202 research outputs found
Rising Powers and State Transformation: The Case of China
This article draws attention to the transformation of statehood under globalisation as a crucial dynamic shaping the emergence and conduct of ‘rising powers’. That states are becoming increasingly fragmented, decentralised and internationalised is noted by some international political economy and global governance scholars, but is neglected in International Relations treatments of rising powers. This article critiques this neglect, demonstrating the importance of state transformation in understanding emerging powers’ foreign and security policies, and their attempts to manage their increasingly transnational interests by promoting state transformation elsewhere, particularly in their near-abroad. It demonstrates the argument using the case of China, typically understood as a classical ‘Westphalian’ state. In reality, the Chinese state’s substantial disaggregation profoundly shapes its external conduct in overseas development assistance and conflict zones like the South China Sea, and in its promotion of extraterritorial governance arrangements in spaces like the Greater Mekong Subregion
The contested rescaling of economic governance in East Asia: a special issue
The special issue this article opens engages with an apparent conundrum that has often puzzled observers of East Asian politics—why, despite the region's considerable economic integration, multilateral economic governance institutions remain largely underdeveloped. The authors argue that this ‘regionalism problématique’ has led to the neglect of prior and more important questions pertaining to how patterns of economic governance, beyond the national scale, are emerging in East Asia and why. In this special issue, the contributors shift analytic focus onto social and political struggles over the scale and instruments of economic governance in East Asia. The contributions identify and explain the emergence of a wide variety of regional modes of economic governance often neglected by the scholarship or erroneously viewed as stepping stones towards ‘deeper’ multilateralism
Entropy Maximization and Instability of Uniformly Magnetized Plasma
A regime where a uniformly magnetized plasma could be unstable to a spatial
perturbation in the magnetic field is explored. In this regime, a uniformly
magnetized state does not maximize the entropy. The physical implication is
discussed in the context of the current generation, the magnetic reconnection,
and the dynamo effect
Magnetic Reynolds number dependence of reconnection rate and flow structure of the self-similar evolution model of fast magnetic reconnection
This paper investigates Magnetic Reynolds number dependence of the
``self-similar evolution model'' (Nitta et al. 2001) of fast magnetic
reconnection. I focused my attention on the flow structure inside and around
the reconnection outflow, which is essential to determine the entire
reconnection system (Nitta et al. 2002). The outflow is consist of several
regions divided by discontinuities, e.g., shocks, and it can be treated by a
shock-tube approximation (Nitta 2004). By solving the junction conditions
(e.g., Rankine-Hugoniot condition), the structure of the reconnection outflow
is obtained. Magnetic reconnection in most astrophysical problems is
characterized by a huge dynamic range of its expansion ( for typical
solar flares) in a free space which is free from any influence of external
circumstances. Such evolution results in a spontaneous self-similar expansion
which is controlled by two intrinsic parameters: the plasma- and the
magnetic Reynolds number. The plasma- dependence had been investigated in
our previous paper. This paper newly clarifies the relation between the
reconnection rate and the inflow structure just outside the Petschek-like slow
shock: As the magnetic Reynolds number increases, strongly converging inflow
toward the Petschek-like slow shock forms, and it significantly reduces the
reconnection rate.Comment: 16 pages. to appear in ApJ (2006 Jan. 20 issue
Toward detailed prominence seismology - I. Computing accurate 2.5D magnetohydrodynamic equilibria
Context. Prominence seismology exploits our knowledge of the linear
eigenoscillations for representative magnetohydro- dynamic models of filaments.
To date, highly idealized models for prominences have been used, especially
with respect to the overall magnetic configurations.
Aims. We initiate a more systematic survey of filament wave modes, where we
consider full multi-dimensional models with twisted magnetic fields
representative of the surrounding magnetic flux rope. This requires the ability
to compute accurate 2.5 dimensional magnetohydrodynamic equilibria that balance
Lorentz forces, gravity, and pressure gradients, while containing density
enhancements (static or in motion).
Methods. The governing extended Grad-Shafranov equation is discussed, along
with an analytic prediction for circular flux ropes for the Shafranov shift of
the central magnetic axis due to gravity. Numerical equilibria are computed
with a finite element-based code, demonstrating fourth order accuracy on an
explicitly known, non-trivial test case.
Results. The code is then used to construct more realistic prominence
equilibria, for all three possible choices of a free flux-function. We quantify
the influence of gravity, and generate cool condensations in hot cavities, as
well as multi- layered prominences.
Conclusions. The internal flux rope equilibria computed here have the
prerequisite numerical accuracy to allow a yet more advanced analysis of the
complete spectrum of linear magnetohydrodynamic perturbations, as will be
demonstrated in the companion paper.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 15 figure
Toward detailed prominence seismology - II. Charting the continuous magnetohydrodynamic spectrum
Starting from accurate MHD flux rope equilibria containing prominence
condensations, we initiate a systematic survey of their linear
eigenoscillations. To quantify the full spectrum of linear MHD eigenmodes, we
require knowledge of all flux-surface localized modes, charting out the
continuous parts of the MHD spectrum. We combine analytical and numerical
findings for the continuous spectrum for realistic prominence configurations.
