91 research outputs found
Dependence of the MHD shock thickness on the finite electrical conductivity
The results of MHD plane shock waves with infinite electrical conductivity
are generalized for a plasma with a finite conductivity. We derive the
adiabatic curves that describe the evolution of the shocked gas as well as the
change in the entropy density. For a parallel shock (i.e., in which the
magnetic field is parallel to the normal to the shock front) we find an
expression for the shock thickness which is a function of the ambient magnetic
field and of the finite electrical conductivity of the plasma. We give
numerical estimates of the physical parameters for which the shock thickness is
of the order of, or greater than, the mean free path of the plasma particles in
a strongly magnetized plasma.Comment: 8 pages, uses standard revtex, to appear in Journal of Plasma Physic
Cosmological Magnetic Fields from Gauge-Mediated Supersymmetry-Breaking Models
We study the generation of primordial magnetic fields, coherent over
cosmologically interesting scales, by gravitational creation of charged scalar
particles during the reheating period. We show that magnetic fields consistent
with those detected by observation may obtained if the particle mean life
\tau_s is in the range 10^{-14} sec \leq \tau_s \leq 10{-7} sec. We apply this
mechanism to minimal gauge mediated supersymmetry-breaking models, in the case
in which the lightest stau \tilde\tau_1 is the next-to-lightest supersymmetric
particle. We show that, for a large range of phenomenologically acceptable
values of the supersymmetry-breaking scale \sqrt{F}, the generated primordial
magnetic field can be strong enough to seed the galactic dynamo.Comment: 12 pages, Latex. Final version accepted for publication in Phys.
Lett.
On the mean field dynamo with Hall effect
We study in the present paper how Hall effect modifies the quenching process
of the electromotive force (e.m.f.) in Mean Field Dynamo (MFD) theories. We
write down the evolution equations for the e.m.f. and for the large and small
scale magnetic helicity, treat Hall effect as a perturbation and integrate the
resulting equations assuming boundary conditions such that the total
divergencies vanish. For force-free large scale magnetic fields, Hall effect
acts by coupling the small scale velocity and magnetic fields. For the range of
parameters considered, the overall effect is a stronger quenching of the e.m.f.
than in standard MHD and a damping of the inverse cascade of magnetic helicity.
In astrophysical environments characterized by the parameters considered here,
Hall effect would produce an earlier quenching of the e.m.f. and consequently a
weaker large scale magnetic field.Comment: 8 pages, 4 figures. Accepted by A&
On the thickness of a mildly relativistic collisional shock wave
We consider an imperfect relativistic fluid which develops a shock wave and
discuss its structure and thickness, taking into account the effects of
viscosity and heat conduction in the form of sound absorption. The junction
conditions and the non linear equations describing the evolution of the shock
are derived with the corresponding Newtonian limit discussed in detail. As
happens in the non relativistic regime, the thickness is inversely proportional
to the discontinuity in the pressure, but new terms of purely relativistic
origin are present. Particularizing for a polytropic gas, it is found that the
pure viscous relativistic shock is thicker than its nonrelativistic
counterpart, while the opposite holds for pure heat conduction.Comment: 11 pages, no figures, title changed, improved introduction and
discussion. New author adde
Primordial magnetic fields from inflation?
The hot plasma above the electroweak scale contains (hyper) charged scalar
particles which are coupled to Abelian gauge fields. Scalars may interact with
gravity in a non-conformally invariant way and thus their fluctuations can be
amplified during inflation. These fluctuations lead to creation of electric
currents and produce inhomogeneous distribution of charge density, resulting in
the generation of cosmological magnetic fields. We address the question whether
these fields can be coherent at large scales so that they may seed the galactic
magnetic fields. Depending upon the mass of the charged scalar and upon various
cosmological (critical fraction of energy density in matter, Hubble constant)
and particle physics parameters we found that the magnetic fields generated in
this way are much larger than vacuum fluctuations. However, their amplitude on
cosmological distances is found to be too small for seeding the galactic
magnetic fields.Comment: 32 pages in RevTex styl
Ohm's Law for Plasma in General Relativity and Cowling's Theorem
The general-relativistic Ohm's law for a two-component plasma which includes
the gravitomagnetic force terms even in the case of quasi-neutrality has been
derived. The equations that describe the electromagnetic processes in a plasma
surrounding a neutron star are obtained by using the general relativistic form
of Maxwell equations in a geometry of slow rotating gravitational object. In
addition to the general-relativistic effect first discussed by Khanna \&
Camenzind (1996) we predict a mechanism of the generation of azimuthal current
under the general relativistic effect of dragging of inertial frames on radial
current in a plasma around neutron star. The azimuthal current being
proportional to the angular velocity of the dragging of inertial
frames can give valuable contribution on the evolution of the stellar magnetic
field if exceeds (
is the number density of the charged particles, is the conductivity of
plasma). Thus in general relativity a rotating neutron star, embedded in
plasma, can in principle generate axial-symmetric magnetic fields even in
axisymmetry. However, classical Cowling's antidynamo theorem, according to
which a stationary axial-symmetric magnetic field can not be sustained against
ohmic diffusion, has to be hold in the general-relativistic case for the
typical plasma being responsible for the rotating neutron star.Comment: Accepted for publication in Astrophysics & Space Scienc
Self consistent estimates of magnetic fields from reheating
We investigate the generation of primordial magnetic fields from stochastic
currents created by the cosmological transition from inflation to reheating. We
consider N charged scalar fields coupled to the electromagnetic field in a
curved background and derive self-consistent equations for the evolution of the
two point functions of the fields, which in the large N limit give a decoupled
set for the scalar and the electromagnetic functions. The main contribution to
the electric current comes from the infrared portion of the spectrum of created
particles, and in this limit the damping of the magnetic field is not due to
normal conductivity but to London currents in the scalar field. For a given set
of the physical parameters of the problem, we solved this equation numerically
and found that, due to the fact that the London currents are oscillating, the
field actually grows exponentially during the time interval in which our
large-N limit equations are valid. Although for the chosen parameters the
induced field is weak, the present uncertainties on their actual values leave
open the possibility for higher intensities.Comment: 25 pages, three figures. One figure added, discussion on non
adiabatic induction of the magnetic field improved. References added. Final
version accepted for publication in Phys. Rev.
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