263 research outputs found
Porting a Hall MHD Code to a Graphic Processing Unit
We present our experience porting a Hall MHD code to a Graphics Processing Unit (GPU). The code is a 2nd order accurate MUSCL-Hancock scheme which makes use of an HLL Riemann solver to compute numerical fluxes and second-order finite differences to compute the Hall contribution to the electric field. The divergence of the magnetic field is controlled with Dedner?s hyperbolic divergence cleaning method. Preliminary benchmark tests indicate a speedup (relative to a single Nehalem core) of 58x for a double precision calculation. We discuss scaling issues which arise when distributing work across multiple GPUs in a CPU-GPU cluster
The role of the Hall effect in the global structure and dynamics of planetary magnetospheres: Ganymede as a case study
We present high resolution Hall MHD simulations of Ganymede's magnetosphere
demonstrating that Hall electric fields in ion-scale magnetic reconnection
layers have significant global effects not captured in resistive MHD
simulations. Consistent with local kinetic simulations of magnetic
reconnection, our global simulations show the development of intense
field-aligned currents along the magnetic separatrices. These currents extend
all the way down to the moon's surface, where they may contribute to Ganymede's
aurora. Within the magnetopause and magnetotail current sheets, Hall currents
in the reconnection plane accelerate ions to the local Alfv\'en speed in the
out-of-plane direction, producing a global system of ion drift belts that
circulates Jovian magnetospheric plasma throughout Ganymede's magnetosphere. We
discuss some observable consequences of these Hall-induced currents and ion
drifts: the appearance of a sub-Jovian "double magnetopause" structure, an
Alfv\'enic ion jet extending across the upstream magnetopause and an asymmetric
pattern of magnetopause Kelvin-Helmholtz waves.Comment: 14 pages, 12 figures; presented at Geospace Environment Modeling
(GEM) workshop (June, 2014) and Fall American Geophysical Union (AGU) meeting
(December, 2014); submitted to Journal of Geophysical Research, December 201
Quantifying the Effect of Non-Larmor Motion of Electrons on the Pressure Tensor
In space plasma, various effects of magnetic reconnection and turbulence
cause the electron motion to significantly deviate from their Larmor orbits.
Collectively these orbits affect the electron velocity distribution function
and lead to the appearance of the "non-gyrotropic" elements in the pressure
tensor. Quantification of this effect has important applications in space and
laboratory plasma, one of which is tracing the electron diffusion region (EDR)
of magnetic reconnection in space observations. Three different measures of
agyrotropy of pressure tensor have previously been proposed, namely,
, and . The multitude of contradictory measures has
caused confusion within the community. We revisit the problem by considering
the basic properties an agyrotropy measure should have. We show that
, and are all defined based on the sum of the
principle minors (i.e. the rotation invariant ) of the pressure tensor. We
discuss in detail the problems of -based measures and explain why they may
produce ambiguous and biased results. We introduce a new measure
constructed based on the determinant of the pressure tensor (i.e. the rotation
invariant ) which does not suffer from the problems of -based
measures. We compare with other measures in 2 and 3-dimension
particle-in-cell magnetic reconnection simulations, and show that can
effectively trace the EDR of reconnection in both Harris and force-free current
sheets. On the other hand, does not show prominent peaks in
the EDR and part of the separatrix in the force-free reconnection simulations,
demonstrating that does not measure all the non-gyrotropic
effects in this case, and is not suitable for studying magnetic reconnection in
more general situations other than Harris sheet reconnection.Comment: accepted by Phys. of Plasm
Does the Hall Effect Solve the Flux Pileup Saturation Problem?
