534 research outputs found
Dissipative Taylor-Couette flows under the influence of helical magnetic fields
The linear stability of MHD Taylor-Couette flows in axially unbounded
cylinders is considered, for magnetic Prandtl number unity. Magnetic fields
varying from purely axial to purely azimuthal are imposed, with a general
helical field parameterized by \beta=B_\phi/B_z. We map out the transition from
the standard MRI for \beta=0 to the nonaxisymmetric Azimuthal MagnetoRotational
Instability (AMRI) for \beta\to \infty. For finite \beta, positive and negative
wave numbers m, corresponding to right and left spirals, are no longer
identical. The transition from \beta=0 to \beta\to\infty includes all the
possible forms of MRI with axisymmetric and nonaxisymmetric modes. For the
nonaxisymmetric modes, the most unstable mode spirals in the opposite direction
to the background field. The standard (\beta=0) MRI is axisymmetric for weak
fields (including the instability with the lowest Reynolds number) but is
nonaxisymmetric for stronger fields. If the azimuthal field is due in part to
an axial current flowing through the fluid itself (and not just along the
central axis), then it is also unstable to the nonaxisymmetric Tayler
instability, which is most effective without rotation. For large \beta this
instability has wavenumber m=1, whereas for \beta\simeq 1 m=2 is most unstable.
The most unstable mode spirals in the same direction as the background field.Comment: 9 pages, 11 figure
Global axisymmetric Magnetorotational Instability with density gradients
We examine global incompressible axisymmetric perturbations of a
differentially rotating MHD plasma with radial density gradients. It is shown
that the standard magnetorotational instability, (MRI) criterion drawn from the
local dispersion relation is often misleading. If the equilibrium magnetic
field is either purely axial or purely toroidal, the problem reduces to finding
the global radial eigenvalues of an effective potential. The standard Keplerian
profile including the origin is mathematically ill-posed, and thus any solution
will depend strongly on the inner boundary. We find a class of unstable modes
localized by the form of the rotation and density profiles, with reduced
dependence on boundary conditions.Comment: 22 pages, 5 figure
The Origin of Solar Activity in the Tachocline
Solar active regions, produced by the emergence of tubes of strong magnetic
field in the photosphere, are restricted to within 35 degrees of the solar
equator. The nature of the dynamo processes that create and renew these fields,
and are therefore responsible for solar magnetic phenomena, are not well
understood. We analyze the magneto-rotational stability of the solar tachocline
for general field geometry. This thin region of strong radial and latitudinal
differential rotation, between the radiative and convective zones, is unstable
at latitudes above 37 degrees, yet is stable closer to the equator. We propose
that small-scale magneto-rotational turbulence prevents coherent magnetic
dynamo action in the tachocline except in the vicinity of the equator, thus
explaining the latitudinal restriction of active regions. Tying the magnetic
dynamo to the tachocline elucidates the physical conditions and processes
relevant to solar magnetism.Comment: 10 pages, 1 figure, accepted for publication in ApJ
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
Relativistic stars with purely toroidal magnetic fields
We investigate the effects of the purely toroidal magnetic field on the
equilibrium structures of the relativistic stars. The master equations for
obtaining equilibrium solutions of relativistic rotating stars containing
purely toroidal magnetic fields are derived for the first time. To solve these
master equations numerically, we extend the Cook-Shapiro-Teukolsky scheme for
calculating relativistic rotating stars containing no magnetic field to
incorporate the effects of the purely toroidal magnetic fields. By using the
numerical scheme, we then calculate a large number of the equilibrium
configurations for a particular distribution of the magnetic field in order to
explore the equilibrium properties. We also construct the equilibrium sequences
of the constant baryon mass and/or the constant magnetic flux, which model the
evolution of an isolated neutron star as it loses angular momentum via the
gravitational waves. Important properties of the equilibrium configurations of
the magnetized stars obtained in this study are summarized as follows ; (1) For
the non-rotating stars, the matter distribution of the stars is prolately
distorted due to the toroidal magnetic fields. (2) For the rapidly rotating
stars, the shape of the stellar surface becomes oblate because of the
centrifugal force. But, the matter distribution deep inside the star is
sufficiently prolate for the mean matter distribution of the star to be
prolate. (3) The stronger toroidal magnetic fields lead to the mass-shedding of
the stars at the lower angular velocity. (4) For some equilibrium sequences of
the constant baryon mass and magnetic flux, the stars can spin up as they lose
angular momentum.Comment: 13 figures, 7 tables, submitted to PR
Investigation of the field-induced ferromagnetic phase transition in spin polarized neutron matter: a lowest order constrained variational approach
In this paper, the lowest order constrained variational (LOCV) method has
been used to investigate the magnetic properties of spin polarized neutron
matter in the presence of strong magnetic field at zero temperature employing
potential. Our results indicate that a ferromagnetic phase transition
is induced by a strong magnetic field with strength greater than ,
leading to a partial spin polarization of the neutron matter. It is also shown
that the equation of state of neutron matter in the presence of magnetic field
is stiffer than the case in absence of magnetic field.Comment: 23 pages, 9 figures Phys. Rev. C (2011) in pres
Outcomes and safety of concomitant nevirapine and rifampicin treatment under programme conditions in Malawi.
