45,073 research outputs found
Magnetohydrodynamic Shallow Water Waves: Linear Analysis
We present a linear analysis of inviscid, incompressible, magnetohydrodynamic
(MHD) shallow water systems. In spherical geometry, a generic property of such
systems is the existence of five wave modes. Three of them (two
magneto-Poincare modes and one magneto-Rossby mode) are previously known. The
other two wave modes are strongly influenced by the magnetic field and
rotation, and have substantially lower angular frequencies; as such, we term
them "magnetostrophic modes". We obtain analytical functions for the velocity,
height and magnetic field perturbations in the limit that the magnitude of the
MHD analogue of Lamb's parameter is large. On a sphere, the magnetostrophic
modes reside near the poles, while the other modes are equatorially confined.
Magnetostrophic modes may be an ingredient in explaining the frequency drifts
observed in Type I X-ray bursts from neutron stars.Comment: 15 pages (emulateapj), 4 figures. Accepted by ApJ (no changes in text
from previous versions, but ApJ wants no acronyms in title). To appear in Oct
2009, Vol 703/704 of the Astrophysical Journa
Black Hole Hair Removal: Non-linear Analysis
BMPV black holes in flat transverse space and in Taub-NUT space have
identical near horizon geometries but different microscopic degeneracies. It
has been proposed that this difference can be accounted for by different
contribution to the degeneracies of these black holes from hair modes, --
degrees of freedom living outside the horizon. In this paper we explicitly
construct the hair modes of these two black holes as finite bosonic and
fermionic deformations of the black hole solution satisfying the full
non-linear equations of motion of supergravity and preserving the supersymmetry
of the original solutions. Special care is taken to ensure that these solutions
do not have any curvature singularity at the future horizon when viewed as the
full ten dimensional geometry. We show that after removing the contribution due
to the hair degrees of freedom from the microscopic partition function, the
partition functions of the two black holes agree.Comment: 40 pages, LaTe
Regional Agglomeration in Portugal: A Linear Analysis
This work aims to study the Portuguese regional agglomeration process, using
the linear form the New Economic Geography models that emphasize the importance
of spatial factors (distance, costs of transport and communication) in
explaining of the concentration of economic activity in certain locations. In a
theoretical context, it is intended to explain the complementarily of
clustering models, associated with the New Economic Geography, and polarization
associated with the Keynesian tradition, describing the mechanisms by which
these processes are based. As a summary conclusion, we can say which the
agglomeration process shows some signs of concentration in Lisboa e Vale do
Tejo (which is evidence of regional divergence in Portugal) and the
productivity factor significantly improves the results that explain the
regional clustering in Portugal (despite being ignored in the models of New
Economic Geography)
Gravitational Fragmentation of Expanding Shells. I. Linear Analysis
We perform a linear perturbation analysis of expanding shells driven by
expansions of HII regions. The ambient gas is assumed to be uniform. As an
unperturbed state, we develop a semi-analytic method for deriving the time
evolution of the density profile across the thickness. It is found that the
time evolution of the density profile can be divided into three evolutionary
phases, deceleration-dominated, intermediate, and self-gravity-dominated
phases. The density peak moves relatively from the shock front to the contact
discontinuity as the shell expands. We perform a linear analysis taking into
account the asymmetric density profile obtained by the semi-analytic method,
and imposing the boundary conditions for the shock front and the contact
discontinuity while the evolutionary effect of the shell is neglected. It is
found that the growth rate is enhanced compared with the previous studies based
on the thin-shell approximation. This is due to the boundary effect of the
contact discontinuity and asymmetric density profile that were not taken into
account in previous works.Comment: 13 pages, 13 figures, to be published in the Astrophysical Journa
Gravitational Fragmentation of Expanding Shells. I. Linear Analysis
We perform a linear perturbation analysis of expanding shells driven by
expansions of HII regions. The ambient gas is assumed to be uniform. As an
unperturbed state, we develop a semi-analytic method for deriving the time
evolution of the density profile across the thickness. It is found that the
time evolution of the density profile can be divided into three evolutionary
phases, deceleration-dominated, intermediate, and self-gravity-dominated
phases. The density peak moves relatively from the shock front to the contact
discontinuity as the shell expands. We perform a linear analysis taking into
account the asymmetric density profile obtained by the semi-analytic method,
and imposing the boundary conditions for the shock front and the contact
discontinuity while the evolutionary effect of the shell is neglected. It is
found that the growth rate is enhanced compared with the previous studies based
on the thin-shell approximation. This is due to the boundary effect of the
contact discontinuity and asymmetric density profile that were not taken into
account in previous works.Comment: 13 pages, 13 figures, to be published in the Astrophysical Journa
Periodicity makes galactic shocks unstable - I. Linear analysis
We study the dynamical stability of stationary galactic spiral shocks. The
steady-state equilibrium flow contains a shock of the type derived by Roberts
in the tightly wound approximation. We find that boundary conditions are
critical in determining whether the solutions are stable or not. Shocks are
unstable if periodic boundary conditions are imposed. For intermediate
strengths of the spiral potential, the instability disappears if boundary
conditions are imposed such that the upstream flow is left unperturbed as in
the classic analysis of D'yakov and Kontorovich. This reconciles apparently
contradictory findings of previous authors regarding the stability of spiral
shocks. This also shows that the instability is distinct from the
Kelvin-Helmholtz instability, confirming the findings of Kim et al. We suggest
that instability is a general characteristics of periodic shocks, regardless of
the presence of shear, and provide a physical picture as to why this is the
case. For strong spiral potentials, high post-shock shear makes the system
unstable also to parasitic Kelvin-Helmholtz instability regardless of the
boundary conditions. Our analysis is performed in the context of a simplified
problem that, while preserving all the important characteristics of the
original problem, strips it from unnecessary complications, and assumes that
the gas is isothermal, non self-gravitating, non-magnetised.Comment: Accepted for publication in MNRA
Regional agglomeration in Portugal: a linear analysis
This work aims to study the Portuguese regional agglomeration process, using the linear form the New Economic Geography models that emphasize the importance of spatial factors (distance, costs of transport and communication) in explaining of the concentration of economic activity in certain locations. In a theoretical context, it is intended to explain the complementarily of clustering models, associated with the New Economic Geography, and polarization associated with the Keynesian tradition, describing the mechanisms by which these processes are based. As a summary conclusion, we can say which the agglomeration process shows some signs of concentration in Lisboa e Vale do Tejo (which is evidence of regional divergence in Portugal) and the productivity factor significantly improves the results that explain the regional clustering in Portugal (despite being ignored in the models of New Economic Geography).agglomeration; Portuguese regions; linear models
Linear analysis of a force reflective teleoperator
Complex force reflective teleoperation systems are often very difficult to analyze due to the large number of components and control loops involved. One mode of a force reflective teleoperator is described. An analysis of the performance of the system based on a linear analysis of the general full order model is presented. Reduced order models are derived and correlated with the full order models. Basic effects of force feedback and position feedback are examined and the effects of time delays between the master and slave are studied. The results show that with symmetrical position-position control of teleoperators, a basic trade off must be made between the intersystem stiffness of the teleoperator, and the impedance felt by the operator in free space
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