184 research outputs found
The Dependence of Dynamo -Effect on Reynolds Numbers, Magnetic Prandtl Number, and the Statistics of MHD Turbulence
We generalize the derivation of dynamo coefficient of Field et al
(1999) to include the following two aspects: first, the de-correlation times of
velocity field and magnetic field are different; second, the magnetic Prandtl
number can be arbitrary. We find that the contributions of velocity field and
magnetic field to the effect are not equal, but affected by their
different statistical properties. In the limit of large kinetic Reynolds number
and large magnetic Reynolds number, -coefficient may not be small if
the de-correlation times of velocity field and magnetic field are shorter than
the eddy turn-over time of the MHD turbulence. We also show that under certain
circumstances, for example if the kinetic helicity and current helicity are
comparable, depends insensitively on magnetic Prandtl number, while if
either the kinetic helicity or the current helicity is dominated by the other
one, a different magnetic Prandtl number will significantly change the dynamo
effect.Comment: 44 pages, 4 figures, to appear in ApJ (vol. 552
Jupiter's magnetosphere
Magnetosphere, ionosphere, and thermal radiation of planet Jupite
Electrodynamic effects of Jupiter's satellite Io
Electrodynamic effects of Jupiters satellite I
The magnetospheric radiation belt and tail plasma sheet
Magnetospheric radiation belt and tail plasma shee
Nonlinear Alpha Effect in Dynamo Theory
We extend the standard two-scale theory of the turbulent dynamo coefficient
to include the nonlinear back reaction of the mean field on
the turbulence. We calculate the turbulent emf as a power series in ,
assuming that the base state of the turbulence () is isotropic, and,
for simplicity, that the magnetic diffusivity equals the kinematic viscosity.
The power series converges for all , and for the special case that the
spectrum of the turbulence is sharply peaked in , our result is proportional
to a tabulated function of the magnetic Reynolds number and the ratio
of (in velocity units) to the rms turbulent velocity .
For (linear regime) we recover the results of Steenbeck et al.
(1966) as modified by Pouquet et al. (1976). For , the usual
astrophysical case, starts to decrease at , dropping
like as . Hence for large ,
saturates at , as estimated by Kraichnan (1979), rather than at
, as inferred by Cattaneo and Hughes (1996) from
their numerical simulations at =100. We plan to carry out simulations with
various values of to investigate the discrepency.Comment: 41 pages, 1 Postscript figure, accepted for publication to Ap
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
B polarization of the CMB from Faraday rotation
We study the effect of Faraday rotation due to a homogeneous magnetic field
on the polarization of the cosmic microwave background (CMB). Scalar
fluctuations give rise only to parity-even E-type polarization of the CMB.
However in the presence of a magnetic field, a non-vanishing parity-odd B-type
polarization component is produced through Faraday rotation. We derive the
exact solution for the E and B modes generated by scalar perturbations
including the Faraday rotation effect of a uniform magnetic field, and evaluate
their cross-correlations with temperature anisotropies. We compute the angular
autocorrelation function of the B-modes in the limit that the Faraday rotation
is small. We find that primordial magnetic fields of present strength around
G rotate E-modes into B-modes with amplitude comparable to those
due to the weak gravitational lensing effect at frequencies around
GHz. The strength of B-modes produced by Faraday rotation scales as
. We evaluate also the depolarizing effect of Faraday rotation upon
the cross correlation between temperature anisotropy and E-type polarization.Comment: 11 pages, 4 figures. Minor changes to match the published versio
Generation of Cosmological Seed Magnetic Fields from Inflation with Cutoff
Inflation has the potential to seed the galactic magnetic fields observed
today. However, there is an obstacle to the amplification of the quantum
fluctuations of the electromagnetic field during inflation: namely the
conformal invariance of electromagnetic theory on a conformally flat underlying
geometry. As the existence of a preferred minimal length breaks the conformal
invariance of the background geometry, it is plausible that this effect could
generate some electromagnetic field amplification. We show that this scenario
is equivalent to endowing the photon with a large negative mass during
inflation. This effective mass is negligibly small in a radiation and matter
dominated universe. Depending on the value of the free parameter of the theory,
we show that the seed required by the dynamo mechanism can be generated. We
also show that this mechanism can produce the requisite galactic magnetic field
without resorting to a dynamo mechanism.Comment: Latex, 16 pages, 2 figures, 4 references added, minor corrections;
v4: more references added, boundary term written in a covariant form,
discussion regarding other gauge fields added, submitted to PRD; v5: matched
with the published versio
Emergence of a Twisted Magnetic Flux Bundle as a Source of Strong Flare Activity
Sunspot proper motions and flares of a super active region NOAA 5395, which
was the biggest and the most flare-active region in the 22nd sunspot cycle,
were analyzed in details. We measured sunspot proper motions by using the
H-alpha - 5.0 A images obtained with the 60-cm Domeless Solar Telescope (DST)
at Hida Observatory, Kyoto University and found some peculiar vortex-like
motions of small satellite spots successively emerged from the leading edge of
this sunspot group. To explain these motions of small sunspots, we proposed a
schematic model of the successive emergence of twisted and winding magnetic
flux ropes coiling around a trunk of magnetic flux tube. The location of the
strongest flare activity was found to coincide with very the site of the
vortex-like motions of sunspots. We conclude that the flare-productive magnetic
shear is produced by the emergence of the twisted magnetic flux bundle.
Magnetic energy is stored in the twisted flux tubes which are originally formed
in the convection zone and released as flares in the course of the emergence of
the twisted flux tubes above the photosphere.Comment: 19 pages LaTeX, 8 figures, ApJ accepted high resolution figures :
e-mail to [email protected]
On a mechanism for enhancing magnetic activity in tidally interacting binaries
We suggest a mechanism for enhancing magnetic activity in tidally interacting
binaries. We suppose that the deviation of the primary star from spherical
symmetry due to the tidal influence of the companion leads to stellar pulsation
in its fundamental mode. It is shown that stellar radial pulsation amplifies
torsional Alfv{\'e}n waves in a dipole-like magnetic field, buried in the
interior, according to the recently proposed swing wave-wave interaction
(Zaqarashvili 2001). Then amplified Alfv{\'e}n waves lead to the onset of
large-scale torsional oscillations, and magnetic flux tubes arising towards the
surface owing to magnetic buoyancy diffuse into the atmosphere producing
enhanced chromospheric and coronal emission.Comment: Accepted in Ap
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