The Dependence of Coronal Loop Heating on the Characteristics of Slow Photospheric Motions

The Parker hypothesis (Parker (1972)) assumes that heating of coronal loops occurs due to reconnection, induced when photospheric motions braid field lines to the point of current sheet formation. In this contribution we address the question of how the nature of photospheric motions affects heating of braided coronal loops. We design a series of boundary drivers and quantify their properties in terms of complexity and helicity injection. We examine a series of long-duration full resistive MHD simulations in which a simulated coronal loop, consisting of initially uniform field lines, is subject to these photospheric flows. Braiding of the loop is continually driven until differences in behaviour induced by the drivers can be characterised. It is shown that heating is crucially dependent on the nature of the photospheric driver - coherent motions typically lead to fewer large energy release events, while more complex motions result in more frequent but less energetic heating events

Magnetic reconnection in flux-tubes undergoing spinning footpoint motions

Aims. Photospheric motions acting on the coronal magnetic field have the potential to build up huge amounts of magnetic energy. The energy may be released through magnetic reconnection, and so a detailed understanding of the 3D process is crucial if its implications for coronal heating are to be fully addressed. Methods. A 3D MHD experiment is described in which misaligned magnetic flux tubes are subjected to simple spinning boundary motions. Results. The resulting shear between adjacent flux systems generates a twisted central separator current sheet that extends vertically throughout the domain. Current density is amplified to a sufficient extent that reconnection begins, and occurs everywhere along the separator current sheet, while the separatrix current sheets that exist in the early stages of the experiment are found to be unimportant in the systems dynamical evolution. In 2D cross-sections, the reconnection process exhibits many similarities to the regime of flux pile-up reconnection

A novel type of intermittency in a nonlinear dynamo in a compressible flow

The transition to intermittent mean--field dynamos is studied using numerical simulations of isotropic magnetohydrodynamic turbulence driven by a helical flow. The low-Prandtl number regime is investigated by keeping the kinematic viscosity fixed while the magnetic diffusivity is varied. Just below the critical parameter value for the onset of dynamo action, a transient mean--field with low magnetic energy is observed. After the transition to a sustained dynamo, the system is shown to evolve through different types of intermittency until a large--scale coherent field with small--scale turbulent fluctuations is formed. Prior to this coherent field stage, a new type of intermittency is detected, where the magnetic field randomly alternates between phases of coherent and incoherent large--scale spatial structures. The relevance of these findings to the understanding of the physics of mean--field dynamo and the physical mechanisms behind intermittent behavior observed in stellar magnetic field variability are discussed.Comment: 19 pages, 13 figure

Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models

The Sun is a magnetic star whose cyclic activity is thought to be linked to internal dynamo mechanisms. A combination of numerical modelling with various levels of complexity is an efficient and accurate tool to investigate such intricate dynamical processes. We investigate the role of the magnetic buoyancy process in 2D Babcock-Leighton dynamo models, by modelling more accurately the surface source term for poloidal field. Methods. To do so, we reintroduce in mean-field models the results of full 3D MHD calculations of the non-linear evolution of a rising flux tube in a convective shell. More specifically, the Babcock-Leighton source term is modified to take into account the delay introduced by the rise time of the toroidal structures from the base of the convection zone to the solar surface. We find that the time delays introduced in the equations produce large temporal modulation of the cycle amplitude even when strong and thus rapidly rising flux tubes are considered. Aperiodic modulations of the solar cycle appear after a sequence of period doubling bifurcations typical of non-linear systems. The strong effects introduced even by small delays is found to be due to the dependence of the delays on the magnetic field strength at the base of the convection zone, the modulation being much less when time delays remain constant. We do not find any significant influence on the cycle period except when the delays are made artificially strong. A possible new origin of the solar cycle variability is here revealed. This modulated activity and the resulting butterfly diagram are then more compatible with observations than what the standard Babcock-Leighton model produces.Comment: 13 pages, 10 figures, accepted for publication in A&

Effects of fieldline topology on energy propagation in the corona

We study the effect of photospheric footpoint motions on magnetic field structures containing magnetic nulls. The footpoint motions are prescribed on the photospheric boundary as a velocity field which entangles the magnetic field. We investigate the propagation of the injected energy, the conversion of energy, emergence of current layers and other consequences of the non-trivial magnetic field topology in this situation. These boundary motions lead initially to an increase in magnetic and kinetic energy. Following this, the energy input from the photosphere is partially dissipated and partially transported out of the domain through the Poynting flux. The presence of separatrix layers and magnetic null-points fundamentally alters the propagation behavior of disturbances from the photosphere into the corona. Depending on the field line topology close to the photosphere, the energy is either trapped or free to propagate into the corona.Comment: 14 pages, 15 figure

