The Experts below are selected from a list of 28611 Experts worldwide ranked by ideXlab platform
Alexandre Marcowith - One of the best experts on this subject based on the ideXlab platform.
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On Fermi acceleration and magnetohydrodynamic instabilities at ultra-relativistic magnetized shock waves
Monthly Notices of the Royal Astronomical Society, 2009Co-Authors: Guy Pelletier, Martin Lemoine, Alexandre MarcowithAbstract:Fermi acceleration can take place at ultra-relativistic shock waves if the upstream or downstream magnetic field has been remodelled so that most of the magnetic power lies on short spatial scales. The relevant conditions under which Fermi acceleration becomes efficient in the presence of both coherent and short-scale turbulent magnetic fields are addressed. Within the magnetohydrodynamic (MHD) approximation, this paper then studies the amplification of a pre-existing magnetic field through the streaming of cosmic rays upstream of a relativistic shock wave. The magnetic field is assumed to be perpendicular in the shock-Front Frame, as generally expected in the limit of large shock Lorentz factor. In the MHD regime, compressive instabilities seeded by the net cosmic-ray charge in the shock precursor (as seen in the shock-Front Frame) develop on the shortest spatial scales but saturate at a moderate level δB/B ∼ 1, which is not sufficient for Fermi acceleration. As we argue, it is possible that other instabilities outside the MHD range provide enough amplification to allow successful Fermi acceleration.
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On Fermi acceleration and MHD-instabilities at ultra-relativistic magnetized shock waves
Monthly Notices of the Royal Astronomical Society, 2008Co-Authors: Guy Pelletier, Martin Lemoine, Alexandre MarcowithAbstract:Fermi acceleration can take place at ultra-relativistic shock waves if theupstream or downstream magnetic field has been remodeled so that most of themagnetic power lies on short spatial scales. The relevant conditions underwhich Fermi acceleration become efficient in the presence of both a coherentand a short scale turbulent magnetic field are addressed. Within the MHDapproximation, this paper then studies the amplification of a pre-existingmagnetic field through the streaming of cosmic rays upstream of a relativisticshock wave. The magnetic field is assumed to be perpendicular in the shockFront Frame, as generally expected in the limit of large shock Lorentz factor.In the MHD regime, compressive instabilities seeded by the net cosmic-raycharge in the shock precursor (as seen in the shock Front Frame) develop on theshortest spatial scales but saturate at a moderate level deltaB/B ~ 1, which isnot sufficient for Fermi acceleration. As we argue, it is possible that otherinstabilities outside the MHD range provide enough amplification to allowsuccessful Fermi acceleration.
Guy Pelletier - One of the best experts on this subject based on the ideXlab platform.
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On Fermi acceleration and magnetohydrodynamic instabilities at ultra-relativistic magnetized shock waves
Monthly Notices of the Royal Astronomical Society, 2009Co-Authors: Guy Pelletier, Martin Lemoine, Alexandre MarcowithAbstract:Fermi acceleration can take place at ultra-relativistic shock waves if the upstream or downstream magnetic field has been remodelled so that most of the magnetic power lies on short spatial scales. The relevant conditions under which Fermi acceleration becomes efficient in the presence of both coherent and short-scale turbulent magnetic fields are addressed. Within the magnetohydrodynamic (MHD) approximation, this paper then studies the amplification of a pre-existing magnetic field through the streaming of cosmic rays upstream of a relativistic shock wave. The magnetic field is assumed to be perpendicular in the shock-Front Frame, as generally expected in the limit of large shock Lorentz factor. In the MHD regime, compressive instabilities seeded by the net cosmic-ray charge in the shock precursor (as seen in the shock-Front Frame) develop on the shortest spatial scales but saturate at a moderate level δB/B ∼ 1, which is not sufficient for Fermi acceleration. As we argue, it is possible that other instabilities outside the MHD range provide enough amplification to allow successful Fermi acceleration.
