86 research outputs found
Numerical simulation of the compressible Orszag-Tang vortex 2. Supersonic flow
The numerical investigation of the Orszag-Tang vortex system in compressible magnetofluids will consider initial conditions with embedded supersonic regions. The simulations have initial average Mach numbers 1.0 and 1.5 and beta 10/3 with Lundquist numbers 50, 100, or 200. The behavior of the system differs significantly from that found previously for the incompressible and subsonic analogs. Shocks form at the downstream boundaries of the embedded supersonic regions outside the central magnetic X-point and produce strong local current sheets which dissipate appreciable magnetic energy. Reconnection at the central X-point, which dominates the incompressible and subsonic systems, peaks later and has a smaller impact as M increases from 0.6 to 1.5. Similarly, correlation between the momentum and magnetic field begins significant growth later than in subsonic and incompressible flows. The shocks bound large compression regions, which dominate the wavenumber spectra of autocorrelations in mass density, velocity, and magnetic field
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
Diagnosing the time-dependence of active region core heating from the emission measure: I. Low-frequency nanoflares
Observational measurements of active region emission measures contain clues
to the time-dependence of the underlying heating mechanism. A strongly
non-linear scaling of the emission measure with temperature indicates a large
amount of hot plasma relative to warm plasma. A weakly non-linear (or linear)
scaling of the emission measure indicates a relatively large amount of warm
plasma, suggesting that the hot active region plasma is allowed to cool and so
the heating is impulsive with a long repeat time. This case is called {\it
low-frequency} nanoflare heating and we investigate its feasibility as an
active region heating scenario here. We explore a parameter space of heating
and coronal loop properties with a hydrodynamic model. For each model run, we
calculate the slope of the emission measure distribution . Our conclusions are: (1) low-frequency nanoflare heating is
consistent with about 36% of observed active region cores when uncertainties in
the atomic data are not accounted for; (2) proper consideration of
uncertainties yields a range in which as many as 77% of observed active regions
are consistent with low-frequency nanoflare heating and as few as zero; (3)
low-frequency nanoflare heating cannot explain observed slopes greater than 3;
(4) the upper limit to the volumetric energy release is in the region of 50 erg
cm to avoid unphysical magnetic field strengths; (5) the heating
timescale may be short for loops of total length less than 40 Mm to be
consistent with the observed range of slopes; (6) predicted slopes are
consistently steeper for longer loops
The effect of the solar corona on the attenuation of small-amplitude prominence oscillations. I. Longitudinal magnetic field
Context. One of the typical features shown by observations of solar
prominence oscillations is that they are damped in time and that the values of
the damping times are usually between one and three times the corresponding
oscillatory period. However, the mechanism responsible for the attenuation is
still not well-known. Aims. Thermal conduction, optically thin or thick
radiation and heating are taken into account in the energy equation, and their
role on the attenuation of prominence oscillations is evaluated. Methods. The
dispersion relation for linear non-adiabatic magnetoacoustic waves is derived
considering an equilibrium made of a prominence plasma slab embedded in an
unbounded corona. The magnetic field is orientated along the direction parallel
to the slab axis and has the same strength in all regions. By solving the
dispersion relation for a fixed wavenumber, a complex oscillatory frequency is
obtained, and the period and the damping time are computed. Results. The effect
of conduction and radiation losses is different for each magnetoacoustic mode
and depends on the wavenumber. In the observed range of wavelengths the
internal slow mode is attenuated by radiation from the prominence plasma, the
fast mode by the combination of prominence radiation and coronal conduction and
the external slow mode by coronal conduction. The consideration of the external
corona is of paramount importance in the case of the fast and external slow
modes, whereas it does not affect the internal slow modes at all. Conclusions.
Non-adiabatic effects are efficient damping mechanisms for magnetoacoustic
modes, and the values of the obtained damping times are compatible with those
observed.Comment: Accepted in A&
Spectroscopic Observations of Hot Lines Constraining Coronal Heating in Solar Active Regions
EUV observations of warm coronal loops suggest that they are bundles of
unresolved strands that are heated impulsively to high temperatures by
nanoflares. The plasma would then have the observed properties (e.g., excess
density compared to static equilibrium) when it cools into the 1-2 MK range. If
this interpretation is correct, then very hot emission should be present
outside of proper flares. It is predicted to be vey faint, however. A critical
element for proving or refuting this hypothesis is the existence of hot, very
faint plasmas which should be at amounts predicted by impulsive heating. We
report on the first comprehensive spectroscopic study of hot plasmas in active
regions. Data from the EIS spectrometer on Hinode were used to construct
emission measure distributions in quiescent active regions in the 1-5 MK
temperature range. The distributions are flat or slowly increasing up to
approximately 3 MK and then fall off rapidly at higher temperatures. We show
that active region models based on impulsive heating can reproduce the observed
EM distributions relatively well. Our results provide strong new evidence that
coronal heating is impulsive in nature.Comment: ApJ, 2009, 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
Smoothed Particle Magnetohydrodynamics III. Multidimensional tests and the div B = 0 constraint
In two previous papers (Price & Monaghan 2004a,b) (papers I,II) we have
described an algorithm for solving the equations of Magnetohydrodynamics (MHD)
using the Smoothed Particle Hydrodynamics (SPH) method. The algorithm uses
dissipative terms in order to capture shocks and has been tested on a wide
range of one dimensional problems in both adiabatic and isothermal MHD. In this
paper we investigate multidimensional aspects of the algorithm, refining many
of the aspects considered in papers I and II and paying particular attention to
the code's ability to maintain the div B = 0 constraint associated with the
magnetic field. In particular we implement a hyperbolic divergence cleaning
method recently proposed by Dedner et al. (2002) in combination with the
consistent formulation of the MHD equations in the presence of non-zero
magnetic divergence derived in papers I and II. Various projection methods for
maintaining the divergence-free condition are also examined. Finally the
algorithm is tested against a wide range of multidimensional problems used to
test recent grid-based MHD codes. A particular finding of these tests is that
in SPMHD the magnitude of the divergence error is dependent on the number of
neighbours used to calculate a particle's properties and only weakly dependent
on the total number of particles. Whilst many improvements could still be made
to the algorithm, our results suggest that the method is ripe for application
to problems of current theoretical interest, such as that of star formation.Comment: Here is the latest offering in my quest for a decent SPMHD algorithm.
