16 research outputs found
Dissipative instability in partially ionised prominence plasmas
We investigate the nature of dissipative instability at the boundary (seen
here as tangential discontinuity) between the viscous corona and the partially
ionised prominence plasma in the incompressible limit. The importance of the
partial ionisation is investigated in terms of the ionisation fraction.
Matching the solutions for the transversal component of the velocity and total
pressure at the interface between the prominence and coronal plasmas, we derive
a dispersion relation whose imaginary part describes the evolution of the
instability. Results are obtained in the limit of weak dissipation. Using
simple analytical methods, we show that dissipative instabilities appear for
flow speeds that are lower than the Kelvin-Helmholtz instability threshold.
While viscosity tends to destabilise the plasma, the effect of partial
ionisation (through the Cowling resistivity) will act towards stabilising the
interface. For ionisation degrees closer to a neutral gas the interface will be
unstable for larger values of equilibrium flow. The same principle is assumed
when studying the appearance of instability at the interface between
prominences and dark plumes. The unstable mode appearing in this case has a
very small growth rate and dissipative instability cannot explain the
appearance of flows in plumes. The present study improves our understanding of
the complexity of dynamical processes at the interface of solar prominences and
solar corona, and the role partial ionisation can have on the stability of the
plasma. Our results clearly show that the problem of partial ionisation
introduces new aspects of plasma stability with consequences on the evolution
of solar prominences.Comment: 8 pages, 2 figure
Mean shear flows generated by nonlinear resonant Alfven waves
In the context of resonant absorption, nonlinearity has two different
manifestations. The first is the reduction in amplitude of perturbations around
the resonant point (wave energy absorption). The second is the generation of
mean shear flows outside the dissipative layer surrounding the resonant point.
Ruderman et al. [Phys. Plasmas 4, 75 (1997)] studied both these effects at the
slow resonance in isotropic plasmas. Clack et al. [Astron. Astrophys. 494}, 317
(2009)] investigated nonlinearity at the Alfven resonance, however, they did
not include the generation of mean shear flow. In this present paper, we
investigate the mean shear flow, analytically, and study its properties. We
find that the flow generated is parallel to the magnetic surfaces and has a
characteristic velocity proportional to , where is
the dimensionless amplitude of perturbations far away from the resonance. This
is, qualitatively, similar to the flow generated at the slow resonance. The
jumps in the derivatives of the parallel and perpendicular components of mean
shear flow across the dissipative layer are derived. We estimate the generated
mean shear flow to be of the order of in both the solar
upper chromosphere and solar corona, however, this value strongly depends on
the choice of boundary conditions. It is proposed that the generated mean shear
flow can produce a Kelvin--Helmholtz instability at the dissipative layer which
can create turbulent motions. This instability would be an additional effect,
as a Kelvin--Helmholtz instability may already exist due to the velocity field
of the resonant Alfven waves. This flow can also be superimposed onto existing
large scale motions in the solar upper atmosphere.Comment: 11 page
Nonlinear resonant absorption of fast magnetoacoustic waves in strongly anisotropic and dispersive plasmas
The nonlinear theory of driven magnetohydrodynamics (MHD) waves in strongly anisotropic and dispersive plasmas, developed for slow resonance by Clack and Ballai [Phys. Plasmas 15, 2310 (2008)] and Alfvén resonance by Clack et al. [Astron. Astrophys. 494, 317 (2009)] , is used to study the weakly nonlinear interaction of fast magnetoacoustic (FMA) waves in a one-dimensional planar plasma. The magnetic configuration consists of an inhomogeneous magnetic slab sandwiched between two regions of semi-infinite homogeneous magnetic plasmas. Laterally driven FMA waves penetrate the inhomogeneous slab interacting with the localized slow or Alfvén dissipative layer and are partly reflected, dissipated, and transmitted by this region. The nonlinearity parameter defined by Clack and Ballai (2008) is assumed to be small and a regular perturbation method is used to obtain analytical solutions in the slow dissipative layer. The effect of dispersion in the slow dissipative layer is to further decrease the coefficient of energy absorption, compared to its standard weakly nonlinear counterpart, and the generation of higher harmonics in the outgoing wave in addition to the fundamental one. The absorption of external drivers at the Alfvén resonance is described within the linear MHD with great accuracy
New approach for analysing dynamical processes on the surface of photospheric vortex tubes
The majority of studies on multi-scale vortex motions employ a
two-dimensional geometry by using a variety of observational and numerical
data. This approach limits the understanding the nature of physical processes
responsible for vortex dynamics. Here we develop a new methodology to extract
essential information from the boundary surface of vortex tubes. 3D
high-resolution magnetoconvection MURaM numerical data has been used to analyse
photospheric intergranular velocity vortices. The Lagrangian Averaged Vorticity
Deviation (LAVD) technique was applied to define the centers of vortex
structures and their boundary surfaces based on the advection of fluid
elements. These surfaces were mapped onto a constructed envelope grid that
allows the study of the key plasma parameters as functions of space and time.
Quantities that help in understanding the dynamics of the plasma, e.g. Lorentz
force, pressure force, plasma- were also determined. Our results suggest
that, while density and pressure have a rather global behaviour, the other
physical quantities undergo local changes, with their magnitude and orientation
changing in space and time. At the surface, the mixing in the horizontal
direction is not efficient, leading to appearance of localized regions with
higher/colder temperatures. In addition, the analysis of the MHD Poynting flux
confirms that the majority of the energy is directed in the horizontal
direction. Our findings also indicate that the pressure and magnetic forces
that drive the dynamics of the plasma on vortex surfaces are unbalanced and
therefore the vortices do not rotate as a rigid body
Nonlinear theory of resonant slow waves in anisotropic and dispersive plasmas
The solar corona is a typical example of a plasma with strongly anisotropic transport processes. The main dissipative mechanisms in the solar corona acting on slow magnetoacoustic waves are the anisotropic thermal conductivity and viscosity [Ballai et al., Phys. Plasmas 5, 252 (1998)] developed the nonlinear theory of driven slow resonant waves in such a regime. In the present paper the nonlinear behavior of driven magnetohydrodynamic waves in the slow dissipative layer in plasmas with strongly anisotropic viscosity and thermal conductivity is expanded by considering dispersive effects due to Hall currents. The nonlinear governing equation describing the dynamics of nonlinear resonant slow waves is supplemented by a term which describes nonlinear dispersion and is of the same order of magnitude as nonlinearity and dissipation. The connection formulas are found to be similar to their nondispersive counterparts
Nonlinear effects in resonant layers in solar and space plasmas
The present paper reviews recent advances in the theory of nonlinear driven
magnetohydrodynamic (MHD) waves in slow and Alfven resonant layers. Simple
estimations show that in the vicinity of resonant positions the amplitude of
variables can grow over the threshold where linear descriptions are valid.
Using the method of matched asymptotic expansions, governing equations of
dynamics inside the dissipative layer and jump conditions across the
dissipative layers are derived. These relations are essential when studying the
efficiency of resonant absorption. Nonlinearity in dissipative layers can
generate new effects, such as mean flows, which can have serious implications
on the stability and efficiency of the resonance