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On Fermi acceleration and MHD-instabilities at ultra-relativistic magnetized shock waves
Monthly Notices of the Royal Astronomical Society, 2008Co-Authors: Guy Pelletier, Martin Lemoine, Alexandre MarcowithAbstract:Fermi acceleration can take place at ultra-relativistic shock waves if theupstream or downstream magnetic field has been remodeled so that most of themagnetic power lies on short spatial scales. The relevant conditions underwhich Fermi acceleration become efficient in the presence of both a coherentand a short scale turbulent magnetic field are addressed. Within the MHDapproximation, this paper then studies the amplification of a pre-existingmagnetic field through the streaming of cosmic rays upstream of a relativisticshock wave. The magnetic field is assumed to be perpendicular in the shockFront Frame, as generally expected in the limit of large shock Lorentz factor.In the MHD regime, compressive instabilities seeded by the net cosmic-raycharge in the shock precursor (as seen in the shock Front Frame) develop on theshortest spatial scales but saturate at a moderate level deltaB/B ~ 1, which isnot sufficient for Fermi acceleration. As we argue, it is possible that otherinstabilities outside the MHD range provide enough amplification to allowsuccessful Fermi acceleration.
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Fermi Acceleration at relativistic Shocks
AIP Conference Proceedings, 2008Co-Authors: Guy Pelletier, Martin Lemoine, A. MarcowithAbstract:After a successful development of theoretical and numerical works on Fermi acceleration at relativistic shocks, some difficulties recently raised with the scattering issue, a crucial aspect of the process. Most pioneering works were developed assuming the scattering off magnetic fluctuations as given. Even in that case, when a mean field is considered, its orientation is mostly perpendicular to the shock normal in the Front Frame, and this tends to quench the scattering process. Solving this difficulty leads to address the issue of the generation of very intense magnetic fluctuations at short wave lengths. The relativistic motion of the shock Front let the cosmic rays to visit upstream during a very short time only, making this generation of magnetic fluctuations very challenging. Anyway there is some hope to solve the problem. Thanks to a recent work by Spitkovsky (2008) \cite{AS}, we know that the process works without any mean field and now we have to investigate up to which intensity the mean field can be amplified for allowing Fermi process with appropriate fast instabilities. In this presentation, the collisionless shock structure in relativistic regime is sketched, the scattering issue is presented, and the instabilities that can provide the expected magnetic field amplification are presented as well. Although there exists observational evidence that particles are accelerated in relativistic flows and are distributed according to a power law suggesting a Fermi process, the drastic conditions for Fermi process to work are not always clearly fulfilled.
Marc Vanderhaeghen - One of the best experts on this subject based on the ideXlab platform.
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Quark transverse charge densities in the Delta(1232) from lattice QCD
Nuclear Physics, 2009Co-Authors: Constantia Alexandrou, Tomasz Korzec, Giannis Koutsou, Cédric Lorcé, John W. Negele, Vladimir Pascalutsa, Antonios Tsapalis, Marc VanderhaeghenAbstract:Abstract We extend the formalism relating electromagnetic form factors to transverse quark charge densities in the light-Front Frame to the case of a spin-3/2 baryon and calculate these transverse densities for the Δ ( 1232 ) isobar using lattice QCD. The transverse charge densities for a transversely polarized spin-3/2 particle are characterized by monopole, dipole, quadrupole, and octupole patterns representing the structure beyond that of a pure point-like spin-3/2 particle. We present lattice QCD results for the Δ-isobar electromagnetic form factors for pion masses down to approximatively 350 MeV for three cases: quenched QCD, two-degenerate flavors of dynamical Wilson quarks, and three flavors of quarks using a mixed action that combines domain wall valence quarks and dynamical staggered sea quarks. We extract transverse quark charge densities from these lattice results and find that the Δ is prolately deformed, as indicated by the fact that the quadrupole moment G E 2 ( 0 ) is larger than the value −3 characterizing a point particle and the fact that the transverse charge density in a Δ + of maximal transverse spin projection is elongated along the axis of the spin.
