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Hydrodynamical numerical methods that converge with high-order hold particular promise for astrophysical studies, as they can in principle reach prescribed accuracy goals with higher computational efficiency than standard second- or third-order approaches. Here we consider the performance and accuracy benefits of Discontinuous Galerkin (DG) methods, which offer a particularly straightforward approach to reach extremely high order. Also, their computational stencil maps well to modern GPU devices, further raising the attractiveness of this approach. However, a traditional weakness of this method lies in the treatment of physical discontinuities such as shocks. We address this by invoking an artificial viscosity field to supply required dissipation where needed, and which can be augmented, if desired, with physical viscosity and thermal conductivity, yielding a high-order treatment of the Navier-Stokes equations for compressible fluids. We show that our approach results in sub-cell shock capturing ability, unlike traditional limiting schemes that tend to defeat the benefits of going to high order in DG in problems featuring many shocks. We demonstrate exponential convergence of our solver as a function of order when applied to smooth flows, such as the Kelvin-Helmholtz reference problem of arXiv:1509.03630. We also demonstrate excellent scalability of our GPU implementation up to hundreds of GPUs distributed on different compute nodes. In a first application to driven, sub-sonic turbulence, we highlight the accuracy advantages of high-order DG compared to traditional second-order accurate methods, and we stress the importance of physical viscosity for obtaining accurate velocity power spectra., Comment: 27 pages, 22 figures, accepted for publication in MNRAS

5 pages, 5 figures; International audience; We report on a two-dimensional (2D) V1–xPtxSe2 alloy that exhibits ferromagnetic order and Rashba spin–orbit coupling. Although ferromagnetism is absent in 1T-VSe2 because of the competition with the charge density wave phase, we demonstrate theoretically and experimentally that the substitution of vanadium by platinum in VSe2 (10–50%) to form a homogeneous 2D alloy restores ferromagnetic order down to one monolayer of V0.65Pt0.35Se2. Moreover, the presence of platinum atoms gives rise to Rashba spin–orbit coupling in (V,Pt)Se2, providing an original platform to study the interplay between ferromagnetism and spin–orbit coupling in the 2D limit.

Large collections of images have become readily available through modern digital catalogs, from sources as diverse as historical photographs, aerial surveys, or user-contributed pictures. Exploiting the quantitative information present in such wide-ranging collections can greatly benefit studies that follow the evolution of landscape features over decades, such as measuring areas of glaciers to study their shrinking under climate change. However, many available images were taken with low-quality lenses and unknown camera parameters. Useful quantitative data may still be extracted, but it becomes important to both account for imperfect optics, and estimate the uncertainty of the derived quantities. In this paper, we present a method to address both these goals, and apply it to the estimation of the area of a landscape feature traced as a polygon on the image of interest. The technique is based on a Bayesian formulation of the camera calibration problem. First, the probability density function (PDF) of the unknown camera parameters is determined for the image, based on matches between 2D (image) and 3D (world) points together with any available prior information. In a second step, the posterior distribution of the feature area of interest is derived from the PDF of camera parameters. In this step, we also model systematic errors arising in the polygon tracing process, as well as uncertainties in the digital elevation model. The resulting area PDF therefore accounts for most sources of uncertainty. We present validation experiments, and show that the model produces accurate and consistent results. We also demonstrate that in some cases, accounting for optical lens distortions is crucial for accurate area determination with consumer-grade lenses. The technique can be applied to many other types of quantitative features to be extracted from photographs when careful error estimation is important.