26 pages, 15 figures, accepted for publication in MNRAS. Version with high
res figures available from
http://www.astro.ex.ac.uk/people/dprice/pubs/spmhd/spmhdpaper3.pd
Exploiting Laboratory And Heliophysics Plasma Synergies
Recent advances in space-based heliospheric observations, laboratory experimentation, and plasma simulation codes are creating an exciting new cross-disciplinary opportunity for understanding fast energy release and transport mechanisms in heliophysics and laboratory plasma dynamics, which had not been previously accessible. This article provides an overview of some new observational, experimental, and computational assets, and discusses current and near-term activities towards exploitation of synergies involving those assets. This overview does not claim to be comprehensive, but instead covers mainly activities closely associated with the authors\u27 interests and reearch. Heliospheric observations reviewed include the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) on the National Aeronautics and Space Administration (NASA) Solar Terrestrial Relations Observatory (STEREO) mission, the first instrument to provide remote sensing imagery observations with spatial continuity extending from the Sun to the Earth, and the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Japanese Hinode spacecraft that is measuring spectroscopically physical parameters of the solar atmosphere towards obtaining plasma temperatures, densities, and mass motions. The Solar Dynamics Observatory (SDO) and the upcoming Solar Orbiter with the Heliospheric Imager (SoloHI) on-board will also be discussed. Laboratory plasma experiments surveyed include the line-tied magnetic reconnection experiments at University of Wisconsin (relevant to coronal heating magnetic flux tube observations and simulations), and a dynamo facility under construction there; the Space Plasma Simulation Chamber at the Naval Research Laboratory that currently produces plasmas scalable to ionospheric and magnetospheric conditions and in the future also will be suited to study the physics of the solar corona; the Versatile Toroidal Facility at the Massachusetts Institute of Technology that provides direct experimental observation of reconnection dynamics; and the Swarthmore Spheromak Experiment, which provides well-diagnosed data on three-dimensional (3D) null-point magnetic reconnection that is also applicable to solar active regions embedded in pre-existing coronal fields. New computer capabilities highlighted include: HYPERION, a fully compressible 3D magnetohydrodynamics (MHD) code with radiation transport and thermal conduction; ORBIT-RF, a 4D Monte-Carlo code for the study of wave interactions with fast ions embedded in background MHD plasmas; the 3D implicit multi-fluid MHD spectral element code, HiFi; and, the 3D Hall MHD code VooDoo. Research synergies for these new tools are primarily in the areas of magnetic reconnection, plasma charged particle acceleration, plasma wave propagation and turbulence in a diverging magnetic field, plasma atomic processes, and magnetic dynamo behavior
Formation of Solar Filaments by Steady and Nonsteady Chromospheric Heating
It has been established that cold plasma condensations can form in a magnetic
loop subject to localized heating of the footpoints. In this paper, we use
grid-adaptive numerical simulations of the radiative hydrodynamic equations to
parametrically investigate the filament formation process in a pre-shaped loop
with both steady and finite-time chromospheric heating. Compared to previous
works, we consider low-lying loops with shallow dips, and use a more realistic
description for the radiative losses. We demonstrate for the first time that
the onset of thermal instability satisfies the linear instability criterion.
The onset time of the condensation is roughly \sim 2 hr or more after the
localized heating at the footpoint is effective, and the growth rate of the
thread length varies from 800 km hr-1 to 4000 km hr-1, depending on the
amplitude and the decay length scale characterizing this localized
chromospheric heating. We show how single or multiple condensation segments may
form in the coronal portion. In the asymmetric heating case, when two segments
form, they approach and coalesce, and the coalesced condensation later drains
down into the chromosphere. With a steady heating, this process repeats with a
periodicity of several hours. While our parametric survey confirms and augments
earlier findings, we also point out that steady heating is not necessary to
sustain the condensation. Once the condensation is formed, it can keep growing
also when the localized heating ceases. Finally, we show that the condensation
can survive continuous buffeting by perturbations resulting from the
photospheric p-mode waves.Comment: 43 pages, 18 figure
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