important heuristic through which to view the holism of change leadership. The implications arising from this study are fourfold: leadership narratives open windows to natural and vicarious meaning making, personal biography is of central rather than peripheral importance, emotional and micropolitical literacy crucial, and professional mentoring a potentially powerful catalyst in building practice wisdom.

text: Context counts : leading educational reform in New Zealand secondary schools Educational leadership and organisational literature is replete with syntheses of effective leader behaviours, change sequences and reform models, yet few studies explore the nuanced manner in which individual leaders make sense of and respond to the dynamic influences shaping their thinking and practice over time. In posing the question, how do principals experience leadership during periods of intentional educational change, this study gains insight into the lived experience of five New Zealand secondary school principals as they lead school wide implementation of Phase 5 Te Kotahitanga, a reform initiative designed to reduce disparities in the educational achievement of indigenous Māori students through the introduction of a culturally responsive pedagogy of relations. The case narratives arising from a three year dialogue with participants examine the genesis of each principal’s commitment to equity in educational access, opportunity and outcome; their evolving thinking and practice; and the manner in which people and structures within each school field enable and constrain their leadership endeavours. Findings demonstrate that leading counter-hegemonic change is a situated, complex and demanding undertaking, the intensity of which eludes much of the leadership and change literature. Hermeneutic reading of these rich qualitative accounts of lived experience suggests that, far from being detached observers, leaders both embody and are embedded in context. This exposes the shortcomings of literature that dichotomises the two and makes Bourdieu’s lens on practice as a temporal, embodied, discursive and social activity an important heuristic through which to view the holism of change leadership. The implications arising from this study are fourfold: leadership narratives open windows to natural and vicarious meaning making, personal biography is of central rather than peripheral importance, emotional and micropolitical literacy crucial, and professional mentoring a potentially powerful catalyst in building practice wisdom.

year: 2019