Angular Momentum Transport by MHD Turbulence in Accretion Disks: Gas Pressure Dependence of the Saturation Level of the Magnetorotational Instability

The saturation level of the magnetorotational instability (MRI) is investigated using three-dimensional MHD simulations. The shearing box approximation is adopted and the vertical component of gravity is ignored, so that the evolution of the MRI is followed in a small local part of the disk. We focus on the dependence of the saturation level of the stress on the gas pressure, which is a key assumption in the standard alpha disk model. From our numerical experiments it is found that there is a weak power-law relation between the saturation level of the Maxwell stress and the gas pressure in the nonlinear regime; the higher the gas pressure, the larger the stress. Although the power-law index depends slightly on the initial field geometry, the relationship between stress and gas pressure is independent of the initial field strength, and is unaffected by Ohmic dissipation if the magnetic Reynolds number is at least 10. The relationship is the same in adiabatic calculations, where pressure increases over time, and nearly-isothermal calculations, where pressure varies little with time. Our numerical results are qualitatively consistent with an idea that the saturation level of the MRI is determined by a balance between the growth of the MRI and the dissipation of the field through reconnection. The quantitative interpretation of the pressure-stress relation, however, may require advances in the theoretical understanding of non-steady magnetic reconnection.Comment: 45 pages, 5 tables, 17 figures, accepted for publication in Ap

Discovery to Development: Insecticides for Malaria Vector Control

This report provides an outline of a program for the discovery of new public health insecticides for malaria vector control. The status of malaria vector control is first reviewed in terms of the chemical, physical chemical, and biochemical properties of the current WHOPES-recommended and approved vector control agents. This review provides a basis for a discussion on the critical need for discovery and development of multiple new chemical malaria vector control agents with novel and diverse modes of action. The Innovative Vector Control Consortium (IVCC) New Active Ingredient Target Product Profile (TPP) describes the essential attributes for a successful new malaria vector control agent and then serves as the basis for development of a discovery cascade. The cascade addresses these attributes experimentally at each stage of the discovery process – from design and assembly of an appropriate collection of chemicals for screening, through development of testing protocols to sort candidates, and into the detailed profiling of advanced pre-development candidates against TPP requirements. In addition, this program defines a staged development system to provide intermediate guidance to the insecticide explorer regarding the progress of their discovery program against the ultimate product goal

Two-Dimensional Hydrodynamic Simulations of Convection in Radiation-Dominated Accretion Disks

The standard equilibrium for radiation-dominated accretion disks has long been known to be viscously, thermally, and convectively unstable, but the nonlinear development of these instabilities---hence the actual state of such disks---has not yet been identified. By performing local two-dimensional hydrodynamic simulations of disks, we demonstrate that convective motions can release heat sufficiently rapidly as to substantially alter the vertical structure of the disk. If the dissipation rate within a vertical column is proportional to its mass, the disk settles into a new configuration thinner by a factor of two than the standard radiation-supported equilibrium. If, on the other hand, the vertically-integrated dissipation rate is proportional to the vertically-integrated total pressure, the disk is subject to the well-known thermal instability. Convection, however, biases the development of this instability toward collapse. The end result of such a collapse is a gas pressure-dominated equilibrium at the original column density.Comment: 10 pages, 7 figures, accepted for publication in ApJ. Please send comments to [email protected]

Bone mineral density, rib pain and other features of the female athlete triad in elite lightweight rowers

Objective: To determine bone mineral density (BMD) and the associations among BMD, menstrual history, disordered eating (DE), training history, intentional weight loss (IWL) and rib pain for the first time in female lightweight rowers. Setting: 9 lightweight rowing clubs, UK. Participants: 29 Caucasian female lightweight rowers volunteered. 21 (12 active, 9 retired) completed the study. Inclusion criteria: female lightweight rowers aged over 18 years. Exclusion criteria: participants with a history of bone disease, used medications known to influence BMD or if they were pregnant, lactating or postmenopausal. Main outcome measures: Dual-energy X-ray absorptiometry measured total body (TB) composition and BMD at the spine, femoral neck (FN), radius and TB. DE, oligomenorrhoea/amenorrhoea years; rib pain and training history. Results: DE was reported in six of the rowers. The active with DE started rowing younger (p<0.05) than those without, and their amount of IWL was associated with Eating Attitudes Test-26 score (p<0.05). Some participants reported a history of oligomenorrhoea/amenorrhoea 17 (76%) and/or rib pain 7 (32%) with those with rib pain having lower spine and TB Z-scores (p<0.05) than those without. Those with oligomenorrhoea/amenorrhoea had lower spine Z-scores (p<0.01) than those without. Twelve participants had low BMD; three at spine; one at FN; and eight at radius. Thirteen per cent of mean total training hours (18.6±9.1 h/week) were spent strength training (2.4±2.2 h/week). Conclusions: Upper body exercises incorporating multidimensional high peak bone strain were not reported and may need to be considered in their strength training to improve radial BMD. Results suggest IWL and high level training at a young age increases the likelihood of DE and there may be a lack of quality nutritional support for these athletes. Thus, multidisciplinary sport science support should be offered at a young age and perhaps also to consider changing the weight rules to prevent the development of the Triad

