3,090 research outputs found
An analytic mean-field -dynamo with a force-free corona
Stellar dynamos are affected by boundary conditions imposed by stellar
coronae. Under some approximations it is possible to find analytical solutions.
Interior dynamo models often consider a current-free coronae without taking
into account the constraints imposed by the presence of currents in the corona.
We aim to analytically evaluate the effect of coronal currents and of an outer
boundary condition on the efficiency of an dynamo. We intend to
estimate the change in geometry and dinamo excitation numbers with respect to
the current-free case. We analytically solve the turbulent dynamo induction
equation for a homogeneous, non-mirror symmetric turbulence, in a spherical
domain surrounded by a linear force-free corona with magnetic field B
satisfying curl B = \beta B. The main result is that the dynamo number is a
decreasing function of \beta. Moreover, if the current is parallel to the field
(\beta > 0) the dynamo number is smaller than in the force-free case. On the
contrary (\beta < 0) the dynamo number is greater than the in force-free case.
We conclude that the presence of currents in the corona needs to be taken into
account because it affects the condition for excitation of a dynamo.Comment: Accepted for publication in A&
Turbulent diffusion and galactic magnetism
Using the test-field method for nearly irrotational turbulence driven by
spherical expansion waves it is shown that the turbulent magnetic diffusivity
increases with magnetic Reynolds numbers. Its value levels off at several times
the rms velocity of the turbulence multiplied by the typical radius of the
expansion waves. This result is discussed in the context of the galactic
mean-field dynamo.Comment: 2 pages, 1 figure, to appear in "Magnetic Fields in Diffuse Media",
proceedings of Joint Discussion at the 2009 XXVII IAU General Assembly in Rio
de Janeiro from 12 to 14 August, 200
Vorticity from irrotationally forced flow
In the interstellar medium the turbulence is believed to be forced mostly
through supernova explosions. In a first approximation these flows can be
written as a gradient of a potential being thus devoid of vorticity. There are
several mechanisms that could lead to vorticity generation, like viscosity and
baroclinic terms, rotation, shear and magnetic fields, but it is not clear how
effective they are, neither is it clear whether the vorticity is essential in
determining the turbulent diffusion acting in the ISM. Here we present a study
of the role of rotation, shear and baroclinicity in the generation of vorticity
in the ISM.Comment: 2 pages, 1 figure, to be published in Proceedings of IAU Symp. 271,
Astrophysical Dynamics: from Stars to Galaxies, ed. N. Brummell and A.S.
Brun, CU
How can vorticity be produced in irrotationally forced flows?
A spherical hydrodynamical expansion flow can be described as the gradient of
a potential. In that case no vorticity should be produced, but several
additional mechanisms can drive its production. Here we analyze the effects of
baroclinicity, rotation and shear in the case of a viscous fluid. Those flows
resemble what happens in the interstellar medium. In fact in this astrophysical
environment supernovae explosion are the dominant flows and, in a first
approximation, they can be seen as spherical. One of the main difference is
that in our numerical study we examine only weakly supersonic flows, while
supernovae explosions are strongly supersonic.Comment: 3 pages, 3 figures, to appear in Proceedings of IAU Symp. 274,
Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and
A. Kosoviche
Influence of Magnetic Helicity in MHD
Observations have shown that the Sun's magnetic field has helical structures.
The helicity content in magnetic field configurations is a crucial constraint
on the dynamical evolution of the system. Since helicity is connected with the
number of links we investigate configurations with interlocked magnetic flux
rings and one with unlinked rings. It turns out that it is not the linking of
the tubes which affects the magnetic field decay, but the content of magnetic
helicity.Comment: 2 pages, 3 figures, proceedings of IAU Symp. 271, Astrophysical
Dynamics: from Stars to Galaxies, ed. N. Brummell and A.S. Brun, CU
Magnetic field decay of three interlocked flux rings with zero linking number
The resistive decay of chains of three interlocked magnetic flux rings is
considered. Depending on the relative orientation of the magnetic field in the
three rings, the late-time decay can be either fast or slow. Thus, the
qualitative degree of tangledness is less important than the actual value of
the linking number or, equivalently, the net magnetic helicity. Our results do
not suggest that invariants of higher order than that of the magnetic helicity
need to be considered to characterize the decay of the field.Comment: 7 pages, 10 figure
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