1,702 research outputs found
Magnetic field evolution and equilibrium configurations in neutron star cores: the effect of ambipolar diffusion
As another step towards understanding the long-term evolution of the magnetic
field in neutron stars, we provide the first simulations of ambipolar diffusion
in a spherical star. Restricting ourselves to axial symmetry, we consider a
charged-particle fluid of protons and electrons carrying the magnetic flux
through a motionless, uniform background of neutrons that exerts a collisional
drag force on the former. We also ignore the possible impact of beta decays,
proton superconductivity, and neutron superfluidity. All initial magnetic field
configurations considered are found to evolve on the analytically expected
time-scales towards "barotropic equilibria" satisfying the "Grad-Shafranov
equation", in which the magnetic force is balanced by the degeneracy pressure
gradient, so ambipolar diffusion is choked. These equilibria are so-called
"twisted torus" configurations, which include poloidal and toroidal components,
the latter restricted to the toroidal volumes in which the poloidal field lines
close inside the star. In axial symmetry, they appear to be stable, although
they are likely to undergo non-axially symmetric instabilities.Comment: MNRAS, accepte
Self-modulation of nonlinear Alfven waves in a strongly magnetized relativistic electron-positron plasma
We study the self-modulation of a circularly polarized Alfven wave in a strongly magnetized relativistic electron-positron plasma with finite temperature. This nonlinear wave corresponds to an exact solution of the equations, with a dispersion relation that has two branches. For a large magnetic field, the Alfven branch has two different zones, which we call the normal dispersion zone (where d omega/dk > 0) and the anomalous dispersion zone (where d omega/dk < 0). A nonlinear Schrodinger equation is derived in the normal dispersion zone of the Alfven wave, where the wave envelope can evolve as a periodic wave train or as a solitary wave, depending on the initial condition. The maximum growth rate of the modulational instability decreases as the temperature is increased. We also study the Alfven wave propagation in the anomalous dispersion zone, where a nonlinear wave equation is obtained. However, in this zone the wave envelope can evolve only as a periodic wave train.CONICyT 21100839 74110049FONDECyT 1110135 1110729 1080658 1121144CNPqEuropean Commission for a Marie Curie International Incoming FellowshipInstitute for Fusion Studie
Self-modulation of nonlinear waves in a weakly magnetized relativistic electron-positron plasma with temperature
We develop a nonlinear theory for self-modulation of a circularly polarized electromagnetic wave in a relativistic hot weakly magnetized electron-positron plasma. The case of parallel propagation along an ambient magnetic field is considered. A nonlinear Schrodinger equation is derived for the complex wave amplitude of a self-modulated wave packet. We show that the maximum growth rate of the modulational instability decreases as the temperature of the pair plasma increases. Depending on the initial conditions, the unstable wave envelope can evolve nonlinearly to either periodic wave trains or solitary waves. This theory has application to high-energy astrophysics and high-power laser physics.CONICyTFONDECyT 1110135 1080658Brazilian agency CNPqBrazilian agency FAPESPMarie Curie International Incoming Fellowshiphospitality of Paris ObservatoryInstitute for Fusion Studie
Humoral and cellular immunopathology of hepatic and cardiac hamster-into-rat xenograft rejection: Marked stimulation of IgM<sup>++bright</sup>/IgD<sup>+dull</sup> splenic B cells
Normal Lewis rat serum contains antibodies (IgM > IgG) that bind to hamster leukocytes and endothelial cells. Transplantation of either the heart or liver from hamster rat results in release of hamster hematolymphoid cells from the graft, which lodge in the recipient spleen (cell migration), where recipient T- and B-cell populations initiate DNA synthesis within one day. There is marked stimulation of splenic IgM++(bright)/IgD+(dull) B cells in the marginal zone and red pulp, which account for 48% of the total splenic blast cell population by 4 days after liver transplantation. CD4+ predominant T-cell proliferation in the splenic periarterial lymphatic sheath and paracortex of peripheral lymph nodes occurs almost simultaneously. The effector phase of rejection in cardiac recipients is dominated by complement-fixing IgM antibodies, which increase daily and result in graft destruction in 3 to 4 days, even in animals treated with FK506. In liver recipients, combined antibody and cellular rejection, associated with graft infiltration by OX8+ natural killer, and fewer W3/25+ (CD4) lymphocytes, are responsible for graft failure in untreated recipients at 6 to 7 days. FK506 inhibits the T-cell response in liver recipients and significantly prolongs graft survival, but does not prevent the rise or deposition of IgM antibodies in the graft. However, a single injection of cyclophosphamide 10 days before transplantation effectively depletes the splenic IgM++(bright)/IgD+(dull) cells and in combination with FK506, results in 100% survival of both cardiac and hepatic xenografts for more than 60 days. Although extrapolation of morphological findings to functional significance is fraught with potential problems, we propose the following mechanisms of xenograft rejection. The reaction initially appears to involve primitive host defense mechanisms, including an IgM-producing subpopulation of splenic B cells and natural killer cells. Based on the reaction and distribution of OX8+ and W3/25+ cells, antibody dependent cell cytotoxicity and delayed-type hypersensitivity responses seem worthy of further investigation as possible effector mechanisms. Effective control of xenograft rejection is likely to require a dual pharmaceutical approach, one to contain T-cell immunity and another to blunt the primitive B-cell response
Stability of axially symmetric magnetic fields in stars
The magnetic fields observed in Ap-stars, white dwarfs, and neutron stars are
known to be stable for long times. However, the physical conditions inside the
stellar interiors that allow these states are still a matter of research. It
has been formally demonstrated that both purely toroidal and purely poloidal
magnetic fields develop instabilities at some point in the star. On the other
hand, numerical simulations have proved the stability of roughly axisymmetric
magnetic field configurations inside stably stratified stars. These
configurations consist of mutually stabilizing toroidal and poloidal components
in a twisted torus shape. Previous studies have proposed rough upper and lower
bounds on the ratio of the magnetic energy in the toroidal and poloidal
components of the magnetic field. With the purpose of mapping out the parameter
space under which such configurations remain stable, we used the Pencil Code to
perform 3D magnetohydrodynamic simulations of the evolution of the magnetic
field in non-rotating, non-degenerate stars in which viscosity is the only
dissipation mechanism, both for stars with a uniform (barotropic) and radially
increasing (stably stratified) specific entropy. Furthermore, we considered
different conditions regarding the degree of stable stratification and the
magnetic energy in each component, roughly confirming the previously suggested
stability boundaries for the magnetic field.Comment: 9 pages, 9 figure
Computational and theoretical study of the wave-particle interaction of protons and waves
We study the wave-particle interaction and the evolution of
electromagnetic waves propagating through a plasma composed of
electrons and protons, using two approaches. First, a quasilinear
kinetic theory has been developed to study the energy transfer
between waves and particles, with the subsequent acceleration and
heating of protons. Second, a one-dimensional hybrid numerical
simulation has been performed, with and without including an
expanding-box model that emulates the spherical expansion of the
solar wind, to investigate the fully nonlinear evolution of this
wave-particle interaction. Numerical results of both approaches
show that there is an anisotropic evolution of proton temperature
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