Nonlinear Alfvén wave dynamics at a 2D magnetic null point: ponderomotive force

Context: In the linear, β = 0 MHD regime, the transient properties of magnetohydrodynamic (MHD) waves in the vicinity of 2D null points are well known. The waves are decoupled and accumulate at predictable parts of the magnetic topology: fast waves accumulate at the null point; whereas Alfvén waves cannot cross the separatricies. However, in nonlinear MHD mode conversion can occur at regions of inhomogeneous Alfvén speed, suggesting that the decoupled nature of waves may not extend to the nonlinear regime. Aims: We investigate the behaviour of low-amplitude Alfvén waves about a 2D magnetic null point in nonlinear, β = 0 MHD. Methods: We numerically simulate the introduction of low-amplitude Alfvén waves into the vicinity of a magnetic null point using the nonlinear LARE2D code. Results: Unlike in the linear regime, we find that the Alfvén wave sustains cospatial daughter disturbances, manifest in the transverse and longitudinal fluid velocity, owing to the action of nonlinear magnetic pressure gradients (viz. the ponderomotive force). These disturbances are dependent on the Alfvén wave and do not interact with the medium to excite magnetoacoustic waves, although the transverse daughter becomes focused at the null point. Additionally, an independently propagating fast magnetoacoustic wave is generated during the early stages, which transports some of the initial Alfvén wave energy towards the null point. Subsequently, despite undergoing dispersion and phase-mixing due to gradients in the Alfvén-speed profile (∇c_A ≠ 0) there is no further nonlinear generation of fast waves. Conclusions: We find that Alfvén waves at 2D cold null points behave largely as in the linear regime, however they sustain transverse and longitudinal disturbances - effects absent in the linear regime - due to nonlinear magnetic pressure gradients

Mortality post inpatient alcohol detoxification: a descriptive case series

<br>Background: Individuals with alcohol dependence often die prematurely. Scotland in particular has high rates of alcohol misuse compared to the rest of the UK and Europe. Although there is some evidence that rates of alcoholrelated harm and alcohol-related deaths are reducing in Europe, there is little evidence of this trend occurring in Scotland. Inpatient detoxification is an important, but expensive, option in the management of alcohol dependence. Little work has been done with regard possible clinical characteristics associated with mortality post discharge in this cohort of patients.</br> <br>Methods: Retrospective electronic records were used to identify patients admitted electively for alcohol detoxification into a psychiatric ward in a district general hospital under the care of a single consultant addiction psychiatrist between 1/1/05 and 31/12/07 inclusively. Demographic and clinical factors were recorded. 5-year mortality data was obtained by linkage to the National Records of Scotland.</br> <br>Results: 25.2% of patients admitted for elective alcohol detoxification died during the 5-year follow up period. 42.9% of deaths (n=12) were due to alcoholic liver disease, one death (3.6%) was of undetermined intent. A higher proportion of the deceased cohort was likely to have continued disulfiram (93% vs. 80%) and acamprosate (81% vs. 60%) than those surviving. There was no statistically significant difference in survival between individuals on medication and those not on medications in our cohort. The average consumption of units of alcohol per week in the deceased cohort was significantly higher than that calculated for the surviving cohort (298.8 units vs. 233.3 units p=0.0137).</br> <br>Conclusion: Inpatient detoxification was associated with significant mortality at 5-year follow up post- discharge. The majority of deaths were due to liver disease while deaths due to accidents and of undetermined intent were relatively infrequent. Increased education and practical strategies such as “space and pace” aimed both at an individual and societal level to reduce alcohol consumption may help to reduce mortality associated with alcohol dependence post inpatient detoxification.</br&gt

