111 research outputs found
Nonaxisymmetric Magnetorotational Instability in Proto-Neutron Stars
We investigate the stability of differentially rotating proto-neutron stars
(PNSs) with a toroidal magnetic field. Stability criteria for nonaxisymmetric
MHD instabilities are derived using a local linear analysis. PNSs are expected
to have much stronger radial shear in the rotation velocity compared to normal
stars. We find that nonaxisymmetric magnetorotational instability (NMRI) with a
large azimuthal wavenumber is dominant over the kink mode () in
differentially rotating PNSs. The growth rate of the NMRI is of the order of
the angular velocity which is faster than that of the kink-type
instability by several orders of magnitude. The stability criteria are
analogous to those of the axisymmetric magnetorotational instability with a
poloidal field, although the effects of leptonic gradients are considered in
our analysis. The NMRI can grow even in convectively stable layers if the
wavevectors of unstable modes are parallel to the restoring force by the
Brunt-V\"ais\"al\"a oscillation. The nonlinear evolution of NMRI could amplify
the magnetic fields and drive MHD turbulence in PNSs, which may lead to
enhancement of the neutrino luminosity.Comment: 24pages, 7figures, Accepted for publication in the Astrophysical
Journal (December 12, 2005
The stability of toroidal fields in stars
We present numerical models of hydromagnetic instabilities under the
conditions prevailing in a stably stratified, non-convective stellar interior,
and compare them with previous results of analytic work on instabilities in
purely toroidal fields. We confirm that an mode (`kink') is the dominant
instability in a toroidal field in which the field strength is proportional to
distance from the axis, such as the field formed by the winding up of a weak
field by differential rotation. We measure the growth rate of the instability
as a function of field strength and rotation rate , and investigate the
effects of a stabilising thermal stratification as well as magnetic and thermal
diffusion on the stability. Where comparison is computationally feasible, the
results agree with analytic predictions.Comment: 13 pages, 14 figures, comments welcom
Bounds on the Magnetic Fields in the Radiative Zone of the Sun
We discuss bounds on the strength of the magnetic fields that could be buried
in the radiative zone of the Sun. The field profiles and decay times are
computed for all axisymmetric toroidal Ohmic decay eigenmodes with lifetimes
exceeding the age of the Sun. The measurements of the solar oblateness yield a
bound <~ 7 MG on the strength of the field. A comparable bound is expected to
come from the analysis of the splitting of the solar oscillation frequencies.
The theoretical analysis of the double diffusive instability also yields a
similar bound. The oblateness measurements at their present level of
sensitivity are therefore not expected to measure a toroidal field
contribution.Comment: 15 pages, 6 figure
Dynamo action by differential rotation in a stably stratified stellar interior
Magnetic fields can be created in stably stratified (non-convective) layers
in a differentially rotating star. A magnetic instability in the toroidal field
(wound up by differential rotation) replaces the role of convection in closing
the field amplification loop. Tayler instability is likely to be the most
relevant magnetic instability. A dynamo model is developed from these
ingredients, and applied to the problem of angular momentum transport in
stellar interiors. It produces a prodominantly horizontal field. This dynamo
process might account for the observed pattern of rotation in the solar core.Comment: Expanded version as accepted by Astron. Astrophy
Non-axisymmetric instabilities of neutron star with toroidal magnetic fields
The aim of this paper is to clarify the stabilities of neutron stars with
strong toroidal magnetic fields against non-axisymmetric perturbation. The
motivation comes from the fact that super magnetized neutron stars of G, magnetars, and magnetized proto-neutron stars born after the
magnetically-driven supernovae are likely to have such strong toroidal magnetic
fields. Long-term, three-dimensional general relativistic magneto-hydrodynamic
simulations are performed, preparing isentropic neutron stars with toroidal
magnetic fields in equilibrium as initial conditions. To explore the effects of
rotations on the stability, simulations are done for both non-rotating and
rigidly rotating models. We find the emergence of the Parker and/or Tayler
instabilities in both the non-rotating and rotating models. For both
non-rotating and rotating models, the Parker instability is the primary
instability as predicted by the local linear perturbation analysis. The
interchange instability also appears in the rotating models. It is found that
rapid rotation is not enough to suppress the Parker instability, and this
finding does not agree with the perturbation analysis. The reason for this is
that rigidly and rapidly rotating stars are marginally stable, and hence, in
the presence of stellar pulsations by which the rotational profile is deformed,
unstable regions with negative gradient of angular momentum profile is
developed. After the onset of the instabilities, a turbulence is excited.