On why the Iron K-shell absorption in AGN is not a signature of the local Warm/Hot Intergalactic Medium

We present a comparison between the 2001 XMM-Newton and 2005 Suzaku observations of the quasar, PG1211+143 at z=0.0809. Variability is observed in the 7 keV iron K-shell absorption line (at 7.6 keV in the quasar frame), which is significantly weaker in 2005 than during the 2001 XMM-Newton observation. From a recombination timescale of <4 years, this implies an absorber density n>0.004 particles/cm3, while the absorber column is 5e22<N_H <1 1e24 particles/cm2. Thus the sizescale of the absorber is too compact (pc scale) and the surface brightness of the dense gas too high (by 9-10 orders of magnitude) to arise from local hot gas, such as the local bubble, group or Warm/Hot Intergalactic Medium (WHIM), as suggested by McKernan et al. (2004, 2005). Instead the iron K-shell absorption must be associated with an AGN outflow with mildly relativistic velocities. Finally we show that the the association of the absorption in PG1211+143 with local hot gas is simply a coincidence, the comparison between the recession and iron K absorber outflow velocities in other AGN does not reveal a one to one kinematic correlation.Comment: accepted for publication in MNRAS LETTERS. 5 pages, 4 figure

Postactivation potentiation and change of direction speed in elite academy rugby players

This study investigated the effect of preceding pro-agility sprints with maximal isometric squats to determine if postactivation potentiation (PAP) could be harnessed in change of direction speed. Sixteen elite under-17 rugby union players (age: 16 +/- 0.41yrs; body mass: 88.7 +/- 12.1kg, height: 1.83 +/- 0.07m) from an Aviva Premiership rugby club were tested. Subjects performed a change of direction specific warm-up, followed by two baseline pro-agility tests. After 10 minutes recovery, 3 x 3-second maximal isometric squats with a 2 minute recovery between sets were completed as a conditioning activity (CA) on a force plate where peak force and mean rate of force development over 300 milliseconds were measured. The pro-agility test was repeated at set time intervals of 1, 3, 5 and 7 minutes following the CA. Overall pro-agility times were significantly slower (p < 0.05) at 1-minute post-CA compared to the baseline (3.3%), with no significant differences occurring at 3, 5 or 7 minutes post-CA. Therefore, it appears that performing multiple sets of maximal isometric squats do not enhance pro-agility performance

Bone mineral density, rib pain and other features of the female athlete triad in elite lightweight rowers

Objective: To determine bone mineral density (BMD) and the associations among BMD, menstrual history, disordered eating (DE), training history, intentional weight loss (IWL) and rib pain for the first time in female lightweight rowers. Setting: 9 lightweight rowing clubs, UK. Participants: 29 Caucasian female lightweight rowers volunteered. 21 (12 active, 9 retired) completed the study. Inclusion criteria: female lightweight rowers aged over 18 years. Exclusion criteria: participants with a history of bone disease, used medications known to influence BMD or if they were pregnant, lactating or postmenopausal. Main outcome measures: Dual-energy X-ray absorptiometry measured total body (TB) composition and BMD at the spine, femoral neck (FN), radius and TB. DE, oligomenorrhoea/amenorrhoea years; rib pain and training history. Results: DE was reported in six of the rowers. The active with DE started rowing younger (p<0.05) than those without, and their amount of IWL was associated with Eating Attitudes Test-26 score (p<0.05). Some participants reported a history of oligomenorrhoea/amenorrhoea 17 (76%) and/or rib pain 7 (32%) with those with rib pain having lower spine and TB Z-scores (p<0.05) than those without. Those with oligomenorrhoea/amenorrhoea had lower spine Z-scores (p<0.01) than those without. Twelve participants had low BMD; three at spine; one at FN; and eight at radius. Thirteen per cent of mean total training hours (18.6±9.1 h/week) were spent strength training (2.4±2.2 h/week). Conclusions: Upper body exercises incorporating multidimensional high peak bone strain were not reported and may need to be considered in their strength training to improve radial BMD. Results suggest IWL and high level training at a young age increases the likelihood of DE and there may be a lack of quality nutritional support for these athletes. Thus, multidisciplinary sport science support should be offered at a young age and perhaps also to consider changing the weight rules to prevent the development of the Triad