Reaction-diffusion systems involving a large number of unknowns and a wide spectrum of scales in space and time model various complex phenomena across different disciplines such as combustion science, plasma physics, or biomedical engineering. The numerical solution of these strongly multi-scale systems of partial differential equations entails specific challenges due to the potentially large stiffness stemming from the broad range of temporal scales in the nonlinear source term or from the presence of steep spatial gradients at the localized reaction fronts, thus hindering high-fidelity solutions at reasonable computational cost. A new generation of techniques featuring adaptation in space and time as well as error control has been introduced recently, yielding accurate solutions to such complex models while considering the entire spectrum of scales. Based on operator splitting, finite volume adaptive multiresolution, and high order time integrators with specific stability properties for each operator, these methods yield a high computational efficiency for stiff reaction-diffusion problems and were specifically designed to carry out large multidimensional simulations on standard architectures such as powerful workstations. While demonstrating the potential of such techniques the data structures of the original implementation, based on trees of pointers, provided limited opportunities for computational efficiency optimizations and posed challenges in terms of parallel programming and load balancing. The present contribution proposes a new implementation of the whole set of numerical methods including Radau5 and Rock4, relying on fully different data structures together with the use of a specific library, TBB, for shared-memory, task-based parallelism with work stealing. The performance of our implementation is assessed in a series of test-cases of increasing difficulty in two and three dimensions on multi-core and many-core architectures, demonstrating high scalability.

Faraday rotation is one of the most widely used observables to infer the strength and configuration of the magnetic field in the ionised gas of the Milky Way and nearby spiral galaxies. Here we compute synthetic Faraday rotation maps at $z=0$ for a set of disk galaxies from the Auriga high-resolution cosmological simulations, for different observer positions within and outside the galaxy. We find that the strength of the Faraday rotation of our simulated galaxies for a hypothetic observer at the solar circle is broadly consistent with the Faraday rotation seen for the Milky Way. The same holds for an observer outside the galaxy and the observed signal of the nearby spiral galaxy M51. However, we also find that the structure and angular power spectra of the synthetic all-sky Faraday rotation maps vary strongly with azimuthal position along the solar circle. We argue that this variation is a result of the structure of the magnetic field of the galaxy that is dominated by an azimuthal magnetic field ordered scales of several kpc, but has radial and vertical magnetic field components that are only ordered on scales of 1-2 kpc. Because the magnetic field strength decreases exponentially with height above the disk, the Faraday rotation for an observer at the solar circle is dominated by the local environment. This represents a severe obstacle for attempts to reconstruct the global magnetic field of the Milky Way from Faraday rotation maps alone without including additional observables., 10 pages, 10 figures, accepted by MNRAS

Current authentications with a secret key are based on the Challenge-Response paradigm. This mechanism avoids the clear transmission of the password in the network. Most protocols in Internet applications integrate this mechanism to authenticate a client before providing a service. Unfortunately this method in the protocol design increases the protocolar exchanges. This contribution is equivalent to Challenge-Response model but it permits to reduce the handshakes. Based on HMAC One-Time-Password (HOTP), the challenge is implicit in the user request. This solution has same security properties of a Challenge-Response authentication and become integrated naturally into Session Initiation Protocol (SIP); SIP is nowadays most popular Voice over IP protocol. Integrated in the existing SIP parameter, the solution doesn't change the SIP signaling, thus permitting a total interoperability between equipments modified or not.

Current authentication and security protocols provide authentication services using either shared secret keys or certificates and public key infrastructures. They usually involve using random values and nonce to prohibit replay attacks and to generate fresh keys per each session. The Challenge-Response authentication mechanisms are the predominant access method for authentication and access control of today Internet applications. These mechanisms provide a proof of knowledge of the secret and then authenticate the communicating entity, which applies a cryptographic algorithm on the shared secret and the challenge sent by the other entity. Unfortunately, basic challenge- response mechanisms, such as HTTP Digest, do not provide mutual authentication and therefore suffer from several attacks, especially the man-in-the-middle and replay attacks. In this paper, we propose a semantic meaning for the challenge becoming used by these mechanisms. The proposed enhancement is completely backward-compatible; an entity aware of our extension connecting to another that does not wish to use or does not support it, will continue the basic authentication process. Moreover, our extension helps in reducing the identity usurpation attacks. A computation of the cryptographic loads and the data transfer demonstrates a negligible overall performance impact on the network and the entities. Key words—Digest and password-based authentication, CHAP (Challenge-Handshake Authentication Protocol), WLAN.

