201 research outputs found
Stationary field-aligned MHD flows at astropauses and in astrotails. Principles of a counterflow configuration between a stellar wind and its interstellar medium wind
A stellar wind passing through the reverse shock is deflected into the
astrospheric tail and leaves the stellar system either as a sub-Alfvenic or as
a super-Alfvenic tail flow. An example is our own heliosphere and its
heliotail. We present an analytical method of calculating stationary,
incompressible, and field-aligned plasma flows in the astrotail of a star. We
present a recipe for constructing an astrosphere with the help of only a few
parameters, like the inner Alfven Mach number and the outer Alfven Mach number,
the magnetic field strength within and outside the stellar wind cavity, and the
distribution of singular points of the magnetic field within these flows.
Within the framework of a one-fluid approximation, it is possible to obtain
solutions of the MHD equations for stationary flows from corresponding static
MHD equilibria, by using noncanonical mappings of the canonical variables. The
canonical variables are the Euler potentials of the magnetic field of
magnetohydrostatic equilibria. Thus we start from static equilibria determined
by the distribution of magnetic neutral points, and assume that the Alfven Mach
number for the corresponding stationary equilibria is finite. The topological
structure determines the geometrical structure of the interstellar gas -
stellar wind interface. Additional boundary conditions like the outer magnetic
field and the jump of the magnetic field across the astropause allow
determination of the noncanonical transformations. This delivers the strength
of the magnetic field at every point in the astrotail region beyond the reverse
shock. The mathematical technique for describing such a scenario is applied to
astrospheres in general, but is also relevant for the heliosphere. It shows the
restrictions of the outer and the inner magnetic field strength in comparison
with the corresponding Alfven Mach numbers in the case of subalfvenic flows.Comment: 19 pages, 17 figures, accepted for publication in A&
Magnetohydrostatic solar prominences in near-potential coronal magnetic fields
We present numerical magnetohydrostatic solutions describing the
gravitationally stratified, bulk equilibrium of cool, dense prominence plasma
embedded in a near-potential coronal field. These solutions are calculated
using the FINESSE magnetohydrodynamics equilibrium solver and describe the
morphologies of magnetic field distributions in and around prominences and the
cool prominence plasma that these fields support. The equilibrium condition for
this class of problem is usually different in distinct subdomains, separated by
free boundaries, across which solutions are matched by suitable continuity or
jump conditions describing force balance. We employ our precise finite element
elliptic solver to calculate solutions not accessible by previous analytical
techniques with temperature or entropy prescribed as free functions of the
magnetic flux function, including a range of values of the polytropic index,
temperature variations mainly across magnetic field lines and photospheric
field profiles sheared close to the polarity inversion line. Out of the many
examples computed here, perhaps the most noteworthy is one which reproduces
precisely the three-part structure often encountered in observations: a cool
dense prominence within a cavity/flux rope embedded in a hot corona. The
stability properties of these new equilibria, which may be relevant to solar
eruptions, can be determined in the form of a full resistive MHD spectrum using
a companion hyperbolic stability solver.Comment: To appear in ApJ August 200
Double-resonant fast particle-wave interaction
In future fusion devices fast particles must be well confined in order to
transfer their energy to the background plasma. Magnetohydrodynamic
instabilities like Toroidal Alfv\'en Eigenmodes or core-localized modes such as
Beta Induced Alfv\'en Eigenmodes and Reversed Shear Alfv\'en Eigenmodes, both
driven by fast particles, can lead to significant losses. This is observed in
many ASDEX Upgrade discharges. The present study applies the drift-kinetic
HAGIS code with the aim of understanding the underlying resonance mechanisms,
especially in the presence of multiple modes with different frequencies. Of
particular interest is the resonant interaction of particles simultaneously
with two different modes, referred to as 'double-resonance'. Various mode
overlapping scenarios with different q profiles are considered. It is found
that, depending on the radial mode distance, double-resonance is able to
enhance growth rates as well as mode amplitudes significantly. Surprisingly, no
radial mode overlap is necessary for this effect. Quite the contrary is found:
small radial mode distances can lead to strong nonlinear mode stabilization of
a linearly dominant mode.Comment: 12 pages, 11 figures; Nuclear Fusion 52 (2012
Numerical simulations of kink instability in line-tied coronal loops
The results from numerical simulations carried out using a new shock-capturing, Lagrangian-remap, 3D MHD code, Lare3d are presented. We study the evolution of the m=1 kink mode instability in a photospherically line-tied coronal loop that has no net axial current. During the non-linear evolution of the kink instability, large current concentrations develop in the neighbourhood of the infinite length mode rational surface. We investigate whether this strong current saturates at a finite value or whether scaling indicates current sheet formation. In particular, we consider the effect of the shear, defined by where is the fieldline twist of the loop, on the current concentration. We also include a non-uniform resistivity in the simulations and observe the amount of free magnetic energy released by magnetic reconnection
Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem
The impulsive phase of a solar flare marks the epoch of rapid conversion of
energy stored in the pre-flare coronal magnetic field. Hard X-ray observations
imply that a substantial fraction of flare energy released during the impulsive
phase is converted to the kinetic energy of mildly relativistic electrons
(10-100 keV). The liberation of the magnetic free energy can occur as the
coronal magnetic field reconfigures and relaxes following reconnection. We
investigate a scenario in which products of the reconfiguration - large-scale
Alfven wave pulses - transport the energy and magnetic-field changes rapidly
through the corona to the lower atmosphere. This offers two possibilities for
electron acceleration. Firstly, in a coronal plasma with beta < m_e/m_p, the
waves propagate as inertial Alfven waves. In the presence of strong spatial
gradients, these generate field-aligned electric fields that can accelerate
electrons to energies on the order of 10 keV and above, including by repeated
interactions between electrons and wavefronts. Secondly, when they reflect and
mode-convert in the chromosphere, a cascade to high wavenumbers may develop.