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Quark transverse charge densities in the from lattice QCD
Nuclear Physics A, 2009Co-Authors: Constantia Alexandrou, Tomasz Korzec, Giannis Koutsou, Cédric Lorcé, John W. Negele, Vladimir Pascalutsa, Antonios Tsapalis, Marc VanderhaeghenAbstract:We extend the formalism relating electromagnetic form factors to transverse quark charge densities in the light-Front Frame to the case of a spin-3/2 baryon and calculate these transverse densities for the $\Delta(1232)$ isobar using lattice QCD. The transverse charge densities for a transversely polarized spin-3/2 particle are characterized by monopole, dipole, quadrupole, and octupole patterns representing the structure beyond that of a pure point-like spin-3/2 particle. We present lattice QCD results for the $\Delta$-isobar electromagnetic form factors for pion masses down to approximatively 350 MeV for three cases: quenched QCD, two-degenerate flavors of dynamical Wilson quarks, and three flavors of quarks using a mixed action that combines domain wall valence quarks and dynamical staggered sea quarks. We extract transverse quark charge densities from these lattice results and find that the $\Delta$ is prolately deformed, as indicated by the fact that the quadrupole moment $G_{E2}(0$) is larger than the value -3 characterizing a point particle and the fact that the transverse charge density in a $\Delta^+$ of maximal transverse spin projection is elongated along the axis of the spin.Comment: 35 pages, 10 figure
Martin Lemoine - One of the best experts on this subject based on the ideXlab platform.
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On Fermi acceleration and magnetohydrodynamic instabilities at ultra-relativistic magnetized shock waves
Monthly Notices of the Royal Astronomical Society, 2009Co-Authors: Guy Pelletier, Martin Lemoine, Alexandre MarcowithAbstract:Fermi acceleration can take place at ultra-relativistic shock waves if the upstream or downstream magnetic field has been remodelled so that most of the magnetic power lies on short spatial scales. The relevant conditions under which Fermi acceleration becomes efficient in the presence of both coherent and short-scale turbulent magnetic fields are addressed. Within the magnetohydrodynamic (MHD) approximation, this paper then studies the amplification of a pre-existing magnetic field through the streaming of cosmic rays upstream of a relativistic shock wave. The magnetic field is assumed to be perpendicular in the shock-Front Frame, as generally expected in the limit of large shock Lorentz factor. In the MHD regime, compressive instabilities seeded by the net cosmic-ray charge in the shock precursor (as seen in the shock-Front Frame) develop on the shortest spatial scales but saturate at a moderate level δB/B ∼ 1, which is not sufficient for Fermi acceleration. As we argue, it is possible that other instabilities outside the MHD range provide enough amplification to allow successful Fermi acceleration.
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On Fermi acceleration and MHD-instabilities at ultra-relativistic magnetized shock waves
Monthly Notices of the Royal Astronomical Society, 2008Co-Authors: Guy Pelletier, Martin Lemoine, Alexandre MarcowithAbstract:Fermi acceleration can take place at ultra-relativistic shock waves if theupstream or downstream magnetic field has been remodeled so that most of themagnetic power lies on short spatial scales. The relevant conditions underwhich Fermi acceleration become efficient in the presence of both a coherentand a short scale turbulent magnetic field are addressed. Within the MHDapproximation, this paper then studies the amplification of a pre-existingmagnetic field through the streaming of cosmic rays upstream of a relativisticshock wave. The magnetic field is assumed to be perpendicular in the shockFront Frame, as generally expected in the limit of large shock Lorentz factor.In the MHD regime, compressive instabilities seeded by the net cosmic-raycharge in the shock precursor (as seen in the shock Front Frame) develop on theshortest spatial scales but saturate at a moderate level deltaB/B ~ 1, which isnot sufficient for Fermi acceleration. As we argue, it is possible that otherinstabilities outside the MHD range provide enough amplification to allowsuccessful Fermi acceleration.
