69 research outputs found
Turbulent Coronal Heating Mechanisms: Coupling of Dynamics and Thermodynamics
Context. Photospheric motions shuffle the footpoints of the strong axial
magnetic field that threads coronal loops giving rise to turbulent nonlinear
dynamics characterized by the continuous formation and dissipation of
field-aligned current sheets where energy is deposited at small-scales and the
heating occurs. Previous studies show that current sheets thickness is orders
of magnitude smaller than current state of the art observational resolution
(~700 km).
Aim. In order to understand coronal heating and interpret correctly
observations it is crucial to study the thermodynamics of such a system where
energy is deposited at unresolved small-scales.
Methods. Fully compressible three-dimensional magnetohydrodynamic simulations
are carried out to understand the thermodynamics of coronal heating in the
magnetically confined solar corona.
Results. We show that temperature is highly structured at scales below
observational resolution and nonhomogeneously distributed so that only a
fraction of the coronal mass and volume gets heated at each time.
Conclusions. This is a multi-thermal system where hotter and cooler plasma
strands are found one next to the other also at sub-resolution scales and
exhibit a temporal dynamics.Comment: A&A Letter, in pres
Turbulence, Energy Transfers and Reconnection in Compressible Coronal Heating Field-line Tangling Models
MHD turbulence has long been proposed as a mechanism for the heating of
coronal loops in the framework of the Parker scenario for coronal heating. So
far most of the studies have focused on its dynamical properties without
considering its thermodynamical and radiative features, because of the very
demanding computational requirements. In this paper we extend this previous
research to the compressible regime, including an energy equation, by using
HYPERION, a new parallelized, viscoresistive, three-dimensional compressible
MHD code. HYPERION employs a Fourier collocation -- finite difference spatial
discretization, and uses a third-order Runge-Kutta temporal discretization. We
show that the implementation of a thermal conduction parallel to the DC
magnetic field induces a radiative emission concentrated at the boundaries,
with properties similar to the chromosphere--transition region--corona system.Comment: 4 pages, 4 figures, Solar Wind 12 proceedings (in press
Viscous, resistive MHD stability computed by spectral techniques
Expansions in Chebyshev polynomials are used to study the linear stability of one dimensional magnetohydrodynamic (MHD) quasi-equilibria, in the presence of finite resistivity and viscosity. The method is modeled on the one used by Orszag in accurate computation of solutions of the Orr-Sommerfeld equation. Two Reynolds like numbers involving Alfven speeds, length scales, kinematic viscosity, and magnetic diffusivity govern the stability boundaries, which are determined by the geometric mean of the two Reynolds like numbers. Marginal stability curves, growth rates versus Reynolds like numbers, and growth rates versus parallel wave numbers are exhibited. A numerical result which appears general is that instability was found to be associated with inflection points in the current profile, though no general analytical proof has emerged. It is possible that nonlinear subcritical three dimensional instabilities may exist, similar to those in Poiseuille and Couette flow
Modeling the Galactic Center Nonthermal Filaments as Magnetized Wake
We simulate the Galactic Center nonthermal filaments as magnetized wakes
formed dynamically from amplification of a weak (tens of G) global
magnetic field through the interaction of molecular clouds with a Galactic
Center wind. One of the key issues in this cometary model is the stability of
the filament against dynamical disruption. Here we show 2-dimensional MHD
simulations for interstellar conditions that are appropriate for the Galactic
Center. The structures eventually disrupt through a shear driven nonlinear
instability but maintain coherence for lengths up to 100 times their width as
observed. The final instability, which destroys the filament through shredding
and plasmoid formation, grows quickly in space (and time) and leads to an
abrupt end to the structure, in accord with observations. As a by-product, the
simulation shows that emission should peak well downstream from the cloud-wind
interaction site.Comment: postscript file, 7 figs (included); Accepted for publication in ApJ
(Part 1
Nonlinear Dynamics of the Parker Scenario for Coronal Heating
The Parker or field line tangling model of coronal heating is studied
comprehensively via long-time high-resolution simulations of the dynamics of a
coronal loop in cartesian geometry within the framework of reduced
magnetohydrodynamics (RMHD). Slow photospheric motions induce a Poynting flux
which saturates by driving an anisotropic turbulent cascade dominated by
magnetic energy. In physical space this corresponds to a magnetic topology
where magnetic field lines are barely entangled, nevertheless current sheets
(corresponding to the original tangential discontinuities hypothesized by
Parker) are continuously formed and dissipated.
