926 research outputs found
Discussion of the Electromotive Force Terms in the Model of Parker-unstable Galactic Disks with Cosmic Rays and Shear
We analyze the electromotive force (EMF) terms and basic assumptions of the
linear and nonlinear dynamo theories in our three-dimensional (3D) numerical
model of the Parker instability with cosmic rays and shear in a galactic disk.
We also apply the well known prescriptions of the EMF obtained by the nonlinear
dynamo theory (Blackman & Field 2002 and Kleeorin et al. 2003) to check if the
EMF reconstructed from their prescriptions corresponds to the EMF obtained
directly from our numerical models. We show that our modeled EMF is fully
nonlinear and it is not possible to apply any of the considered nonlinear
dynamo approximations due to the fact that the conditions for the scale
separation are not fulfilled.Comment: 15 pages, 12 figure
The alpha-effect and current helicity for fast sheared rotators
We explore the alpha-effect and the small-scale current helicity, for the
case of weakly compressible magnetically driven turbulence that is subjected to
the differential rotation. No restriction is applied to the amplitude of
angular velocity, i.e., the derivations presented are valid for an arbitrary
Coriolis number, though the differential rotation itself is assumed to be weak.
The expressions obtained are used to explore the possible distributions of
alpha-effect and current helicity in convection zones (CZ) of the solar-type
stars. The implications of the obtained results to the mean-field dynamo models
are discussed.Comment: 20 pages, 6 figure
The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
A numerical model of isotropic homogeneous turbulence with helical forcing is
investigated. The resulting flow, which is essentially the prototype of the
alpha^2 dynamo of mean-field dynamo theory, produces strong dynamo action with
an additional large scale field on the scale of the box (at wavenumber k=1;
forcing is at k=5). This large scale field is nearly force-free and exceeds the
equipartition value. As the magnetic Reynolds number R_m increases, the
saturation field strength and the growth rate of the dynamo increase. However,
the time it takes to built up the large scale field from equipartition to its
final super-equipartition value increases with magnetic Reynolds number. The
large scale field generation can be identified as being due to nonlocal
interactions originating from the forcing scale, which is characteristic of the
alpha-effect. Both alpha and turbulent magnetic diffusivity eta_t are
determined simultaneously using numerical experiments where the mean-field is
modified artificially. Both quantities are quenched in a R_m-dependent fashion.
The evolution of the energy of the mean field matches that predicted by an
alpha^2 dynamo model with similar alpha and eta_t quenchings. For this model an
analytic solution is given which matches the results of the simulations. The
simulations are numerically robust in that the shape of the spectrum at large
scales is unchanged when changing the resolution from 30^3 to 120^3 meshpoints,
or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity)
from 1 to 100. Increasing the forcing wavenumber to 30 (i.e. increasing the
scale separation) makes the inverse cascade effect more pronounced, although it
remains otherwise qualitatively unchanged.Comment: 21 pages, 26 figures, ApJ (accepted
The Accretion Rates and Spectral Energy Distributions of BL Lacertae Objects
We investigate the relationship between accretion rates and the spectral
energy distributions (SEDs) of BL Lac objects, using a sample of objects for
which published information on the host galaxies, emission-line luminosities,
and peak frequencies and luminosities of their SEDs are available. The sample
is composed of 43 BL Lac objects which have a relatively continuous
distribution of peak frequencies. Under the assumption that the observed
emission lines are photoionized by the central accretion disk, we use the line
luminosities to estimate the accretion luminosities and hence accretion rates.
We find that low frequency-peaked BL Lac objects (LBLs) span a wide range of
accretion rates, whereas high frequency-peaked BL Lac objects (HBLs) cover a
more restricted range of lower values. There appears to be a continuous
distribution of accretion rates between the two subclasses of BL Lac objects.
We find that the peak frequency of the SED, \pknu, correlates with the
accretion rate, approximately with the form \pknu\propto \Lambda^{-3} in HBLs
and \pknu \propto \Lambda^{-0.25} in LBLs, where . The peak luminosity of the SED is also correlated with .
These results suggest that the accretion rate influences the shape of the SED
in BL Lac objects. They also support models which couple the jet and the
accretion disk. We present a physical scenario to account for the empirical
trends.Comment: 6 pages in emulateapj.sty, 3 figures 1 table. The Astrophysical
Journal (in press
Negative Domain Wall Resistance in Ferromagnets
The electrical resistance of a diffusive ferromagnet with magnetic domain
walls is studied theoretically, taking into account the spatial dependence of
the magnetization. The semiclassical domain wall resistance is found to be
either negative or positive depending on the difference between the
spin-dependent scattering life-times. The predictions can be tested
experimentally by transport studies in doped ferromagnets.Comment: 4 pages, 2 figures, accepted Phys. Rev. Let
Magnetic field generation in fully convective rotating spheres
Magnetohydrodynamic simulations of fully convective, rotating spheres with
volume heating near the center and cooling at the surface are presented. The
dynamo-generated magnetic field saturates at equipartition field strength near
the surface. In the interior, the field is dominated by small-scale structures,
but outside the sphere by the global scale. Azimuthal averages of the field
reveal a large-scale field of smaller amplitude also inside the star. The
internal angular velocity shows some tendency to be constant along cylinders
and is ``anti-solar'' (fastest at the poles and slowest at the equator).Comment: 12 pages, 11 figures, 2 tables, to appear in the 10 Feb issue of Ap
Hemophagocytic syndrome caused by primary herpes simplex virus 1 infection: report of a first case
Introduction: Hemophagocytic syndrome represents a severe hyperinflammatory condition by activated macrophages. Leading viral triggering agents are Epstein-Barr virus (EBV), cytomegalovirus (CMV), and adenovirus. Materials and methods: We present a patient with Wegener's granulomatosis on azathioprine and prednisone medication, who developed a life-threatening hemophagocytic syndrome. Positive plasma polymerase chain reaction (PCR) with negative serology revealed a primary, disseminated infection with herpes simplex virus-1 as the triggering pathogen. After treatment with acyclovir, high-dose steroids, immunoglobulins, and etoposide, the patient recovered. Conclusion: Early diagnosis of potentially underlying infections of hemophagocytic syndrome influences the therapeutic approach. It is important to consider a variety of infectious agents, particularly in immunosuppressed individuals. The reported case emphasizes the importance of screening for herpes simplex virus
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