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Fermi Acceleration at relativistic Shocks
AIP Conference Proceedings, 2008Co-Authors: Guy Pelletier, Martin Lemoine, A. MarcowithAbstract:After a successful development of theoretical and numerical works on Fermi acceleration at relativistic shocks, some difficulties recently raised with the scattering issue, a crucial aspect of the process. Most pioneering works were developed assuming the scattering off magnetic fluctuations as given. Even in that case, when a mean field is considered, its orientation is mostly perpendicular to the shock normal in the Front Frame, and this tends to quench the scattering process. Solving this difficulty leads to address the issue of the generation of very intense magnetic fluctuations at short wave lengths. The relativistic motion of the shock Front let the cosmic rays to visit upstream during a very short time only, making this generation of magnetic fluctuations very challenging. Anyway there is some hope to solve the problem. Thanks to a recent work by Spitkovsky (2008) \cite{AS}, we know that the process works without any mean field and now we have to investigate up to which intensity the mean field can be amplified for allowing Fermi process with appropriate fast instabilities. In this presentation, the collisionless shock structure in relativistic regime is sketched, the scattering issue is presented, and the instabilities that can provide the expected magnetic field amplification are presented as well. Although there exists observational evidence that particles are accelerated in relativistic flows and are distributed according to a power law suggesting a Fermi process, the drastic conditions for Fermi process to work are not always clearly fulfilled.
Constantia Alexandrou - One of the best experts on this subject based on the ideXlab platform.
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Quark transverse charge densities in the Delta(1232) from lattice QCD
Nuclear Physics, 2009Co-Authors: Constantia Alexandrou, Tomasz Korzec, Giannis Koutsou, Cédric Lorcé, John W. Negele, Vladimir Pascalutsa, Antonios Tsapalis, Marc VanderhaeghenAbstract:Abstract We extend the formalism relating electromagnetic form factors to transverse quark charge densities in the light-Front Frame to the case of a spin-3/2 baryon and calculate these transverse densities for the Δ ( 1232 ) isobar using lattice QCD. The transverse charge densities for a transversely polarized spin-3/2 particle are characterized by monopole, dipole, quadrupole, and octupole patterns representing the structure beyond that of a pure point-like spin-3/2 particle. We present lattice QCD results for the Δ-isobar electromagnetic form factors for pion masses down to approximatively 350 MeV for three cases: quenched QCD, two-degenerate flavors of dynamical Wilson quarks, and three flavors of quarks using a mixed action that combines domain wall valence quarks and dynamical staggered sea quarks. We extract transverse quark charge densities from these lattice results and find that the Δ is prolately deformed, as indicated by the fact that the quadrupole moment G E 2 ( 0 ) is larger than the value −3 characterizing a point particle and the fact that the transverse charge density in a Δ + of maximal transverse spin projection is elongated along the axis of the spin.
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Quark transverse charge densities in the from lattice QCD
Nuclear Physics A, 2009Co-Authors: Constantia Alexandrou, Tomasz Korzec, Giannis Koutsou, Cédric Lorcé, John W. Negele, Vladimir Pascalutsa, Antonios Tsapalis, Marc VanderhaeghenAbstract:We extend the formalism relating electromagnetic form factors to transverse quark charge densities in the light-Front Frame to the case of a spin-3/2 baryon and calculate these transverse densities for the $\Delta(1232)$ isobar using lattice QCD. The transverse charge densities for a transversely polarized spin-3/2 particle are characterized by monopole, dipole, quadrupole, and octupole patterns representing the structure beyond that of a pure point-like spin-3/2 particle. We present lattice QCD results for the $\Delta$-isobar electromagnetic form factors for pion masses down to approximatively 350 MeV for three cases: quenched QCD, two-degenerate flavors of dynamical Wilson quarks, and three flavors of quarks using a mixed action that combines domain wall valence quarks and dynamical staggered sea quarks. We extract transverse quark charge densities from these lattice results and find that the $\Delta$ is prolately deformed, as indicated by the fact that the quadrupole moment $G_{E2}(0$) is larger than the value -3 characterizing a point particle and the fact that the transverse charge density in a $\Delta^+$ of maximal transverse spin projection is elongated along the axis of the spin.Comment: 35 pages, 10 figure