It is well known that magnetic flux pileup can significantly speed up the rate of magnetic reconnection in high Lundquist number resistive MHD,allowing reconnection to proceed at a rate which is insensitive to the plasma resistivity over a wide range of Lundquist number. Hence, pileup is a possible solution to the Sweet-Parker time scale problem. Unfortunately, pileup tends to saturate above a critical value of the Lundquist number, S_c, where the value ofS_c depends on initial and boundary conditions, with Sweet-Parker scaling returning above S_c. It has been argued (see Dorelli and Bim [2003] and Dorelli [2003]) that the Hall effect can allow flux pileup to saturate (when the scale of the current sheet approaches ion inertial scale, di) before the reconnection rate begins to stall. However, the resulting saturated reconnection rate, while insensitive to the plasma resistivity, was found to depend strongly on the di. In this presentation, we revisit the problem of magnetic island coalescence (which is a well known example of flux pileup reconnection), addressing the dependence of the maximum coalescence rate on the ratio of di in the "large island" limit in which the following inequality is always satisfied: l_eta di lambda, where I_eta is the resistive diffusion length and lambda is the island wavelength
Tracing magnetic separators and their dependence on IMF clock angle in global magnetospheric simulations
A new, efficient, and highly accurate method for tracing magnetic separators
in global magnetospheric simulations with arbitrary clock angle is presented.
The technique is to begin at a magnetic null and iteratively march along the
separator by finding where four magnetic topologies meet on a spherical
surface. The technique is verified using exact solutions for separators
resulting from an analytic magnetic field model that superposes dipolar and
uniform magnetic fields. Global resistive magnetohydrodynamic simulations are
performed using the three-dimensional BATS-R-US code with a uniform
resistivity, in eight distinct simulations with interplanetary magnetic field
(IMF) clock angles ranging from 0 (parallel) to 180 degrees (anti-parallel).
Magnetic nulls and separators are found in the simulations, and it is shown
that separators traced here are accurate for any clock angle, unlike the last
closed field line on the Sun-Earth line that fails for southward IMF. Trends in
magnetic null locations and the structure of magnetic separators as a function
of clock angle are presented and compared with those from the analytic field
model. There are many qualitative similarities between the two models, but
quantitative differences are also noted. Dependence on solar wind density is
briefly investigated.Comment: 10 pages, 10 figures, Presented at 2012 AGU Fall Meeting and 2013
Geospace Environment Modeling (GEM) Worksho
A Simple GPU-Accelerated Two-Dimensional MUSCL-Hancock Solver for Ideal Magnetohydrodynamics
We describe our experience using NVIDIA's CUDA (Compute Unified Device Architecture) C programming environment to implement a two-dimensional second-order MUSCL-Hancock ideal magnetohydrodynamics (MHD) solver on a GTX 480 Graphics Processing Unit (GPU). Taking a simple approach in which the MHD variables are stored exclusively in the global memory of the GTX 480 and accessed in a cache-friendly manner (without further optimizing memory access by, for example, staging data in the GPU's faster shared memory), we achieved a maximum speed-up of approx. = 126 for a sq 1024 grid relative to the sequential C code running on a single Intel Nehalem (2.8 GHz) core. This speedup is consistent with simple estimates based on the known floating point performance, memory throughput and parallel processing capacity of the GTX 480
Radios communautaires de Dakar: communication pour le développement et extraversion
Au lendemain de la libération des ondes au Sénégal à la fin des années 1990, les radios communautaires, issues de mouvements sociaux et associatifs, émergent. Simultanément, les politiques de développement se redéfinissent autour du « développer local », devenant de façon circonstancielle le point d’ancrage des nouvelles radios à travers des « partenariats de développement ». S’inscrivant dans la continuité de l’histoire de la radio en Afrique, une histoire commencée sous l’égide coloniale, ces radios communautaires réarticulent des pratiques culturelles teintées d’extraversion et d’ambivalence. Ayant comme source principale des entretiens effectués au cours de
l’automne 2009, ce mémoire s’attelle à explorer la réalité des radios communautaires de Dakar, la capitale du Sénégal. Plus particulièrement, il s’agit d’interroger les relations
qu’elles entretiennent avec leurs « partenaires d’appuis » afin de dévoiler les codes et pratiques du milieu. En interrogeant les rapports de pouvoir, de collaboration et
d’affirmation, ce mémoire se penchera sur les problématiques de « glocalisation » et d’ « extraversion ». Ces phénomènes propres à un local entrepris par un international seront appréhendés afin de mettre en avant les attitudes de négociations et d’appropriation des radios communautaires face au milieu du développement. Les travaux de Jean-François Bayart et Achille Mbembe appuieront notre propos pour une compréhension du milieu valorisant les acteurs et leurs modes d’action. Les réflexions méthodologiques et théoriques s’efforceront de retranscrire la complexité du milieu représenté
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