Contrary to the axisymmetric case, the magnetic fields never reach an
equilibrium state after the development of the turbulence. This conclusion
suggests that three-dimensional simulation is indispensable for exploring the
formation of magnetars or prominence activities of magnetars such as giant
flares.Comment: 19 pages, 11 figures, to be published in A&
Magnetic field evolution in neutron stars
Neutron stars contain persistent, ordered magnetic fields that are the
strongest known in the Universe. However, their magnetic fluxes are similar to
those in magnetic A and B stars and white dwarfs, suggesting that flux
conservation during gravitational collapse may play an important role in
establishing the field, although it might also be modified substantially by
early convection, differential rotation, and magnetic instabilities. The
equilibrium field configuration, established within hours (at most) of the
formation of the star, is likely to be roughly axisymmetric, involving both
poloidal and toroidal components. The stable stratification of the neutron star
matter (due to its radial composition gradient) probably plays a crucial role
in holding this magnetic structure inside the star. The field can evolve on
long time scales by processes that overcome the stable stratification, such as
weak interactions changing the relative abundances and ambipolar diffusion of
charged particles with respect to neutrons. These processes become more
effective for stronger magnetic fields, thus naturally explaining the magnetic
energy dissipation expected in magnetars, at the same time as the longer-lived,
weaker fields in classical and millisecond pulsars.Comment: To appear in Astronomische Nachrichten (Astronomical Notes) as part
of the Proceedings of the 5th Potsdam Thinkshop, "Meridional Circulation,
Differential Rotation, Solar and Stellar Activity", held 2007 June 24-29. 5
pages, no figure
Stellar evolution of massive stars with a radiative alpha-omega dynamo
Models of rotationally-driven dynamos in stellar radiative zones have
suggested that magnetohydrodynamic transport of angular momentum and chemical
composition can dominate over the otherwise purely hydrodynamic processes. A
proper consideration of the interaction between rotation and magnetic fields is
therefore essential. Previous studies have focused on a magnetic model where
the magnetic field strength is derived as a function of the stellar structure
and angular momentum distribution. We have adapted our one-dimensional stellar
rotation code, RoSE, to model the poloidal and toroidal magnetic field
strengths with a pair of time-dependent advection-diffusion equations coupled
to the equations for the evolution of the angular momentum distribution and
stellar structure. This produces a much more complete, though still reasonably
simple, model for the magnetic field evolution. Our model reproduces well
observed surface nitrogen enrichment of massive stars in the Large Magellanic
Cloud. In particular it reproduces a population of slowly-rotating
nitrogen-enriched stars that cannot be explained by rotational mixing alone
alongside the traditional rotationlly-enriched stars. The model further
predicts a strong mass-dependency for the dynamo-driven field. Above a
threshold mass, the strength of the magnetic dynamo decreases abruptly and so
we predict that more massive stars are much less likely to support a
dynamo-driven field than less massive stars.Comment: Accepted for publication in MNRAS. 15 pages, 13 figure
Magnetohydrodynamic relaxation of AGN ejecta: radio bubbles in the intracluster medium
X-ray images of galaxy clusters often display underdense bubbles which are
apparently inflated by AGN outflow. I consider the evolution of the magnetic
field inside such a bubble, using a mixture of analytic and numerical methods.
It is found that the field relaxes into an equilibrium filling the entire
volume of the bubble. The timescale on which this happens depends critically on
the magnetisation and helicity of the outflow as well as on properties of the
surrounding ICM. If the outflow is strongly magnetised, the magnetic field
undergoes reconnection on a short timescale, magnetic energy being converted
into heat whilst the characteristic length scale of the field rises; this
process stops when a global equilibrium is reached. The strength of the
equilibrium field is determined by the magnetic helicity injected into the
bubble by the AGN: if the outflow has a consistent net flux and consequently a
large helicity then a global equilibrium will be reached on a short timescale,
whereas a low-helicity outflow results in no global equilibrium being reached
and at the time of observation reconnection will be ongoing. However, localised
flux-tube equilibria will form. If, on the other hand, the outflow is very
weakly magnetised, no reconnection occurs and the magnetic field inside the
bubble remains small-scale and passive. These results have implications for the
internal composition of the bubbles, their interaction with ICM -- in
particular to explain how bubbles could move a large distance through the ICM
without breaking up -- as well as for the cooling flow problem in general. In
addition, reconnection sites in a bubble could be a convenient source of
energetic particles, circumventing the problem of synchrotron emitters having a
shorter lifetime than the age of the bubble they inhabit.Comment: MNRAS accepted. 15 pages, 10 figures
Magnetic field structure of relativistic jets without current sheets
We present an analytical class of equilibrium solutions for the structure of
relativistic sheared and rotating magnetized jets that contain no boundary
current sheets. We demonstrate the overall dynamical stability of these
solutions and, most importantly, a better numerical resistive stability than
the commonly employed force-free structures which inevitably require the
presence of dissipative surface currents. The jet is volumetrically confined by
the external pressure, with no pressure gradient on the surface. We calculate
the expected observed properties of such jets. Given the simplicity of these
solution we suggest them as useful initial conditions for relativistic jet
simulations.Comment: 13 pages, 13 figures, Accepted by MNRA
Active region formation through the negative effective magnetic pressure instability
The negative effective magnetic pressure instability operates on scales
encompassing many turbulent eddies and is here discussed in connection with the
formation of active regions near the surface layers of the Sun. This
instability is related to the negative contribution of turbulence to the mean
magnetic pressure that causes the formation of large-scale magnetic structures.
For an isothermal layer, direct numerical simulations and mean-field
simulations of this phenomenon are shown to agree in many details in that their
onset occurs at the same depth. This depth increases with increasing field
strength, such that the maximum growth rate of this instability is independent
of the field strength, provided the magnetic structures are fully contained
within the domain. A linear stability analysis is shown to support this
finding. The instability also leads to a redistribution of turbulent intensity
and gas pressure that could provide direct observational signatures.Comment: 19 pages, 10 figures, submitted to Solar Physic
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