Current sheets are the result of the nonlinear cascade that transfers energy
from the scale of convective motions () down to the dissipative
scales, where it is finally converted to heat and/or particle acceleration.
Current sheets constitute the dissipative structure of the system, and the
associated magnetic reconnection gives rise to impulsive ``bursty'' heating
events at the small scales. This picture is consistent with the slender loops
observed by state-of-the-art (E)UV and X-ray imagers which, although apparently
quiescent, shine bright in these wavelengths with little evidence of entangled
features.
The different regimes of weak and strong MHD turbulence that develop, and
their influence on coronal heating scalings, are shown to depend on the loop
parameters, and this dependence is quantitatively characterized: weak
turbulence regimes and steeper spectra occur in {\it stronger loop fields} and
lead to {\it larger heating rates} than in weak field regions.Comment: 22 pages, 18 figures, uses emulateapj, for mpeg file associated to
Figure 17e see (temporarily) http://www.df.unipi.it/~rappazzo/arxiv/jfl.mpg,
ApJ, in pres
Coronal Heating, Weak MHD Turbulence and Scaling Laws
Long-time high-resolution simulations of the dynamics of a coronal loop in
cartesian geometry are carried out, within the framework of reduced
magnetohydrodynamics (RMHD), to understand coronal heating driven by motion of
field lines anchored in the photosphere. We unambiguously identify MHD
anisotropic turbulence as the physical mechanism responsible for the transport
of energy from the large scales, where energy is injected by photospheric
motions, to the small scales, where it is dissipated. As the loop parameters
vary different regimes of turbulence develop: strong turbulence is found for
weak axial magnetic fields and long loops, leading to Kolmogorov-like spectra
in the perpendicular direction, while weaker and weaker regimes (steeper
spectral slopes of total energy) are found for strong axial magnetic fields and
short loops. As a consequence we predict that the scaling of the heating rate
with axial magnetic field intensity , which depends on the spectral index
of total energy for given loop parameters, must vary from for weak
fields to for strong fields at a given aspect ratio. The predicted
heating rate is within the lower range of observed active region and quiet Sun
coronal energy losses.Comment: 4 pages, 5 figures, uses emulateapj, complies with published versio
Magnetic Effects at the Edge of the Solar System: MHD Instabilities, the de Laval nozzle Effect and an Extended Jet
To model the interaction between the solar wind and the interstellar wind,
magnetic fields must be included. Recently Opher et al. 2003 found that, by
including the solar magnetic field in a 3D high resolution simulation using the
University of Michigan BATS-R-US code, a jet-sheet structure forms beyond the
solar wind Termination Shock. Here we present an even higher resolution
three-dimensional case where the jet extends for beyond the Termination
Shock. We discuss the formation of the jet due to a de Laval nozzle effect and
it's su bsequent large period oscillation due to magnetohydrodynamic
instabilities. To verify the source of the instability, we also perform a
simplified two dimensional-geometry magnetohydrodynamic calculation of a plane
fluid jet embedded in a neutral sheet with the profiles taken from our 3D
simulation. We find remarkable agreement with the full three-dimensional
evolution. We compare both simulations and the temporal evolution of the jet
showing that the sinuous mode is the dominant mode that develops into a
velocity-shear-instability with a growth rate of . As a result, the outer edge of the heliosphere
presents remarkable dynamics, such as turbulent flows caused by the motion of
the jet. Further study, e.g., including neutrals and the tilt of the solar
rotation from the magnetic axis, is required before we can definitively address
how this outer boundary behaves. Already, however, we can say that the magnetic
field effects are a major player in this region changing our previous notion of
how the solar system ends.Comment: 24 pages, 13 figures, accepted for publication in Astrophysical
Journal (2004
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