Sedentary time in older men and women: an international consensus statement and research priorities

Sedentary time is a modifiable determinant of poor health, and in older adults, reducing sedentary time may be an important first step in adopting and maintaining a more active lifestyle. The primary purpose of this consensus statement is to provide an integrated perspective on current knowledge and expert opinion pertaining to sedentary behaviour in older adults on the topics of measurement, associations with health outcomes, and interventions. A secondary yet equally important purpose is to suggest priorities for future research and knowledge translation based on gaps identified. A five-step Delphi consensus process was used. Experts in the area of sedentary behaviour and older adults (n=15) participated in three surveys, an in-person consensus meeting, and a validation process. The surveys specifically probed measurement, health outcomes, interventions, and research priorities. The meeting was informed by a literature review and conference symposium, and it was used to create statements on each of the areas addressed in this document. Knowledge users (n=3) also participated in the consensus meeting. Statements were then sent to the experts for validation. It was agreed that self-report tools need to be developed for understanding the context in which sedentary time is accumulated. For health outcomes, it was agreed that the focus of sedentary time research in older adults needs to include geriatric-relevant health outcomes, that there is insufficient evidence to quantify the dose-response relationship, that there is a lack of evidence on sedentary time from older adults in assisted facilities, and that evidence on the association between sedentary time and sleep is lacking. For interventions, research is needed to assess the impact that reducing sedentary time, or breaking up prolonged bouts of sedentary time has on geriatric-relevant health outcomes. Research priorities listed for each of these areas should be considered by researchers and funding agencies

Sheep Updates 2007 - part 4

This session covers eight papers from different authors: GRAZING 1. The impact of high dietary salt and its implications for the management of livestock grazing saline land, Dean Thomas, Dominique Blache, Dean Revell, Hayley Norman, Phil Vercoe, Zoey Durmic, Serina Digby, Di Mayberry, Megan Chadwick, Martin Sillence and David Masters, CRC for Plant-based Management of Dryland Salinity, Faculty of Natural & Agricultural Sciences, The University of Western Australia, WA. 2. Sustainable Grazing on Saline Lands - outcomes from the WA1 research project, H.C. Norman1,2, D.G. Masters1,2, R. Silberstein1,2, F. Byrne2,3, P.G.H. Nichols2,4, J. Young3, L. Atkins1,2, M.G. Wilmot1,2, A.J. Rintoul1,2, T. Lambert1,2, D.R. McClements2,4, P. Raper4, P. Ward1,2, C. Walton5 and T. York6 1CSIRO Centre for Environment and Life Sciences, Wembley, WA 2CRC for Plant-based Management of Dryland Salinity. 3School of Agricultural and Resource Economics, University of Western Australia. 4Department of Agriculture and Food WA. 5Condering Hills, Yealering. 6Anameka Farms, Tammin. MEAT QUALITY 3. Development of intramuscular fat in prime lambs, young sheep and beef cattle, David Pethick1, David Hopkins2 and Malcolm McPhee3,1School of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, WA, 2NSW Department of Primary Industries, Cowra, NSW,3NSW Dept. of Primary Industries, University of New England, Armidale, NSW, 4. Importance of drinking water temperature for managing heat stress in sheep, Savage DB, Nolan JV, Godwin IR, Aoetpah A, Nguyen T, Baillie N and Lawler C University of New England, Armidale, NSW, Australia EWE MANAGEMENT TOOLS 5. E - sheep Management of Pregnant Merino Ewes and their Finishing Lambs, Ken GeentyA, John SmithA, Darryl SmithB, Tim DyallA and Grant UphillA A Sheep CRC and CSIRO Livestock Industries, Chiswick, NSW B Turretfield Research Station, SARDI, Roseworthy, SA 6. Is it important to manage ewes to CS targets? John Young, Farming Systems Analysis Service, Kojonup, WA MULESING 7. Mulesing accreditation - Vital for Wool\u27s Future, Dr Michael Paton, Department of Agriculture and Food WA, 8. Mulesing Alternatives, Jules Dorrian, Affiliation Project Manager Blowfly Control Australian Wool Inovatio