242 research outputs found
Gradient expansion, curvature perturbations and magnetized plasmas
The properties of magnetized plasmas are always investigated under the
hypothesis that the relativistic inhomogeneities stemming from the fluid
sources and from the geometry itself are sufficiently small to allow for a
perturbative description prior to photon decoupling. The latter assumption is
hereby relaxed and pre-decoupling plasmas are described within a suitable
expansion where the inhomogeneities are treated to a given order in the spatial
gradients. It is argued that the (general relativistic) gradient expansion
shares the same features of the drift approximation, customarily employed in
the description of cold plasmas, so that the two schemes are physically
complementary in the large-scale limit and for the low-frequency branch of the
spectrum of plasma modes. The two-fluid description, as well as the
magnetohydrodynamical reduction, are derived and studied in the presence of the
spatial gradients of the geometry. Various solutions of the coupled system of
evolution equations in the anti-Newtonian regime and in the quasi-isotropic
approximation are presented. The relation of this analysis to the so-called
separate Universe paradigm is outlined. The evolution of the magnetized
curvature perturbations in the nonlinear regime is addressed for the magnetized
adiabatic mode in the plasma frame.Comment: 40 pages, no figure
Relation of Astrophysical Turbulence and Magnetic Reconnection
Astrophysical fluids are generically turbulent and this must be taken into
account for most transport processes. We discuss how the preexisting turbulence
modifies magnetic reconnection and how magnetic reconnection affects the MHD
turbulent cascade. We show the intrinsic interdependence and interrelation of
magnetic turbulence and magnetic reconnection, in particular, that strong
magnetic turbulence in 3D requires reconnection and 3D magnetic turbulence
entails fast reconnection. We follow the approach in Eyink, Lazarian & Vishniac
2011 to show that the expressions of fast magnetic reconnection in Lazarian &
Vishniac 1999 can be recovered if Richardson diffusion of turbulent flows is
used instead of ordinary Ohmic diffusion. This does not revive, however, the
concept of magnetic turbulent diffusion which assumes that magnetic fields can
be mixed up in a passive way down to a very small dissipation scales. On the
contrary, we are dealing the reconnection of dynamically important magnetic
field bundles which strongly resist bending and have well defined mean
direction weakly perturbed by turbulence. We argue that in the presence of
turbulence the very concept of flux-freezing requires modification. The
diffusion that arises from magnetic turbulence can be called reconnection
diffusion as it based on reconnection of magnetic field lines. The reconnection
diffusion has important implications for the continuous transport processes in
magnetized plasmas and for star formation. In addition, fast magnetic
reconnection in turbulent media induces the First order Fermi acceleration of
energetic particles, can explain solar flares and gamma ray bursts. However,
the most dramatic consequence of these developments is the fact that the
standard flux freezing concept must be radically modified in the presence of
turbulence.Comment: 8 pages, 4 figures, Physics of Plasma
Parameter dependence of magnetized CMB observables
Pre-decoupling magnetic fields affect the scalar modes of the geometry and
produce observable effects which can be constrained also through the use of
current (as opposed to forthcoming) data stemming from the Cosmic Microwave
Background observations. The dependence of the temperature and polarization
angular power spectra upon the parameters of an ambient magnetic field is
encoded in the scaling properties of a set of basic integrals whose derivation
is simplified in the limit of small angular scales. The magnetically-induced
distortions patterns of the relevant observables can be computed analytically
by employing scaling considerations which are corroborated by numerical
results.Comment: 48 pages, 11 figures; corrected minor typos; discussions added; to
appear in Physical Revie
Magneto-acoustic wave energy from numerical simulations of an observed sunspot umbra
We aim at reproducing the height dependence of sunspot wave signatures
obtained from spectropolarimetric observations through 3D MHD numerical
simulations. A magneto-static sunspot model based on the properties of the
observed sunspot is constructed and perturbed at the photosphere introducing
the fluctuations measured with the \SiI\ 10827 \AA\ line. The results
of the simulations are compared with the oscillations observed simultaneously
at different heights from the \HeI\ 10830 \AA\ line, the \CaIIH\ core
and the \FeI\ blends in the wings of the \CaIIH\ line. The simulations show a
remarkable agreement with the observations. They reproduce the velocity maps
and power spectra at the formation heights of the observed lines, as well as
the phase and amplification spectra between several pair of lines. We find that
the stronger shocks at the chromosphere are accompanied with a delay between
the observed signal and the simulated one at the corresponding height,
indicating that shocks shift the formation height of the chromospheric lines to
higher layers. Since the simulated wave propagation matches very well the
properties of the observed one, we are able to use the numerical calculations
to quantify the energy contribution of the magneto-acoustic waves to the
chromospheric heating in sunspots. Our findings indicate that the energy
supplied by these waves is too low to balance the chromospheric radiative
losses. The energy contained at the formation height of the lowermost \SiI\
10827 \AA\ line in the form of slow magneto-acoustic waves is already
insufficient to heat the higher layers, and the acoustic energy which reaches
the chromosphere is around 3-9 times lower than the required amount of energy.
