685 research outputs found
Resistive ballooning modes in line-tied coronal arcades
The equations describing the linear evolution of resistive ballooning modes are obtained by using a modified WKB expansion in the short perpendicular wavelength, while variations of the perturbations along the field are described by a slowly varying amplitude, on which the tying boundary conditions are imposed. In general, given an equilibrium, there are certain ranges of magnetic surfaces for which the system predicts instability even without dissipation. The main conclusion is that within the resistive MHD approximation cylindrically symmetric arcades with pressure falling with radius are unstable to resistive localized modes; the growth rates, close to ideal marginal stability, are large, so that it would appear that energy could be released during 10 to 100 Alfven times. The wavelength of the modes is expected to be limited by the ion gyroradius, when stabilizing drift effects must be taken into account. The nonlinear evolution of resistive ballooning modes should be studied to assess their overall relevance to the violent and rapidly evolving phenomena observed on the sun
Documentazione e ricostruzione 3D di un menhir inedito rinvenuto in localitĂ âSa Perda Fittaâ, SantâAnna Arresi (Sardegna sud-occidentale)
The Authors illustrate the documentation process of a menhir not recorded in earlier studies, which was fortuitously discovered in Sa Perda Fitta, in the western landscape of SantâAnna Arresi, a small municipality of Sulcis (south-western Sardinia). This area was the subject of a detailed study by the Authors for their PhD dissertation at the Universidad de Granada, whose purpose was the detection and analysis of settlement patterns of human communities from the Neolithic to the Bronze Age. This territory, bounded on the East by the Sulcis massif and on the West by the Gulf of Palmas, is characterized by the presence of coastal ponds which have fostered the presence and the settlement of human groups since the Neolithic. The place name Sa Perda Fitta (in the Sardinian language âstone fixed into the groundâ) has aroused a great deal of interest as it suggests the presence of at least one menhir, a widespread prehistoric monument on the island from the Neolithic to the Chalcolithic age. Morphology and typology of the monument was examined and a 3D model created by the photogrammetric processing of a digital photo dataset using Agisoft Photoscan 3D Pro, an automatic 3D reconstruction Structure from Motion (SfM) based software offering a significant and innovative contribution to the recording and dissemination of archaeological data
Shell-models of RMHD turbulence and the heating of solar coronal loops
A simplified non-linear numerical model for the development of incompressible
magnetohydrodynamics (MHD) in the presence of a strong magnetic field B0 and
stratification, nicknamed Shell-Atm, is presented. In planes orthogonal to the
mean field, the non-linear incompressible dynamics is replaced by 2D
shell-models for the complex variables u and b, allowing one to reach large
Reynolds numbers while at the same time carrying out sufficiently long time
integrations to obtain a good statistics at moderate computational cost. The
shell-models of different planes are coupled by Alfven waves propagating along
B0. The model may be applied to open or closed magnetic field configurations
where the axial field dominates and the plasma pressure is low; here we apply
it to the specific case of a magnetic loop of the solar corona heated via
turbulence driven by photospheric motions, and we use statistics for its
analysis. The Alfven waves interact non-linearly and form turbulent spectra in
the directions perpendicular and, via propagation, also parallel to the mean
field. A heating function is obtained, and is shown to be intermittent; the
average heating is consistent with values required for sustaining a hot corona,
and is proportional to the aspect ratio of the loop to the power -1.5;
characteristic properties of heating events are distributed as power-laws.
Cross-correlations show a delay of dissipation compared to energy content.Comment: 12 pages, 16 figures, accepted for publication in Ap
Propagation and dissipation of Alfvén waves in stellar atmospheres permeated by isothermal winds
We investigate the nonlinear evolution of Alfvén waves in a radially stratified isothermal atmosphere with wind, from the atmospheric base out to the Alfvénic point. Nonlinear interactions, triggered by wave reflection due to the atmospheric gradients, are assumed to occur mainly in directions perpendicular to the mean radial magnetic field. The nonlinear coupling between waves propagating in opposite directions is modeled by a phenomenological term, containing an integral turbulent length scale, which acts as a dissipative coefficient for waves of a given frequency. Although the wind acceleration profile is not determined self-consistently one may estimate the dissipation rate inside the layer and follow the evolution of an initial frequency spectrum. Reflection of low frequency waves drives dissipation across the whole spectrum, and steeper gradients, i.e. lower coronal temperatures, enhance the dissipation rate. Moreover, when reasonable wave amplitudes are considered, waves of all frequencies damp at the same rate and the spectrum is not modified substantially during propagation. Therefore the sub-Alfvénic coronal layer acts differently when waves interact nonlinearly, no longer behaving as a frequency dependent filter once reflection-generated nonlinear interactions are included, at least within the classes of models discussed here
Non linear evolution of Alfvén wave in the solar atmosphere
We investigate the non-linear evolution of AlfvĂ©n waves in the solar atmosphere and wind, from the photosphere out to the Earthâs orbit. Photosphere and chromosphere are modeled as isothermal layers in static equilibrium, connecting across the transition region with a corona and wind. Nonlinear coupling between waves propagating in opposite directions is modeled by a phenomenological term containing an integral turbulent length scale. Spectrum modifications remain similar to what is found in a linear analysis and some characteristic features - i.e. oscillations at higher frequencies- persist despite nonlinear interactions
Heating of coronal loops: weak MHD turbulence and scaling laws
To understand the nonlinear dynamics of the Parker scenario for coronal
heating, long-time high-resolution simulations of the dynamics of a coronal
loop in cartesian geometry are carried out. A loop is modeled as a box extended
along the direction of the strong magnetic field in which the system is
embedded. At the top and bottom plates, which represent the photosphere,
velocity fields mimicking photospheric motions are imposed.
We show that the nonlinear dynamics is described by different regimes of MHD
anisotropic turbulence, with spectra characterized by intertial range power
laws whose indexes range from Kolmogorov-like values () up to . We briefly describe the bearing for coronal heating rates.Comment: 8 pages, 4 figure
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
- âŠ