6,323 research outputs found
Magnetic self-organisation in Hall-dominated magnetorotational turbulence
The magnetorotational instability (MRI) is the most promising mechanism by
which angular momentum is efficiently transported outwards in astrophysical
discs. However, its application to protoplanetary discs remains problematic.
These discs are so poorly ionised that they may not support magnetorotational
turbulence in regions referred to as `dead zones'. It has recently been
suggested that the Hall effect, a non-ideal magnetohydrodynamic (MHD) effect,
could revive these dead zones by enhancing the magnetically active column
density by an order of magnitude or more. We investigate this idea by
performing local, three-dimensional, resistive Hall-MHD simulations of the MRI
in situations where the Hall effect dominates over Ohmic dissipation. As
expected from linear stability analysis, we find an exponentially growing
instability in regimes otherwise linearly stable in resistive MHD. However,
instead of vigorous and sustained magnetorotational turbulence, we find that
the MRI saturates by producing large-scale, long-lived, axisymmetric structures
in the magnetic and velocity fields. We refer to these structures as zonal
fields and zonal flows, respectively. Their emergence causes a steep reduction
in turbulent transport by at least two orders of magnitude from extrapolations
based upon resistive MHD, a result that calls into question contemporary models
of layered accretion. We construct a rigorous mean-field theory to explain this
new behaviour and to predict when it should occur. Implications for
protoplanetary disc structure and evolution, as well as for theories of planet
formation, are briefly discussed.Comment: 18 pages, 16 figures, accepted for publication in MNRA
Simulations of Field Driven Domain Wall Interactions in Ferromagnetic Nanowires
The interaction of domain walls in a single ferromagnetic nanowire has been observed with micromagnetic simulation. Domain walls separating domains of opposite magnetization move towards each other when an external field is applied along the long axis of the wire resulting in a collision. The final magnetic state of the wire after the collision will contain either zero (domain wall annihilation) or two (domain wall conservation) domain walls. Here we explore the behavior that determines the final state, showing that it depends on the initial domain wall configuration, the speed the domain walls are moving with before the collision, and the dimensions of the nanowire. A model is also presented which helps to determine the repulsive force the conserved domain walls exert on each other
Linear Vlasov theory of a magnetised, thermally stratified atmosphere
The stability of a collisionless, magnetised plasma to local convective
disturbances is examined, with a focus on kinetic and finite-Larmor-radius
effects. Specific application is made to the outskirts of galaxy clusters,
which contain hot and tenuous plasma whose temperature increases in the
direction of gravity. At long wavelengths (the "drift-kinetic" limit), we
obtain the kinetic version of the magnetothermal instability (MTI) and its
Alfv\'enic counterpart (Alfv\'enic MTI), which were previously discovered and
analysed using a magnetofluid (i.e. Braginskii) description. At sub-ion-Larmor
scales, we discover an overstability driven by the electron temperature
gradient of kinetic-Alfv\'en drift waves -- the electron MTI (eMTI) -- whose
growth rate is even larger than the standard MTI. At intermediate scales, we
find that ion finite-Larmor-radius effects tend to stabilise the plasma. We
discuss the physical interpretation of these instabilities in detail, and
compare them both with previous work on magnetised convection in a collisional
plasma and with temperature-gradient-driven drift-wave instabilities well-known
to the magnetic-confinement-fusion community. The implications of having both
fluid and kinetic scales simultaneously driven unstable by the same temperature
gradient are briefly discussed.Comment: 51 pages, 9 figures; to appear in Journal of Plasma Physic
Combined effects of a converging beam of light and mirror misalignment in Michelson interferometry
Expressions have been derived and calculations have been made which show that combined effects lead to asymmetric interferograms and reduction in power at zero path difference. Criteria are given for estimating maximum allowable mirror misalignment
Flexible arms provide constant force for pressure switch calibration
In-place calibration of a pressure switch is provided by a system of radially oriented flexing arms which, when rotated at a known velocity, convert the centrifugal force of the arms to a linear force along the shaft. The linear force, when applied to a pressure switch diaphragm, can then be calculated
Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability
In collisionless and weakly collisional plasmas, such as hot accretion flows
onto compact objects, the magnetorotational instability (MRI) can differ
significantly from the standard (collisional) MRI. In particular, pressure
anisotropy with respect to the local magnetic-field direction can both change
the linear MRI dispersion relation and cause nonlinear modifications to the
mode structure and growth rate, even when the field and flow perturbations are
small. This work studies these pressure-anisotropy-induced nonlinearities in
the weakly nonlinear, high-ion-beta regime, before the MRI saturates into
strong turbulence. Our goal is to better understand how the saturation of the
MRI in a low collisionality plasma might differ from that in the collisional
regime. We focus on two key effects: (i) the direct impact of self-induced
pressure-anisotropy nonlinearities on the evolution of an MRI mode, and (ii)
the influence of pressure anisotropy on the "parasitic instabilities" that are
suspected to cause the mode to break up into turbulence. Our main conclusions
are: (i) The mirror instability regulates the pressure anisotropy in such a way
that the linear MRI in a collisionless plasma is an approximate nonlinear
solution once the mode amplitude becomes larger than the background field (just
as in MHD). This implies that differences between the collisionless and
collisional MRI become unimportant at large amplitudes. (ii) The break up of
large amplitude MRI modes into turbulence via parasitic instabilities is
similar in collisionless and collisional plasmas. Together, these conclusions
suggest that the route to magnetorotational turbulence in a collisionless
plasma may well be similar to that in a collisional plasma, as suggested by
recent kinetic simulations. As a supplement to these findings, we offer
guidance for the design of future kinetic simulations of magnetorotational
turbulence.Comment: Submitted to Journal of Plasma Physic
Why we need to see the dark matter to understand the dark energy
The cosmological concordance model contains two separate constituents which
interact only gravitationally with themselves and everything else, the dark
matter and the dark energy. In the standard dark energy models, the dark matter
makes up some 20% of the total energy budget today, while the dark energy is
responsible for about 75%. Here we show that these numbers are only robust for
specific dark energy models and that in general we cannot measure the abundance
of the dark constituents separately without making strong assumptions.Comment: 4 pages, to be published in the Journal of Physics: Conference Series
as a contribution to the 2007 Europhysics Conference on High Energy Physic
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