Modern astrophysical simulations aim to accurately model an ever-growing array of physical processes, including the interaction of fluids with magnetic fields, under increasingly stringent performance and scalability requirements driven by present-day trends in computing architectures. Discontinuous Galerkin methods have recently gained some traction in astrophysics, because of their arbitrarily high order and controllable numerical diffusion, combined with attractive characteristics for high performance computing. In this paper, we describe and test our implementation of a discontinuous Galerkin (DG) scheme for ideal magnetohydrodynamics in the AREPO-DG code. Our DG-MHD scheme relies on a modal expansion of the solution on Legendre polynomials inside the cells of an Eulerian octree-based AMR grid. The divergence-free constraint of the magnetic field is enforced using one out of two distinct cell-centred schemes: either a Powell-type scheme based on nonconservative source terms, or a hyperbolic divergence cleaning method. The Powell scheme relies on a basis of locally divergence-free vector polynomials inside each cell to represent the magnetic field. Limiting prescriptions are implemented to ensure non-oscillatory and positive solutions. We show that the resulting scheme is accurate and robust: it can achieve high-order and low numerical diffusion, as well as accurately capture strong MHD shocks. In addition, we show that our scheme exhibits a number of attractive properties for astrophysical simulations, such as lower advection errors and better Galilean invariance at reduced resolution, together with more accurate capturing of barely resolved flow features. We discuss the prospects of our implementation, and DG methods in general, for scalable astrophysical simulations., 38 pages, 29 figures, 2 tables, accepted for publication in MNRAS

The circumgalactic medium (CGM) is one of the frontiers of galaxy formation and intimately connected to the galaxy via accretion of gas on to the galaxy and gaseous outflows from the galaxy. Here we analyse the magnetic field in the CGM of the Milky Way-like galaxies simulated as part of the \textsc{Auriga} project that constitutes a set of high resolution cosmological magnetohydrodynamical zoom simulations. We show that before $z=1$ the CGM becomes magnetised via galactic outflows that transport magnetised gas from the disk into the halo. At this time the magnetisation of the CGM closely follows its metal enrichment. We then show that at low redshift an in-situ turbulent dynamo that operates on a timescale of Gigayears further amplifies the magnetic field in the CGM and saturates before $z=0$. The magnetic field strength reaches a typical value of $0.1\,\mu G$ at the virial radius at $z=0$ and becomes mostly uniform within the virial radius. Its Faraday rotation signal is in excellent agreement with recent observations. For most of its evolution the magnetic field in the CGM is an unordered small scale field. Only strong coherent outflows at low redshift are able to order the magnetic field in parts of the CGM that are directly displaced by these outflows., Comment: 14 pages, 15 figures, accepted by MNRAS, updated Fig. 15 in proofs with errorbars

The magnetic fields observed in the Milky~Way and nearby galaxies appear to be in equipartition with the turbulent, thermal, and cosmic ray energy densities, and hence are expected to be dynamically important. However, the origin of these strong magnetic fields is still unclear, and most previous attempts to simulate galaxy formation from cosmological initial conditions have ignored them altogether. Here, we analyse the magnetic fields predicted by the simulations of the Auriga Project, a set of 30 high-resolution cosmological zoom simulations of Milky~Way-like galaxies, carried out with a moving-mesh magneto-hydrodynamics code and a detailed galaxy formation physics model. We find that the magnetic fields grow exponentially at early times owing to a small-scale dynamo with an e-folding time of roughly $100\,\rm{Myr}$ in the center of halos until saturation occurs around $z=2-3$, when the magnetic energy density reaches about $10\%$ of the turbulent energy density with a typical strength of $10-50\,\rm{\mu G}$. In the galactic centers the ratio between magnetic and turbulent energy remains nearly constant until $z=0$. At larger radii, differential rotation in the disks leads to linear amplification that typically saturates around $z=0.5$ to $z=0$. The final radial and vertical variations of the magnetic field strength can be well described by two joint exponential profiles, and are in good agreement with observational constraints. Overall, the magnetic fields have only little effect on the global evolution of the galaxies as it takes too long to reach equipartition. We also demonstrate that our results are well converged with numerical resolution., Comment: 16 pages, 16 figures, accepted by MNRAS, section 4.1 (turbulent dynamo) extended substantially, now includes magnetic and kinetic power spectra