271 research outputs found
Coupling the solar surface and the corona: coronal rotation, Alfv\'en wave-driven polar plumes
The dynamical response of the solar corona to surface and sub-surface
perturbations depends on the chromospheric stratification, and specifically on
how efficiently these layers reflect or transmit incoming Alfv\'en waves. While
it would be desirable to include the chromospheric layers in the numerical
simulations used to study such phenomena, that is most often not feasible. We
defined and tested a simple approximation allowing the study of coronal
phenomena while taking into account a parametrised chromospheric reflectivity.
We addressed the problems of the transmission of the surface rotation to the
corona and that of the generation of polar plumes by Alfv\'en waves (Pinto et
al., 2010, 2011). We found that a high (yet partial) effective chromospheric
reflectivity is required to properly describe the angular momentum balance in
the corona and the way the surface differential rotation is transmitted
upwards. Alfv\'en wave-driven polar plumes maintain their properties for a wide
range of values for the reflectivity, but they become bursty (and eventually
disrupt) when the limit of total reflection is attained.Comment: Solar Wind 13: Proceedings of the Thirteenth International Solar Wind
Conferenc
On the two-dimensional state in driven magnetohydrodynamic turbulence
The dynamics of the two-dimensional (2D) state in driven tridimensional (3D)
incompressible magnetohydrodynamic turbulence is investigated through
high-resolution direct numerical simulations and in the presence of an external
magnetic field at various intensities. For such a flow the 2D state (or slow
mode) and the 3D modes correspond respectively to spectral fluctuations in the
plan and in the area . It is shown that if
initially the 2D state is set to zero it becomes non negligible in few turnover
times particularly when the external magnetic field is strong. The maintenance
of a large scale driving leads to a break for the energy spectra of 3D modes;
when the driving is stopped the previous break is removed and a decay phase
emerges with alfv\'enic fluctuations. For a strong external magnetic field the
energy at large perpendicular scales lies mainly in the 2D state and in all
situations a pinning effect is observed at small scales.Comment: 11 pages, 11 figure
Finite dissipation and intermittency in magnetohydrodynamics
We present an analysis of data stemming from numerical simulations of
decaying magnetohydrodynamic (MHD) turbulence up to grid resolution of 1536^3
points and up to Taylor Reynolds number of 1200. The initial conditions are
such that the initial velocity and magnetic fields are helical and in
equipartition, while their correlation is negligible. Analyzing the data at the
peak of dissipation, we show that the dissipation in MHD seems to asymptote to
a constant as the Reynolds number increases, thereby strengthening the
possibility of fast reconnection events in the solar environment for very large
Reynolds numbers. Furthermore, intermittency of MHD flows, as determined by the
spectrum of anomalous exponents of structure functions of the velocity and the
magnetic field, is stronger than for fluids, confirming earlier results;
however, we also find that there is a measurable difference between the
exponents of the velocity and those of the magnetic field, as observed recently
in the solar wind. Finally, we discuss the spectral scaling laws that arise in
this flow.Comment: 4 pages, 4 figure
Phenomenology for the decay of energy-containing eddies in homogeneous MHD turbulence
We evaluate a number of simple, one‐point phenomenological models for the decay of energy‐containing eddies in magnetohydrodynamic(MHD) and hydrodynamicturbulence. The MHDmodels include effects of cross helicity and Alfvénic couplings associated with a constant mean magnetic field, based on physical effects well‐described in the literature. The analytic structure of three separate MHDmodels is discussed. The single hydrodynamic model and several MHDmodels are compared against results from spectral‐method simulations. The hydrodynamic model phenomenology has been previously verified against experiments in wind tunnels, and certain experimentally determined parameters in the model are satisfactorily reproduced by the present simulation. This agreement supports the suitability of our numerical calculations for examining MHDturbulence, where practical difficulties make it more difficult to study physical examples. When the triple‐decorrelation time and effects of spectral anisotropy are properly taken into account, particular MHDmodels give decay rates that remain correct to within a factor of 2 for several energy‐halving times. A simple model of this type is likely to be useful in a number of applications in space physics, astrophysics, and laboratory plasma physics where the approximate effects of turbulence need to be included
Coronal heating in coupled photosphere-chromosphere-coronal systems: turbulence and leakage
Coronal loops act as resonant cavities for low frequency fluctuations that
are transmitted from the deeper layers of the solar atmosphere and are
amplified in the corona, triggering nonlinear interactions. However trapping is
not perfect, some energy leaks down to the chromosphere, thus limiting the
turbulence development and the associated heating. We consider the combined
effects of turbulence and leakage in determining the energy level and
associated heating rate in models of coronal loops which include the
chromosphere and transition region. We use a piece-wise constant model for the
Alfven speed and a Reduced MHD - Shell model to describe the interplay between
turbulent dynamics in the direction perpendicular to the mean field and
propagation along the field. Turbulence is sustained by incoming fluctuations
which are equivalent, in the line-tied case, to forcing by the photospheric
shear flows. While varying the turbulence strength, we compare systematically
the average coronal energy level (E) and dissipation rate (D) in three models
with increasing complexity: the classical closed model, the semi-open corona
model, and the corona-chromosphere (or 3-layer) model, the latter two models
allowing energy leakage. We find that:
(i) Leakage always plays a role (even for strong turbulence), E and D are
systematically lower than in the line-tied model. (ii) E is close to the
resonant prediction, i.e., assuming effective turbulent correlation time longer
than the Alfven coronal crossing time (Ta). (iii) D is close to the value given
by the ratio of photospheric energy divided by Ta (iv) The coronal spectra
exibits an inertial range with 5/3 spectral slope, and a large scale peak of
trapped resonant modes that inhibit nonlinear couplings. (v) In the realistic
3-layer model, the two-component spectrum leads to a damping time equal to the
Kolmogorov time reduced by a factor u_rms/Va_coronaComment: 15 pages, 15 figures, Accepted for publication in A&
Spectral energy dynamics in magnetohydrodynamic turbulence
Spectral direct numerical simulations of incompressible MHD turbulence at a
resolution of up to collocation points are presented for a
statistically isotropic system as well as for a setup with an imposed strong
mean magnetic field. The spectra of residual energy,
, and total energy,
, are observed to scale self-similarly in
the inertial range as ,
(isotropic case) and ,
(anisotropic case, perpendicular to the mean
field direction). A model of dynamic equilibrium between kinetic and magnetic
energy, based on the corresponding evolution equations of the eddy-damped
quasi-normal Markovian (EDQNM) closure approximation, explains the findings.
The assumed interplay of turbulent dynamo and Alfv\'en effect yields
which is confirmed by the simulations.Comment: accepted for publication by PR
Strong Imbalanced Turbulence
We consider stationary, forced, imbalanced, or cross-helical MHD Alfvenic
turbulence where the waves traveling in one direction have higher amplitudes
than the opposite waves. This paper is dedicated to so-called strong
turbulence, which cannot be treated perturbatively. Our main result is that the
anisotropy of the weak waves is stronger than the anisotropy of a strong waves.
We propose that critical balance, which was originally conceived as a causality
argument, has to be amended by what we call a propagation argument. This
revised formulation of critical balance is able to handle the imbalanced case
and reduces to old formulation in the balanced case. We also provide
phenomenological model of energy cascading and discuss possibility of
self-similar solutions in a realistic setup of driven turbulence.Comment: this is shorter, 5 page version of what is to appear in ApJ 682, Aug.
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