1,758 research outputs found
Particle orbits at the magnetopause: Kelvin-Helmholtz induced trapping
The Kelvin-Helmholtz instability (KHI) is a known mechanism for penetration
of solar wind matter into the magnetosphere. Using three-dimensional, resistive
magnetohydrodynamic simulations, the double mid-latitude reconnection (DMLR)
process was shown to efficiently exchange solar wind matter into the
magnetosphere, through mixing and reconnection. Here, we compute test particle
orbits through DMLR configurations. In the instantaneous electromagnetic
fields, charged particle trajectories are integrated using the guiding centre
approximation. The mechanisms involved in the electron particle orbits and
their kinetic energy evolutions are studied in detail, to identify specific
signatures of the DMLR through particle characteristics. The charged particle
orbits are influenced mainly by magnetic curvature drifts. We identify complex,
temporarily trapped, trajectories where the combined electric field and
(reconnected) magnetic field variations realize local cavities where particles
gain energy before escaping. By comparing the orbits in strongly deformed
fields due to the KHI development, with the textbook mirror-drift orbits
resulting from our initial configuration, we identify effects due to current
sheets formed in the DMLR process. We do this in various representative stages
during the DMLR development.Comment: Matching accepted version in AGU JGR: Space Physic
Simulations of MHD Instabilities in Intracluster Medium Including Anisotropic Thermal Conduction
We perform a suite of simulations of cooling cores in clusters of galaxies in
order to investigate the effect of the recently discovered heat flux buoyancy
instability (HBI) on the evolution of cores. Our models follow the
3-dimensional magnetohydrodynamics (MHD) of cooling cluster cores and capture
the effects of anisotropic heat conduction along the lines of magnetic field,
but do not account for the cosmological setting of clusters or the presence of
AGN. Our model clusters can be divided into three groups according to their
final thermodynamical state: catastrophically collapsing cores, isothermal
cores, and an intermediate group whose final state is determined by the initial
configuration of magnetic field. Modeled cores that are reminiscent of real
cluster cores show evolution towards thermal collapse on a time scale which is
prolonged by a factor of ~2-10 compared with the zero-conduction cases. The
principal effect of the HBI is to re-orient field lines to be perpendicular to
the temperature gradient. Once the field has been wrapped up onto spherical
surfaces surrounding the core, the core is insulated from further conductive
heating (with the effective thermal conduction suppressed to less than 1/100th
of the Spitzer value) and proceeds to collapse. We speculate that, in real
clusters, the central AGN and possibly mergers play the role of "stirrers,"
periodically disrupting the azimuthal field structure and allowing thermal
conduction to sporadically heat the core.Comment: 16 pages, 3 tables, 17 figures, accepted to ApJ with minor revisions,
to appear in Volume 704, Oct 20, 2009 issu
Divergence-Free Adaptive Mesh Refinement for Magnetohydrodynamics
In this paper we present a full-fledged scheme for the second order accurate,
divergence-free evolution of vector fields on an adaptive mesh refinement (AMR)
hierarchy. We focus here on adaptive mesh MHD. The scheme is based on making a
significant advance in the divergence-free reconstruction of vector fields. In
that sense, it complements the earlier work of Balsara and Spicer (1999) where
we discussed the divergence-free time-update of vector fields which satisfy
Stoke's law type evolution equations. Our advance in divergence-free
reconstruction of vector fields is such that it reduces to the total variation
diminishing (TVD) property for one-dimensional evolution and yet goes beyond it
in multiple dimensions. Divergence-free restriction is also discussed. An
electric field correction strategy is presented for use on AMR meshes. The
electric field correction strategy helps preserve the divergence-free evolution
of the magnetic field even when the time steps are sub-cycled on refined
meshes. The above-mentioned innovations have been implemented in Balsara's
RIEMANN framework for parallel, self-adaptive computational astrophysics which
supports both non-relativistic and relativistic MHD. Several rigorous, three
dimensional AMR-MHD test problems with strong discontinuities have been run
with the RIEMANN framework showing that the strategy works very well.Comment: J.C.P., figures of reduced qualit
Assessing the role of oxygen on ring current formation and evolution through numerical experiments
