655 research outputs found
Modeling the Parker instability in a rotating plasma screw pinch
We analytically and numerically study the analogue of the Parker (magnetic
buoyancy) instability in a uniformly rotating plasma screw pinch confined in a
cylinder. Uniform plasma rotation is imposed to create a centrifugal
acceleration, which mimics the gravity required for the classical Parker
instability. The goal of this study is to determine how the Parker instability
could be unambiguously identified in a weakly magnetized, rapidly rotating
screw pinch, in which the rotation provides an effective gravity and a radially
varying azimuthal field is controlled to give conditions for which the plasma
is magnetically buoyant to inward motion. We show that an axial magnetic field
is also required to circumvent conventional current driven magnetohydrodynamic
(MHD) instabilities such as the sausage and kink modes that would obscure the
Parker instability. These conditions can be realized in the Madison Plasma
Couette Experiment (MPCX). Simulations are performed using the extended MHD
code NIMROD for an isothermal compressible plasma model. Both linear and
nonlinear regimes of the instability are studied, and the results obtained for
the linear regime are compared with analytical results from a slab geometry.
Based on this comparison, it is found that in a cylindrical pinch the magnetic
buoyancy mechanism dominates at relatively large Mach numbers (M>5), while at
low Mach numbers (M<1) the instability is due to the curvature of magnetic
field lines. At intermediate values of Mach number (1<M<5) the Coriolis force
has a strong stabilizing effect on the plasma. A possible scenario for
experimental demonstration of the Parker instability in MPCX is discussed
Magnetic Reconnection Triggered by the Parker Instability in the Galaxy: Two-Dimensional Numerical Magnetohydrodynamic Simulations and Application to the Origin of X-Ray Gas in the Galactic Halo
We propose the Galactic flare model for the origin of the X-ray gas in the
Galactic halo. For this purpose, we examine the magnetic reconnection triggered
by Parker instability (magnetic buoyancy instability), by performing the
two-dimensional resistive numerical magnetohydrodynamic simulations. As a
result of numerical simulations, the system evolves as following phases: Parker
instability occurs in the Galactic disk. In the nonlinear phase of Parker
instability, the magnetic loop inflates from the Galactic disk into the
Galactic halo, and collides with the anti-parallel magnetic field, so that the
current sheets are created in the Galactic halo. The tearing instability
occurs, and creates the plasmoids (magnetic islands). Just after the plasmoid
ejection, further current-sheet thinning occurs in the sheet, and the anomalous
resistivity sets in. Petschek reconnection starts, and heats the gas quickly in
the Galactic halo. It also creates the slow and fast shock regions in the
Galactic halo. The magnetic field (G), for example, can heat the
gas ( cm) to temperature of K via the
reconnection in the Galactic halo. The gas is accelerated to Alfv\'en velocity
( km s). Such high velocity jets are the evidence of the
Galactic flare model we present in this paper, if the Doppler shift of the
bipolar jet is detected in the Galactic halo. Full size figures are available
at http://www.kwasan.kyoto-u.ac.jp/~tanuma/study/ApJ2002/ApJ2002.htmlComment: 13 pages, 12 figures, uses emulateapj.sty, accepted by Ap
Two-step Emergence of the Magnetic Flux Sheet from the Solar Convection Zone
We perform two-dimensional MHD simulations on the solar flux emergence. We
set the initial magnetic flux sheet at z=-20,000 km in the convection zone. The
flux sheet rises through the convective layer due to the Parker instability,
however, decelerates beneath the photosphere because the plasma on the flux
sheet piles up owing to the convectively stable photosphere above. Meanwhile,
the flux sheet becomes locally unstable to the Parker instability within the
photosphere, and the further evolution to the corona occurs (two-step emergence
model). We carry out a parameter survey to investigate the condition for this
two-step model. We find that magnetic fluxes which form active regions are
likely to have undergone the two-step emergence. The condition for the two-step
emergence is 10^21 - 10^22 Mx with 10^4 G at z=-20,000 km in the convection
zone.Comment: 41 pages, 15 figures, 1 table, Accepted for publication in Ap
A Comparative Study of the Parker Instability under Three Models of the Galactic Gravity
To examine how non-uniform nature of the Galactic gravity might affect length
and time scales of the Parker instability, we took three models of gravity,
uniform, linear and realistic ones. To make comparisons of the three gravity
models on a common basis, we first fixed the ratio of magnetic pressure to gas
pressure at = 0.25, that of cosmic-ray pressure at = 0.4, and
the rms velocity of interstellar clouds at = 6.4 km s, and then
adjusted parameters of the gravity models in such a way that the resulting
