343 research outputs found
Finite Element Analysis of Electric Fracture Properties in Modified Small Punch Testing of Piezoceramic Plates
Magnetic flux emergence in granular convection: Radiative MHD simulations and observational signatures
We study the emergence of magnetic flux from the near-surface layers of the
solar convection zone into the photosphere. To model magnetic flux emergence,
we carried out a set of numerical radiative magnetohydrodynamics simulations.
Our simulations take into account the effects of compressibility, energy
exchange via radiative transfer, and partial ionization in the equation of
state. All these physical ingredients are essential for a proper treatment of
the problem. Furthermore, the inclusion of radiative transfer allows us to
directly compare the simulation results with actual observations of emerging
flux. We find that the interaction between the magnetic flux tube and the
external flow field has an important influence on the emergent morphology of
the magnetic field. Depending on the initial properties of the flux tube (e.g.
field strength, twist, entropy etc.), the emergence process can also modify the
local granulation pattern. The emergence of magnetic flux tubes with a flux of
Mx disturbs the granulation and leads to the transient appearance of
a dark lane, which is coincident with upflowing material. These results are
consistent with observed properties of emerging magnetic flux.Comment: To appear in A&
Evolution and Flare Activity of Delta-Sunspots in Cycle 23
The emergence and magnetic evolution of solar active regions (ARs) of
beta-gamma-delta type, which are known to be highly flare-productive, were
studied with the SOHO/MDI data in Cycle 23. We selected 31 ARs that can be
observed from their birth phase, as unbiased samples for our study. From the
analysis of the magnetic topology (twist and writhe), we obtained the following
results. i) Emerging beta-gamma-delta ARs can be classified into three
topological types as "quasi-beta", "writhed" and "top-to-top". ii) Among them,
the "writhed" and "top-to-top" types tend to show high flare activity. iii) As
the signs of twist and writhe agree with each other in most cases of the
"writhed" type (12 cases out of 13), we propose a magnetic model in which the
emerging flux regions in a beta-gamma-delta AR are not separated but united as
a single structure below the solar surface. iv) Almost all the "writhed"-type
ARs have downward knotted structures in the mid portion of the magnetic flux
tube. This, we believe, is the essential property of beta-gamma-delta ARs. v)
The flare activity of beta-gamma-delta ARs is highly correlated not only with
the sunspot area but also with the magnetic complexity. vi) We suggest that
there is a possible scaling-law between the flare index and the maximum umbral
area
Oscillation of a small Hα surge in a solar polar coronal hole
Hα surges (i.e. cool/dense collimated plasma ejections) may act as a guide for a propagation of magnetohydrodynamic waves. We report a high-resolution observation of a surge observed with 1.6 m Goode Solar Telescope (GST) on 2009 August 26, from 18:20 UT to 18:45 UT. Characteristics of plasma motions in the surge are determined with the normalizing radial gradient filter and the Fourier motion filter. The shape of the surge is found to change from a ‘C’ shape to an inverse ‘C’ shape after a formation of a cusp, a signature of reconnection. There are apparent upflows seen above the cusp top and downflows below it. The upflows show rising and rotational motions in the right-hand direction, with the rotational speed decreasing with height. Near the cusp top, we find a transverse oscillation of the surge, with the period of∼2 min. There is no change of the oscillation phase below the cusp top, but above the top a phase change is identified, giving a vertical phase speed about 86 km s−1. As the height increases, the initial amplitude of the oscillation increases, and the oscillation damping time decreases from 5.13 to 1.18 min. We conclude that the oscillation is a propagating kink wave that is possibly excited by the repetitive spontaneous magnetic reconnection
Simulation of a flux emergence event and comparison with observations by Hinode
We study the observational signature of flux emergence in the photosphere
using synthetic data from a 3D MHD simulation of the emergence of a twisted
flux tube. Several stages in the emergence process are considered. At every
stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5
{\AA} and Fe I 6302.5 {\AA} and degrade the data to the spatial and spectral
resolution of Hinode's SOT/SP. Then, following observational practice, we apply
Milne-Eddington-type inversions to the synthetic spectra in order to retrieve
various atmospheric parameters and compare the results with recent Hinode
observations. During the emergence sequence, the spectral lines sample
different parts of the rising flux tube, revealing its twisted structure. The
horizontal component of the magnetic field retrieved from the simulations is
close to the observed values. The flattening of the flux tube in the
photosphere is caused by radiative cooling, which slows down the ascent of the
tube to the upper solar atmosphere. Consistent with the observations, the
rising magnetized plasma produces a blue shift of the spectral lines during a
large part of the emergence sequence.Comment: A&A Letter, 3 figure
Simulation of the Formation of a Solar Active Region
We present a radiative magnetohydrodynamics simulation of the formation of an
Active Region on the solar surface. The simulation models the rise of a buoyant
magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the
solar photosphere. The rise of the magnetic plasma in the convection zone is
accompanied by predominantly horizontal expansion. Such an expansion leads to a
scaling relation between the plasma density and the magnetic field strength
such that . The emergence of magnetic flux into the
photosphere appears as a complex magnetic pattern, which results from the
interaction of the rising magnetic field with the turbulent convective flows.