The contribution of the magnetic energy is even lower.Comment: Accepted for publication in The Astrophysical Journa
Faraday rotation, stochastic magnetic fields and CMB maps
The high- and low-frequency descriptions of the pre-decoupling plasma are
deduced from the Vlasov-Landau treatment generalized to curved space-times and
in the presence of the relativistic fluctuations of the geometry. It is
demonstrated that the interplay between one-fluid and two-fluid treatments is
mandatory for a complete and reliable calculation of the polarization
observables. The Einstein-Boltzmann hierarchy is generalized to handle the
dispersive propagation of the electromagnetic disturbances in the
pre-decoupling plasma. Given the improved physical and numerical framework, the
polarization observables are computed within the magnetized CDM
paradigm (mCDM). In particular, the Faraday-induced B-mode is
consistently estimated by taking into account the effects of the magnetic
fields on the initial conditions of the Boltzmann hierarchy, on the dynamical
equations and on the dispersion relations. The complete calculations of the
angular power spectra constitutes the first step for the derivation of
magnetized maps of the CMB temperature and polarization which are here obtained
for the first time and within the minimal mCDM model. The obtained
results set the ground for direct experimental scrutiny of large-scale
magnetism via the low and high frequency instruments of the Planck explorer
satellite.Comment: 53 pages, 15 included figure
Mathematical models of magnetospheric convection and its coupling to the ionosphere
Mathematical models of magnetospheric convection and its coupling to ionospher
Classical diamagnetism, magnetic interaction energies, and repulsive forces in magnetized plasmas
The Bohr-van Leeuwen theorem is often summarized as saying that there is no
classical magnetic susceptibility, in particular no diamagnetism. This is
seriously misleading. The theorem assumes position dependent interactions but
this is not required by classical physics. Since the work of Darwin in 1920 it
has been known that the magnetism due to classical charged point particles can
only be described by allowing velocity dependent interactions in the
Lagrangian. Legendre transformation to an approximate Hamiltonian can give an
estimate of the Darwin diamagnetism for a system of charged point particles.
Comparison with experiment, however, requires knowledge of the number of
classically behaving electrons in the sample. A new repulsive effective
many-body force, which should be relevant in plasmas, is predicted by the
Hamiltonian.Comment: added references, revise
Cosmic polarimetry in magnetoactive plasmas
Polarimetry of the Cosmic Microwave Background (CMB) represents one of the
possible diagnostics aimed at testing large-scale magnetism at the epoch of the
photon decoupling. The propagation of electromagnetic disturbances in a
magnetized plasma leads naturally to a B-mode polarization whose angular power
spectrum is hereby computed both analytically and numerically. Combined
analyses of all the publicly available data on the B-mode polarization are
presented, for the first time, in the light of the magnetized CDM
scenario. Novel constraints on pre-equality magnetism are also derived in view
of the current and expected sensitivities to the B-mode polarization.Comment: 34 pages, 13 figure
Structural Invariance of Sunspot Umbrae Over the Solar Cycle: 1993-2004
Measurements of maximum magnetic flux, minimum intensity, and size are
presented for 12 967 sunspot umbrae detected on the NASA/NSO
spectromagnetograms between 1993 and 2004 to study umbral structure and
strength during the solar cycle. The umbrae are selected using an automated
thresholding technique. Measured umbral intensities are first corrected for a
confirming observation of umbral limb-darkening. Log-normal fits to the
observed size distribution confirm that the size spectrum shape does not vary
with time. The intensity-magnetic flux relationship is found to be steady over
the solar cycle. The dependence of umbral size on the magnetic flux and minimum
intensity are also independent of cycle phase and give linear and quadratic
relations, respectively. While the large sample size does show a low amplitude
oscillation in the mean minimum intensity and maximum magnetic flux correlated
with the solar cycle, this can be explained in terms of variations in the mean
umbral size. These size variations, however, are small and do not substantiate
a meaningful change in the size spectrum of the umbrae generated by the Sun.
Thus, in contrast to previous reports, the observations suggest the equilibrium
structure, as testified by the invariant size-magnetic field relationship, as
well as the mean size (i.e. strength) of sunspot umbrae do not significantly
depend on solar cycle phase.Comment: 17 pages, 6 figures. Published in Solar Physic
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