We address the effect of ionospheric outflow and magnetospheric ion composition on the physical processes that control the development of the 5 August 2011 magnetic storm. Simulations with the Space Weather Modeling Framework are used to investigate the global dynamics and energization of ions throughout the magnetosphere during storm time, with a focus on the formation and evolution of the ring current. Simulations involving multifluid (with variable H+/O+ ratio in the inner magnetosphere) and singleâfluid (with constant H+/O+ ratio in the inner magnetosphere) MHD for the global magnetosphere with inner boundary conditions set either by specifying a constant ion density or by physicsâbased calculations of the ion fluxes reveal that dynamical changes of the ion composition in the inner magnetosphere alter the total energy density of the magnetosphere, leading to variations in the magnetic field as well as particle drifts throughout the simulated domain. A low oxygen to hydrogen ratio and outflow resulting from a constant ion density boundary produced the most disturbed magnetosphere, leading to a stronger ring current but misses the timing of the storm development. Conversely, including a physicsâbased solution for the ionospheric outflow to the magnetosphere system leads to a reduction in the crossâpolar cap potential (CPCP). The increased presence of oxygen in the inner magnetosphere affects the global magnetospheric structure and dynamics and brings the nightside reconnection point closer to the Earth. The combination of reduced CPCP together with the formation of the reconnection line closer to the Earth yields less adiabatic heating in the magnetotail and reduces the amount of energetic plasma that has access to the inner magnetosphere.Key PointsLow O+/H+ ratio produced stronger ring currentInclusion of physicsâbased ionospheric outflow leads to a reduction in the CPCPOxygen presence is linked to a nightside reconnection point closer to the EarthPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112251/1/jgra51856.pd
A Multiscale Approach Using Patches of Finite Elements for Solving Wave Propagation Problems in Microwave Discharge Plasma
We consider the development of an efficient numerical method for the simulation of microwave discharge plasmas. The method uses the idea of finite element patch and can deal with very disparate length scales of the plasma. In this paper, the time-domain Maxwell's equations, which are coupled with the plasma transport equations via the time-varying electron current density, are solved with a two-level Schwarz type algorithm based on a variational formulation of the standard Yee scheme. The patch of finite elements is used to calculate in an iterative manner the solution in the plasma region where a better precision is required. This numerical approach provides the Yee scheme with an efficient local-grid refinement capacity while preserving its stability. A numerical analysis shows its accuracy and computational efficiency on nested Cartesian grids. Simulation of a microwave breakdown in air under atmospheric pressure is then performed and results are discussed. We believe that both the inherent versatility with regard to the variational formulation and the efficiency of the proposed method can make it particularly suitable in modeling of microwave discharge plasmas by providing more insights of their nature and behavior
Investigation of upwind, multigrid, multiblock numerical schemes for three dimensional flows. Volume 1: Runge-Kutta methods for a thin layer Navier-Stokes solver
A state-of-the-art computer code has been developed that incorporates a modified Runge-Kutta time integration scheme, upwind numerical techniques, multigrid acceleration, and multi-block capabilities (RUMM). A three-dimensional thin-layer formulation of the Navier-Stokes equations is employed. For turbulent flow cases, the Baldwin-Lomax algebraic turbulence model is used. Two different upwind techniques are available: van Leer's flux-vector splitting and Roe's flux-difference splitting. Full approximation multi-grid plus implicit residual and corrector smoothing were implemented to enhance the rate of convergence. Multi-block capabilities were developed to provide geometric flexibility. This feature allows the developed computer code to accommodate any grid topology or grid configuration with multiple topologies. The results shown in this dissertation were chosen to validate the computer code and display its geometric flexibility, which is provided by the multi-block structure
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Gaussian Process Modeling for Upsampling Algorithms With Applications in Computer Vision and Computational Fluid Dynamics
Across a variety of fields, interpolation algorithms have been used to upsample lowresolution or coarse data fields. In this work, novel Gaussian Process based methodsare employed to solve a variety of upsampling problems. Specifically threeapplications are explored: coarse data prolongation in Adaptive Mesh Refinement(AMR) in the field of Computational Fluid Dynamics, accurate document imageupsampling to enhance Optical Character Recognition (OCR) accuracy, and fastand accurate Single Image Super Resolution (SISR). For AMR, a new, efficient,and â3rd order accurateâ algorithm called GP-AMR is presented. Next, a novel,non-zero mean, windowed GP model is generated to upsample low resolution documentimages to generate a higher OCR accuracy, when compared to the industrystandard. Finally, a hybrid GP convolutional neural network algorithm is used togenerate a computationally efficient and high quality SISR model
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