Resistive magnetohydrodynamic reconnection : resolving long-term, chaotic dynamics

We acknowledge financial support from the EC FP7/2007-2013 Grant Agreement SWIFF (No. 263340) and from project GOA/2009/009 (KU Leuven). This research has been funded by the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office (IAP P7/08 CHARM). Part of the simulations used the infrastructure of the VSC-Flemish Supercomputer Center, funded by the Hercules Foundation and the Flemish Government-Department EWI. Another part of the simulations was done at the former Danish Center for Scientific Computing at Copenhagen University which is now part of DeIC Danish e-Infrastructure Cooperation.In this paper, we address the long-term evolution of an idealised double current system entering reconnection regimes where chaotic behavior plays a prominent role. Our aim is to quantify the energetics in high magnetic Reynolds number evolutions, enriched by secondary tearing events, multiple magnetic island coalescence, and compressive versus resistive heating scenarios. Our study will pay particular attention to the required numerical resolutions achievable by modern (grid-adaptive) computations, and comment on the challenge associated with resolving chaotic island formation and interaction. We will use shock-capturing, conservative, grid-adaptive simulations for investigating trends dominated by both physical (resistivity) and numerical (resolution) parameters, and confront them with (visco-)resistive magnetohydrodynamic simulations performed with very different, but equally widely used discretization schemes. This will allow us to comment on the obtained evolutions in a manner irrespective of the adopted discretization strategy. Our findings demonstrate that all schemes used (finite volume based shock-capturing, high order finite differences, and particle in cell-like methods) qualitatively agree on the various evolutionary stages, and that resistivity values of order 0.001 already can lead to chaotic island appearance. However, none of the methods exploited demonstrates convergence in the strong sense in these chaotic regimes. At the same time, nonperturbed tests for showing convergence over long time scales in ideal to resistive regimes are provided as well, where all methods are shown to agree. Both the advantages and disadvantages of specific discretizations as applied to this challenging problem are discussed.Publisher PDFPeer reviewe

Radiation of electrons in Weibel-generated fields: a general case

Weibel instability turns out to be the a ubiquitous phenomenon in High-Energy Density environments, ranging from astrophysical sources, e.g., gamma-ray bursts, to laboratory experiments involving laser-produced plasmas. Relativistic particles (electrons) radiate in the Weibel-produced magnetic fields in the Jitter regime. Conventionally, in this regime, the particle deflections are considered to be smaller than the relativistic beaming angle of 1/$\gamma$ ($\gamma$ being the Lorentz factor of an emitting particle) and the particle distribution is assumed to be isotropic. This is a relatively idealized situation as far as lab experiments are concerned. We relax the assumption of the isotropy of radiating particle distribution and present the extension of the jitter theory amenable for comparisons with experimental data.Comment: Proceedings of International Conference on HEDP/HEDLA-0

Demographic reconstruction from ancient DNA supports rapid extinction of the great auk

The great auk was once abundant and distributed across the North Atlantic. It is now extinct, having been heavily exploited for its eggs, meat, and feathers. We investigated the impact of human hunting on its demise by integrating genetic data, GPS-based ocean current data, and analyses of population viability. We sequenced complete mitochondrial genomes of 41 individuals from across the species’ geographic range and reconstructed population structure and population dynamics throughout the Holocene. Taken together, our data do not provide any evidence that great auks were at risk of extinction prior to the onset of intensive human hunting in the early 16th century. In addition, our population viability analyses reveal that even if the great auk had not been under threat by environmental change, human hunting alone could have been sufficient to cause its extinction. Our results emphasise the vulnerability of even abundant and widespread species to intense and localised exploitation

Measurement of B(D_s+ -> mu+ nu_mu)/B(D_s+ -> phi mu+ nu_mu) and Determination of the Decay Constant f_{D_s}

We have observed $23.2 \pm 6.0_{-0.9}^{+1.0}$ purely-leptonic decays of $D_s^+ -> \mu^+ \nu_\mu$ from a sample of muonic one prong decay events detected in the emulsion target of Fermilab experiment E653. Using the $D_s^+ -> \phi \mu^+ \nu_\mu$ yield measured previously in this experiment, we obtain $B(D_s^+ --> \mu^+ \nu_\mu) / B(D_s^+ --> \phi \mu^+ \nu_\mu) =0.16 \pm 0.06 \pm 0.03$. In addition, we extract the decay constant $f_{D_s}=194 \pm 35 \pm 20 \pm 14 MeV$.Comment: 15 pages including one figur

