Contributions of divergent and nondivergent winds to the kinetic energy balance of a severe storm environment

Divergent and rotational components of the synoptic scale kinetic energy balance are presented using rawinsonde data at 3 and 6 h intervals from the Atmospheric Variability Experiment (AVE 4). Two intense thunderstorm complexes occurred during the period. Energy budgets are described for the entire computational region and for limited volumes that enclose and move with the convection. Although small in magnitude, the divergent wind component played an important role in the cross contour generation and horizontal flux divergence of kinetic energy. The importance of V sub D appears directly to the presence and intensity of convection within the area. Although K sub D usually comprised less than 10 percent of the total kinetic energy content within the storm environment, as much as 87 percent of the total horizontal flux divergence and 68 percent of the total cross contour generation was due to the divergent component in the upper atmosphere. Generation of kinetic energy by the divergent component appears to be a major factor in the creation of an upper level wind maximum on the poleward side of one of the complexes. A random error analysis is presented to assess confidence limits in the various energy parameters

Coronal heating in multiple magnetic threads

Context. Heating the solar corona to several million degrees requires the conversion of magnetic energy into thermal energy. In this paper, we investigate whether an unstable magnetic thread within a coronal loop can destabilise a neighbouring magnetic thread. Aims. By running a series of simulations, we aim to understand under what conditions the destabilisation of a single magnetic thread can also trigger a release of energy in a nearby thread. Methods. The 3D magnetohydrodynamics code, Lare3d, is used to simulate the temporal evolution of coronal magnetic fields during a kink instability and the subsequent relaxation process. We assume that a coronal magnetic loop consists of non-potential magnetic threads that are initially in an equilibrium state. Results. The non-linear kink instability in one magnetic thread forms a helical current sheet and initiates magnetic reconnection. The current sheet fragments, and magnetic energy is released throughout that thread. We find that, under certain conditions, this event can destabilise a nearby thread, which is a necessary requirement for starting an avalanche of energy release in magnetic threads. Conclusions. It is possible to initiate an energy release in a nearby, non-potential magnetic thread, because the energy released from one unstable magnetic thread can trigger energy release in nearby threads, provided that the nearby structures are close to marginal stability

Thermal and non-thermal emission from reconnecting twisted coronal loops

Twisted magnetic fields should be ubiquitous in flare-producing active regions where the magnetic fields are strongly non-potential. It has been shown that reconnection in helical magnetic coronal loops results in plasma heating and particle acceleration distributed within a large volume, including the lower coronal and chromospheric sections of the loops. This scenario can be an alternative to the standard flare model, where particles are accelerated only in a small volume located in the upper corona. We use a combination of MHD simulations and test-particle methods, which describe the development of kink instability and magnetic reconnection in twisted coronal loops using resistive compressible MHD, and incorporate atmospheric stratification and large-scale loop curvature. The resulting distributions of hot plasma let us estimate thermal X-ray emission intensities. The electric and magnetic fields obtained are used to calculate electron trajectories using the guiding-centre approximation. These trajectories combined with the MHD plasma density distributions let us deduce synthetic HXR bremsstrahlung intensities. Our simulations emphasise that the geometry of the emission patterns produced by hot plasma in flaring twisted coronal loops can differ from the actual geometry of the underlying magnetic fields. The twist angles revealed by the emission threads (SXR) are consistently lower than the field-line twist present at the onset of the kink-instability. HXR emission due to the interaction of energetic electrons with the stratified background are concentrated at the loop foot-points in these simulations, even though the electrons are accelerated everywhere within the coronal volume of the loop. The maximum of HXR emission consistently precedes that of SXR emission, with the HXR light-curve being approximately proportional to the temporal derivative of the SXR light-curve.Comment: (accepted for publication on A&A

Rational curves on smooth hypersurfaces of low degree

We study the family of rational curves on arbitrary smooth hypersurfaces of low degree using tools from analytic number theory

School leadership and ‘language games’ in neoliberal times: A critical ethnographic case study of the Independent Public School policy in Western Australia

This thesis is a critical policy ethnography (CPE) of school leaders in three low socio-economic indicator (SEI) government schools in Western Australia (WA) that have achieved Independent Public School (IPS) status under the state’s IPS policy. It draws on the stories of school leaders to understand the logics, processes, and tensions they experience in enacting this policy maneuver, and how it is negotiated and resolved at the school level (Ball, 2003). The introduction of WA’s IPS policy occurs in the context of a distinct and well-documented shift in the ideological forces driving education policy in the Western world. Such a shift has resulted in the ascendancy of neoliberalism as the dominant discourse within government education policy formation. Further, this shift is clearly evident in responses provided by the school leaders throughout this thesis. Central to this thesis is the argument that independent or autonomous government schools are part of what Lyotard (1984) terms ‘language games.’ These language games occur within a broader set of neoliberal discourses driven by the idea of ‘homo economicus,’ which governs the ways in which individuals conceive of themselves and society. Such a reconceptualization of homo economicus represents an elemental disruption of democracy as individuals within the neoliberal language game strategize for themselves (Dilts, 2011). The use of CPE provides an opportunity to locate the daily experiences of school leaders in the context of these broader ideological shifts as it relates to the enactment of the IPS policy at three school sites. Ethnography also allows an anthropological approach to the study through seeking to describe participants’ actions, intentions, motives, and reasons. In selecting this particular methodological approach, the voices of participants are given center stage. CPE is a methodology that critically examines the ways in which official policy discourses constitute the lived realities of individuals. In this case, the formal school leadership is comprised of heads of learning areas, deputy principals and principals. The thesis makes a specific contribution to research by examining the broader effects of neoliberal language games through the enactment of the IPS policy in WA via a range of primary and secondary sources. In particular, it examines the effect of the enactment of the IPS policy from the perspective of school leaders in socially disadvantaged WA government high schools

Cubic hypersurfaces and a version of the circle method for number fields

A version of the Hardy–Littlewood circle method is developed for number fields K/QK/Q and is used to show that nonsingular projective cubic hypersurfaces over KK always have a KK-rational point when they have dimension at least 88