density scale heights for the three models may all have the same value of 160
pc. Performing linear stability analyses onto equilibrium states under the
three models with the typical ISM conditions, we calculate the maximum growth
rate and corresponding length scale for each of the gravity models. Under the
uniform gravity the Parker instability has the growth time of 1.2
years and the length scale of 1.6 kpc for symmetric mode. Under the realistic
gravity it grows in 1.8 years for both symmetric and
antisymmetric modes, and develops density condensations at intervals of 400 pc
for the symmetric mode and 200 pc for the antisymmetric one. A simple change of
the gravity model has thus reduced the growth time by almost an order of
magnitude and its length scale by factors of four to eight. These results
suggest that an onset of the Parker instability in the ISM may not necessarily
be confined to the regions of high and .Comment: Accepted for publication in ApJ, using aaspp4.sty, 18 text pages with
9 figure
The effect of cosmic-ray diffusion on the Parker instability
The Parker instability, which has been considered as a process governing the structure of the interstellar medium, is induced by the buoyancy of magnetic fields and cosmic rays. In previous studies, while the magnetic field has been fully incorporated in the context of isothermal magnetohydrodynamics, cosmic rays have normally been treated with the simplifying assumption of infinite diffusion along magnetic field lines but no diffusion across them. The cosmic-ray diffusion is, however, finite. In this work, we fully take into account the diffusion process of cosmic rays in a linear stability analysis of the Parker instability. Cosmic rays are described with the diffusion-convection equation. With realistic values of cosmic-ray diffusion coefficients expected in the interstellar medium, we show that the result of previous studies with the simplifying assumption about cosmic-ray diffusion applies well. The finiteness of the parallel diffusion decreases the growth rate of the Parker instability, while the relatively smaller perpendicular diffusion has no significant effect. We discuss the implication of our result on the role of the Parker instability in the interstellar mediumopen373
3D Magneto-Hydrodynamic Simulations of Parker Instability with Cosmic Rays
This study investigates Parker instability in an interstellar medium (ISM)
near the Galactic plane using three-dimensional magneto-hydrodynamic
simulations. Parker instability arises from the presence of a magnetic field in
a plasma, wherein the magnetic buoyant pressure expels the gas and cause the
gas to move along the field lines. The process is thought to induce the
formation of giant molecular clouds in the Galaxy. In this study, the effects
of cosmic-ray (CR) diffusion are examined. The ISM at equilibrium is assumed to
comprise a plasma fluid and a CR fluid at various temperatures, with a uniform
magnetic field passing through it in the azimuthal direction of the Galactic
disk. After a small perturbation, the unstable gas aggregates at the footpoint
of the magnetic fields and forms dense blobs. The growth rate of the
instability increases with the strength of the CR diffusion. The formation of
dense clouds is enhanced by the effect of cosmic rays (CRs), whereas the shape
of the clouds depends sensitively on the initial conditions of perturbation.Comment: 4 pages, Computer Physics Communications 2011, 182, p177-17
Resistive MHD simulations of the Parker instability in galactic disks
Parker instability leads to the formation of tangential discontinuities in a
magnetic field and subsequent magnetic reconnection due to a numerical and/or
an explicit resistivity. In this paper we investigate the role of the uniform,
localized and numerical resistivity on the diffusion of magnetic field lines
during the growth phase of Parker instability modes. We propose a new method to
quantify the diffusion of magnetic field lines which is attributed to the
presence of resistivity in ideal and non-ideal MHD codes. The method relies (1)
on integration of magnetic lines in between periodic boundaries, (2) on
measurements of the dispersion of magnetic field lines with the left and the
right periodic boundaries and (3) on a statistical analysis of shifts of a
large set of magnetic lines. The proposed method makes it possible to detect
topological evolution of magnetic field. We perform a series of resistive MHD
simulations of the Parker instability in uniformly rotating galactic disks. We
follow the topological evolution of the magnetic field evolving due to the
Parker instability and relate it to the ratio of total to uniform magnetic
field in galactic disks. We find that after the onset of the Parker
instability, the magnetic field becomes first tangled and later on it evolves
toward a uniform state due to the presence of resistivity. A similar effect of
a varying contribution of a turbulent magnetic field is observed in arms and
inter-arm regions of galaxies.Comment: 12 pages, 13 eps figures, accepted for publication in A&
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