A bright millisecond radio burst of extragalactic origin

Pulsar surveys offer one of the few opportunities to monitor even a small fraction (~0.00001) of the radio sky for impulsive burst-like events with millisecond durations. In analysis of archival survey data, we have discovered a 30-Jy dispersed burst of duration <5 ms located three degrees from the Small Magellanic Cloud. The burst properties argue against a physical association with our Galaxy or the Small Magellanic Cloud. Current models for the free electron content in the Universe imply a distance to the burst of <1 Gpc No further bursts are seen in 90-hr of additional observations, implying that it was a singular event such as a supernova or coalescence of relativistic objects. Hundreds of similar events could occur every day and act as insightful cosmological probes.Comment: 18 pages, 4 figures. Accepted by Science. Published electronically via Science Express on September 27, 200

Induced antiferromagnetism and large magnetoresistances in RuSr2(Nd,Y,Ce)2Cu2O10-d ruthenocuprates

RuSr2(Nd,Y,Ce)2Cu2O10-d ruthenocuprates have been studied by neutron diffraction, magnetotransport and magnetisation measurements and the electronic phase diagram is reported. Separate Ru and Cu spin ordering transitions are observed, with spontaneous Cu antiferromagnetic order for low hole doping levels p, and a distinct, induced-antiferromagnetic Cu spin phase in the 0.02 < p < 0.06 pseudogap region. This ordering gives rise to large negative magnetoresistances which vary systematically with p in the RuSr2Nd1.8-xY0.2CexCu2O10-d series. A collapse of the magnetoresistance (MR) and magnetisation in the pre-superconducting region may signify the onset of superconducting fluctuations.Comment: 22 pages, 11 figure

On the periodicity of oscillatory reconnection

Context. Oscillatory reconnection is a time-dependent magnetic reconnection mechanism that naturally produces periodic outputs from aperiodic drivers. Aims. This paper aims to quantify and measure the periodic nature of oscillatory reconnection for the first time. Methods. We solve the compressible, resistive, nonlinear magnetohydrodynamics (MHD) equations using 2.5D numerical simulations. Results. We identify two distinct periodic regimes: the impulsive and stationary phases. In the impulsive phase, we find the greater the amplitude of the initial velocity driver, the longer the resultant current sheet and the earlier its formation. In the stationary phase, we find that the oscillations are exponentially decaying and for driving amplitudes 6.3−126.2 kms−1, we measure stationary-phase periods in the range 56.3−78.9 s, i.e. these are high frequency (0.01−0.02 Hz) oscillations. In both phases, we find that the greater the amplitude of the initial velocity driver, the shorter the resultant period, but note that different physical processes and periods are associated with both phases. Conclusions. We conclude that the oscillatory reconnection mechanism behaves akin to a damped harmonic oscillator

Meeting update: faecal microbiota transplantation––bench, bedside, courtroom?

A group of stakeholders met, under the aegis of the British Society of Gastroenterology, to discuss the current landscape of faecal microbiota transplant- ation (FMT) within the UK and beyond. The meeting covered a wide range of topics, ranging from the practical aspects of establishing an FMT service and regu- latory issues relating to its delivery, to research implications and likely future directions

Non-symmetric magnetohydrostatic equilibria:a multigrid approach

Aims. Linear magnetohydrostatic (MHS) models of solar magnetic fields balance plasma pressure gradients, gravity and Lorentz forces where the current density is composed of a linear force-free component and a cross-field component that depends on gravitational stratification. In this paper, we investigate an efficient numerical procedure for calculating such equilibria.Methods. The MHS equations are reduced to two scalar elliptic equations – one on the lower boundary and the other within the interior of the computational domain. The normal component of the magnetic field is prescribed on the lower boundary and a multigrid method is applied on both this boundary and within the domain to find the poloidal scalar potential. Once solved to a desired accuracy, the magnetic field, plasma pressure and density are found using a finite difference method.Results. We investigate the effects of the cross-field currents on the linear MHS equilibria. Force-free and non-force-free examples are given to demonstrate the numerical scheme and an analysis of speed-up due to parallelization on a graphics processing unit (GPU) is presented. It is shown that speed-ups of ×30 are readily achievable