SETTING: Thyolo District Hospital, rural Malawi. OBJECTIVES: To report on 1) clinical, immunological and virological outcomes and 2) safety among human immunodeficiency virus (HIV) infected patients with tuberculosis (TB) who received concurrent nevirapine (NVP) and rifampicin (RMP) based treatment. DESIGN: Retrospective cohort study. METHODS: Analysis of programme data, June-December 2007. RESULTS: Of a total of 156 HIV-infected TB patients who started NVP-based antiretroviral treatment, 136 (87%) completed TB treatment successfully, 16 (10%) died and 5 (4%) were transferred out. Mean body weight and CD4 gain (adults) were respectively 4.4 kg (95%CI 3.3-5.4) and 140 cells/mm(3) (95%CI 117-162). Seventy-four per cent of patients who completed TB treatment and had a viral load performed (n = 74) had undetectable levels (<50 copies/ml), while 17 (22%) had a viral load of 50-1000 copies/ml. Hepatotoxicity was present in 2 (1.3%) patients at baseline. Two patients developed Grade 2 and one developed Grade 3 alanine transaminase enzyme elevations during TB treatment (incidence rate per 10 years of follow-up 4.2, 95%CI 1.4-13.1). There were no reported deaths linked to hepatotoxicity. CONCLUSIONS: In a rural district in Malawi, concomitant NVP and RMP treatment is associated with good TB treatment outcomes and appears safe. Further follow-up of patients would be useful to ascertain the longer-term effects of this concurrent treatment
A comparison of incompressible limits for resistive plasmas
The constraint of incompressibility is often used to simplify the
magnetohydrodynamic (MHD) description of linearized plasma dynamics because it
does not affect the ideal MHD marginal stability point. In this paper two
methods for introducing incompressibility are compared in a cylindrical plasma
model: In the first method, the limit is taken, where
is the ratio of specific heats; in the second, an anisotropic mass
tensor is used, with the component parallel to the magnetic
field taken to vanish, . Use of resistive MHD reveals
the nature of these two limits because the Alfv\'en and slow magnetosonic
continua of ideal MHD are converted to point spectra and moved into the complex
plane. Both limits profoundly change the slow-magnetosonic spectrum, but only
the second limit faithfully reproduces the resistive Alfv\'en spectrum and its
wavemodes. In ideal MHD, the slow magnetosonic continuum degenerates to the
Alfv\'en continuum in the first method, while it is moved to infinity by the
second. The degeneracy in the first is broken by finite resistivity. For
numerical and semi-analytical study of these models, we choose plasma
equilibria which cast light on puzzling aspects of results found in earlier
literature.Comment: 14 pages, 10 figure
The published research paper: is it an important indicator of successful operational research at programme level?
Is a published research paper an important indicator of successful operational research at programme level in low-income countries? In academia, publishing in peer-reviewed scientific journals is highly encouraged and strongly pursued for academic recognition and career progression. In contrast, for those who engage in operational research at programme level, there is often no necessity or reward for publishing the results of research studies; it may even be criticized as being an unnecessary detraction from programme-related work. We present arguments to support publishing operational research from low-income countries; we highlight some of the main reasons for failure of publication at programme level and suggest ways forward
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