First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes

We overview nonequilibrium Green function combined with density functional theory (NEGF-DFT) modeling of independent electron and phonon transport in nanojunctions with applications focused on a new class of thermoelectric devices where a single molecule is attached to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make possible injection of evanescent wavefunctions from ZGNRs, so that their overlap within the molecular region generates a peak in the electronic transmission. Additionally, the spatial symmetry properties of the transverse propagating states in the ZGNR electrodes suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with diminished phonon conductance through a ZGNR/molecule/ZGNR inhomogeneous structure, yields the thermoelectric figure of merit ZT~0.5 at room temperature and 0.5<ZT<2.5 below liquid nitrogen temperature. The reliance on evanescent mode transport and symmetry of propagating states in the electrodes makes the electronic-transport-determined power factor in this class of devices largely insensitive to the type of sufficiently short conjugated organic molecule, which we demonstrate by showing that both 18-annulene and C10 molecule sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can search for molecules that will further reduce the phonon thermal conductance (in the denominator of ZT) while keeping the electronic power factor (in the nominator of ZT) optimized. We also show how often employed Brenner empirical interatomic potential for hydrocarbon systems fails to describe phonon transport in our single-molecule nanojunctions when contrasted with first-principles results obtained via NEGF-DFT methodology.Comment: 20 pages, 6 figures; mini-review article prepared for the special issue of the Journal of Computational Electronics on "Simulation of Thermal, Thermoelectric, and Electrothermal Phenomena in Nanostructures", edited by I. Knezevic and Z. Aksamij

Load management strategy for Particle-In-Cell simulations in high energy particle acceleration

International audienceIn the wake of the intense effort made for the experimental CILEX project, numerical simulation campaigns have been carried out in order to finalize the design of the facility and to identify optimal laser and plasma parameters. These simulations bring, of course, important insight into the fundamental physics at play. As a by-product, they also characterize the quality of our theoretical and numerical models. In this paper, we compare the results given by different codes and point out algorithmic limitations both in terms of physical accuracy and computational performances. These limitations are illustrated in the context of electron l aser w ake f ield a cceleration (LWFA). The main limitation we identify in state-of-the-art P article- I n- C ell (PIC) codes is computational load imbalance. We propose an innovative algorithm to deal with this specific issue as well as milestones towards a modern, accurate high-performance PIC code for high energy particle acceleration

From cooperative data collection to full collaboration and co-management: a synthesis of the 2014 ICES symposium on fishery-dependent information

In this paper, we synthesize information presented at the 2nd Fishery Dependent Information (FDI) Conference, held in Rome, Italy, from 2 to 6 March 2014. We review current issues and advances in the collection, interpretation and application of fishery-dependent data, and highlight emergent findings in the field. Key issues include (i) the design and collection of data associated with commercial and recreational fisheries and the use of these data to support conventional and novel approaches to fisheries science and management and (ii) the role of fishers in co-management and policy setting. We noted that since the 2010 FDI conference a paradigm shift towards full engagement of key stakeholders started to take place. It also became evident that trust between stakeholders, managers, and scientists is necessary to develop efficient fishery monitoring programmes. While building such trust among key players often begins in informal settings, eventually one must evolve structured, formalized, and agreed processes for such interactions. We also conclude that because of the diversity of fisheries any determination of “best practices” may be difficult. Instead, we provide a list of “best principles” emerged from the conference

Physical activity in the elderly is associated with improved executive function and processing speed : the LADIS Study

Objectives Physical activity reduces the risk of cognitive decline but may affect cognitive domains differently. We examined whether physical activity modifies processing speed, executive function and memory in a population of non-dementia elderly subjects with age-related white matter changes (ARWMC). Methods Data from the Leukoaraiosis And DISability (LADIS) study, a multicenter, European prospective cohort study aimed at examining the role of ARWMC in transition to disability, was used. Subjects in the LADIS study were clinically assessed yearly for 3-years including MRI at baseline and 3-year follow-up. Physical activity was assessed at baseline, and cognitive compound scores at baseline and 3-year assessment were used. Results Two-hundred-eighty-two subjects (age, y (mean (SD)): 73.1 (\ub15.1); gender (f/m): 164/118); MMSE (mean (SD)): 28.3 (\ub11.7)) who had not progressed to MCI or dementia, were included. Multiple variable linear regression analysis with baseline MMSE, education, gender, age, stroke, diabetes and ARWMC rating as covariates revealed that physical activity was associated with better scores at baseline and 3-year follow-up for executive function (baseline: \u3b2: 0.39, 95% CI: 0.13-0.90, p-=-0.008; follow-up: \u3b2: 0.24, 95% CI: 0.10-0.38, p-=-0.001) and processing speed (baseline: \u3b2: 0.48, 95% CI: 0.14-0.89, p-=-0.005; follow-up: \u3b2: 0.15, 95% CI: 0.02-0.29, p-=-0.02) but not memory. When including baseline cognitive score as a covariate in the analysis of 3-year follow-up scores, executive function remained significant (\u3b2: 0.11, 95% CI: 0-0.22, p-=-0.04). Conclusion Our findings confirm previous findings of a positive effect of physical activity on cognitive functions in elderly subjects, and further extends these by showing that the association is also present in patients with ARWMC