169 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
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&
Phenomenology for the decay of energy-containing eddies in homogeneous MHD turbulence
We evaluate a number of simple, oneâpoint phenomenological models for the decay of energyâcontaining eddies in magnetohydrodynamic(MHD) and hydrodynamicturbulence. The MHDmodels include effects of cross helicity and AlfvĂ©nic couplings associated with a constant mean magnetic field, based on physical effects wellâdescribed in the literature. The analytic structure of three separate MHDmodels is discussed. The single hydrodynamic model and several MHDmodels are compared against results from spectralâmethod 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 tripleâdecorrelation 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 energyâhalving 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
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
Impact of observational uncertainties on universal scaling of MHD turbulence
Scaling exponents are the central quantitative prediction of theories of
turbulence and in-situ satellite observations of the high Reynolds number solar
wind flow have provided an extensive testbed of these. We propose a general,
instrument independent method to estimate the uncertainty of velocity field
fluctuations. We obtain the systematic shift that this uncertainty introduces
into the observed spectral exponent. This shift is essential for the correct
interpretation of observed scaling exponents. It is sufficient to explain the
contradiction between spectral features of the Elsasser fields observed in the
solar wind with both theoretical models and numerical simulations of
Magnetohydrodynamic turbulence
Nonlinear cascades in two-dimensional turbulent magnetoconvection
The dynamics of spectral transport in two-dimensional turbulent convection of
electrically conducting fluids is studied by means of direct numerical
simulations (DNS) in the frame of the magnetohydrodynamic (MHD) Boussinesq
approximation. The system performs quasi-oscillations between two different
regimes of small-scale turbulence: one dominated by nonlinear MHD interactions,
the other governed by buoyancy forces. The self-excited change of turbulent
states is reported here for the first time. The process is controlled by the
ideal invariant cross-helicity, . The observations are explained by the interplay of
convective driving with the nonlinear spectral transfer of total MHD energy and
cross-helicity.Comment: 4 pages, 4 figures, accepted for publication in PR
The interplay between helicity and rotation in turbulence: implications for scaling laws and small-scale dynamics
Invariance properties of physical systems govern their behavior: energy
conservation in turbulence drives a wide distribution of energy among modes,
observed in geophysical or astrophysical flows. In ideal hydrodynamics, the
role of helicity conservation (correlation between velocity and its curl,
measuring departures from mirror symmetry) remains unclear since it does not
alter the energy spectrum. However, with solid body rotation, significant
differences emerge between helical and non-helical flows. We first outline
several results, like the energy and helicity spectral distribution and the
breaking of strict universality for the individual spectra. Using massive
numerical simulations, we then show that small-scale structures and their
intermittency properties differ according to whether helicity is present or
not, in particular with respect to the emergence of Beltrami-core vortices
(BCV) that are laminar helical vertical updrafts. These results point to the
discovery of a small parameter besides the Rossby number; this could relate the
problem of rotating helical turbulence to that of critical phenomena, through
renormalization group and weak turbulence theory. This parameter can be
associated with the adimensionalized ratio of the energy to helicity flux to
small scales, the three-dimensional energy cascade being weak and self-similar
Numerical solutions of the three-dimensional magnetohydrodynamic alpha-model
We present direct numerical simulations and alpha-model simulations of four
familiar three-dimensional magnetohydrodynamic (MHD) turbulence effects:
selective decay, dynamic alignment, inverse cascade of magnetic helicity, and
the helical dynamo effect. The MHD alpha-model is shown to capture the
long-wavelength spectra in all these problems, allowing for a significant
reduction of computer time and memory at the same kinetic and magnetic Reynolds
numbers. In the helical dynamo, not only does the alpha-model correctly
reproduce the growth rate of magnetic energy during the kinematic regime, but
it also captures the nonlinear saturation level and the late generation of a
large scale magnetic field by the helical turbulence.Comment: 12 pages, 19 figure
Coherence and Intermittency of Electron Density in Small-Scale Interstellar Turbulence
Spatial intermittency in decaying kinetic Alfven wave turbulence is
investigated to determine if it produces non Gaussian density fluctuations in
the interstellar medium. Non Gaussian density fluctuations have been inferred
from pulsar scintillation scaling. Kinetic Alfven wave turbulence characterizes
density evolution in magnetic turbulence at scales near the ion gyroradius. It
is shown that intense localized current filaments in the tail of an initial
Gaussian probability distribution function possess a sheared magnetic field
that strongly refracts the random kinetic Alfven waves responsible for
turbulent decorrelation. The refraction localizes turbulence to the filament
periphery, hence it avoids mixing by the turbulence. As the turbulence decays
these long-lived filaments create a non Gaussian tail. A condition related to
the shear of the filament field determines which fluctuations become coherent
and which decay as random fluctuations. The refraction also creates coherent
structures in electron density. These structures are not localized. Their
spatial envelope maps into a probability distribution that decays as density to
the power -3. The spatial envelope of density yields a Levy distribution in the
density gradient.Comment: 31 pages, 3 figures. Replacement contains short additions to Secs. 6
and
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