This will also accelerate electrons by turbulence, in a medium with a locally
high electron number density. This concept, which bridges MHD-based and
particle-based views of a flare, provides an interpretation of the
recently-observed rapid variations of the line-of-sight component of the
photospheric magnetic field across the flare impulsive phase, and offers
solutions to some perplexing flare problems, such as the flare "number problem"
of finding and resupplying sufficient electrons to explain the impulsive-phase
hard X-ray emission.Comment: 31 pages, 6 figure
Three-dimensional stability of magnetically confined mountains on accreting neutron stars
We examine the hydromagnetic stability of magnetically confined mountains,
which arise when material accumulates at the magnetic poles of an accreting
neutron star. We extend a previous axisymmetric stability analysis by
performing three-dimensional simulations using the ideal-magnetohydrodynamic
(ideal-MHD) code \textsc{zeus-mp}, investigating the role played by boundary
conditions, accreted mass, stellar curvature, and (briefly) toroidal magnetic
field strength. We find that axisymmetric equilibria are susceptible to the
undular sub-mode of the Parker instability but are not disrupted. The
line-tying boundary condition at the stellar surface is crucial in stabilizing
the mountain. The nonlinear three-dimensional saturation state of the
instability is characterized by a small degree of nonaxisymmetry (\la 0.1 per
cent) and a mass ellipticity of for an accreted mass of
. Hence there is a good prospect of detecting
gravitational waves from accreting millisecond pulsars with long-baseline
interferometers such as Advanced LIGO. We also investigate the ideal-MHD
spectrum of the system, finding that long-wavelength poloidal modes are
suppressed in favour of toroidal modes in the nonaxisymmetric saturation state.Comment: accepted by MNRA
Pressure-driven instabilities in astrophysical jets
Astrophysical jets are widely believed to be self-collimated by the
hoop-stress due to the azimuthal component of their magnetic field. However
this implies that the magnetic field is largely dominated by its azimuthal
component in the outer jet region. In the fusion context, it is well-known that
such configurations are highly unstable in static columns, leading to plasma
disruption. It has long been pointed out that a similar outcome may follow for
MHD jets, and the reasons preventing disruption are still not elucidated,
although some progress has been accomplished in the recent years.
In these notes, I review the present status of this open problem for
pressure-driven instabilities, one of the two major sources of ideal MHD
instability in static columns (the other one being current-driven
instabilities).
I first discuss in a heuristic way the origin of these instabilities.
Magnetic resonances and magnetic shear are introduced, and their role in
pressure-driven instabilities discussed in relation to Suydam's criterion. A
dispersion relation is derived for pressure-driven modes in the limit of large
azimuthal magnetic fields, which gives back the two criteria derived by
Kadomtsev for this instability. The growth rates of these instabilities are
expected to be short in comparison with the jet propagation time.
What is known about the potential stabilizing role of the axial velocity of
jets is then reviewed. In particular, a nonlinear stabilization mechanism
recently identified in the fusion literature is discussed.
Key words: Ideal MHD: stability, pressure-driven modes; Jets: stabilityComment: 20 pages, 3 figures. Lecture given at the JETSET European school
"Numerical MHD and Instabilities". To be published by Springer in the
"Lectures notes in physics" serie
Global solutions to the three-dimensional full compressible magnetohydrodynamic flows
The equations of the three-dimensional viscous, compressible, and heat
conducting magnetohydrodynamic flows are considered in a bounded domain. The
viscosity coefficients and heat conductivity can depend on the temperature. A
solution to the initial-boundary value problem is constructed through an
approximation scheme and a weak convergence method. The existence of a global
variational weak solution to the three-dimensional full magnetohydrodynamic
equations with large data is established
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