The equations governing all eigenmodes for translationally symmetric,
gravitating equilibria containing an axial shear flow, are analyzed, along with
their flux-surface localized limit. The analysis is valid for general 2.5D
equilibria, where either density, entropy, or temperature vary from one flux
surface to another. We analyze the mode couplings caused by the poloidal
variation in the flux rope equilibria, by performing a small gravity parameter
expansion. We contrast the analytical results with continuous spectra obtained
numerically. For equilibria where the density is a flux function, we show that
continuum modes can be overstable, and we present the stability criterion for
these convective continuum instabilities. Furthermore, for all equilibria, a
four-mode coupling scheme between an Alfvenic mode of poloidal mode number m
and three neighboring (m-1, m, m+1) slow modes is identified, occurring in the
vicinity of rational flux surfaces. For realistically prominence equilibria,
this coupling is shown to play an important role, from weak to stronger gravity
parameter g values. The analytic predictions for small g are compared with
numerical spectra, and progressive deviations for larger g are identified. The
unstable continuum modes could be relevant for short-lived prominence
configurations. The gaps created by poloidal mode coupling in the continuous
spectrum need further analysis, as they form preferred frequency ranges for
global eigenoscillations.Comment: Accepted by Astronmy & Astrophysics, 21 pages, 15 figure
Unstable magnetohydrodynamical continuous spectrum of accretion disks. A new route to magnetohydrodynamical turbulence in accretion disks
We present a detailed study of localised magnetohydrodynamical (MHD)
instabilities occuring in two--dimensional magnetized accretion disks. We model
axisymmetric MHD disk tori, and solve the equations governing a
two--dimensional magnetized accretion disk equilibrium and linear wave modes
about this equilibrium. We show the existence of novel MHD instabilities in
these two--dimensional equilibria which do not occur in an accretion disk in
the cylindrical limit. The disk equilibria are numerically computed by the
FINESSE code. The stability of accretion disks is investigated analytically as
well as numerically. We use the PHOENIX code to compute all the waves and
instabilities accessible to the computed disk equilibrium. We concentrate on
strongly magnetized disks and sub--Keplerian rotation in a large part of the
disk. These disk equilibria show that the thermal pressure of the disk can only
decrease outwards if there is a strong gravitational potential. Our theoretical
stability analysis shows that convective continuum instabilities can only
appear if the density contours coincide with the poloidal magnetic flux
contours. Our numerical results confirm and complement this theoretical
analysis. Furthermore, these results show that the influence of gravity can
either be stabilizing or destabilizing on this new kind of MHD instability. In
the likely case of a non--constant density, the height of the disk should
exceed a threshold before this type of instability can play a role. This
localised MHD instability provides an ideal, linear route to MHD turbulence in
strongly magnetized accretion disk tori.Comment: 20 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Magnetohydrodynamic equilibria of a cylindrical plasma with poloidal mass flow and arbitrary cross section shape
The equilibrium of a cylindrical plasma with purely poloidal mass flow and
cross section of arbitrary shape is investigated within the framework of the
ideal MHD theory. For the system under consideration it is shown that only
incompressible flows are possible and, conscequently, the general two
dimensional flow equilibrium equations reduce to a single second-order
quasilinear partial differential equation for the poloidal magnetic flux
function , in which four profile functionals of appear. Apart from
a singularity occuring when the modulus of Mach number associated with the
Alfv\'en velocity for the poloidal magnetic field is unity, this equation is
always elliptic and permits the construction of several classes of analytic
solutions. Specific exact equlibria for a plasma confined within a perfectly
conducting circular cylindrical boundary and having i) a flat current density
and ii) a peaked current density are obtained and studied.Comment: Accepted to Plasma Physics & Controlled Fusion, 14 pages, revte
Feature Lines for Illustrating Medical Surface Models: Mathematical Background and Survey
This paper provides a tutorial and survey for a specific kind of illustrative
visualization technique: feature lines. We examine different feature line
methods. For this, we provide the differential geometry behind these concepts
and adapt this mathematical field to the discrete differential geometry. All
discrete differential geometry terms are explained for triangulated surface
meshes. These utilities serve as basis for the feature line methods. We provide
the reader with all knowledge to re-implement every feature line method.
Furthermore, we summarize the methods and suggest a guideline for which kind of
surface which feature line algorithm is best suited. Our work is motivated by,
but not restricted to, medical and biological surface models.Comment: 33 page
Turbulent Magnetic Reconnection in Two Dimensions
Two-dimensional numerical simulations of the effect of background turbulence
on 2D resistive magnetic reconnection are presented. For sufficiently small
values of the resistivity () and moderate values of the turbulent power
(), the reconnection rate is found to have a much weaker dependence
on than the Sweet-Parker scaling of and is even consistent
with an independent value. For a given value of , the dependence
of the reconnection rate on the turbulent power exhibits a critical threshold
in above which the reconnection rate is significantly enhanced.Comment: Accepted to MNRA
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