Small-scale magnetic elements at the surface first appear, followed by their
gradual coalescence into larger magnetic concentrations, which eventually
results in the formation of a pair of opposite polarity spots. Although the
mean flow pattern in the vicinity of the developing spots is directed radially
outward, correlations between the magnetic field and velocity field
fluctuations allow the spots to accumulate flux. Such correlations result from
the Lorentz-force driven, counter-streaming motion of opposite-polarity
fragments. The formation of the simulated Active Region is accompanied by
transient light bridges between umbrae and umbral dots. Together with recent
sunspot modeling, this work highlights the common magnetoconvective origin of
umbral dots, light bridges and penumbral filaments.Comment: Accepted for publication in Ap
Plasmoid-Induced-Reconnection and Fractal Reconnection
As a key to undertanding the basic mechanism for fast reconnection in solar
flares, plasmoid-induced-reconnection and fractal reconnection are proposed and
examined. We first briefly summarize recent solar observations that give us
hints on the role of plasmoid (flux rope) ejections in flare energy release. We
then discuss the plasmoid-induced-reconnection model, which is an extention of
the classical two-ribbon-flare model which we refer to as the CSHKP model. An
essential ingredient of the new model is the formation and ejection of a
plasmoid which play an essential role in the storage of magnetic energy (by
inhibiting reconnection) and the induction of a strong inflow into reconnection
region. Using a simple analytical model, we show that the plasmoid ejection and
acceleration are closely coupled with the reconnection process, leading to a
nonlinear instability for the whole dynamics that determines the macroscopic
reconnection rate uniquely. Next we show that the current sheet tends to have a
fractal structure via the following process path: tearing, sheet thinning,
Sweet- Parker sheet, secondary tearing, further sheet thinning... These
processes occur repeatedly at smaller scales until a microscopic plasma scale
(either the ion Larmor radius or the ion inertial length) is reached where
anomalous resistivity or collisionless reconnection can occur. The current
sheet eventually has a fractal structure with many plasmoids (magnetic islands)
of different sizes. When these plasmoids are ejected out of the current sheets,
fast reconnection occurs at various different scales in a highly time dependent
manner. Finally, a scenario is presented for fast reconnection in the solar
corona on the basis of above plasmoid-induced-reconnection in a fractal current
sheet.Comment: 9 pages, 11 figures, with using eps.sty; Earth, Planets and Space in
press; ps-file is also available at
http://stesun8.stelab.nagoya-u.ac.jp/~tanuma/study/shibata2001
The dynamical disconnection of sunspots from their magnetic roots
After a dynamically active emergence phase, magnetic flux at the solar
surface soon ceases to show strong signs of the subsurface dynamics of its
parent magnetic structure. This indicates that some kind of disconnection of
the emerged flux from its roots in the deep convection zone should take place.
We propose a mechanism for the dynamical disconnection of the surface flux
based upon the buoyant upflow of plasma along the field lines. Such flows arise
in the upper part of a rising flux loop during the final phases of its buoyant
ascent towards the surface. The combination of the pressure buildup by the
upflow and the cooling of the upper layers of an emerged flux tube by radiative
losses at the surface lead to a progressive weakening of the magnetic field in
several Mm depth. When the field strength has become sufficiently low,
convective motions and the fluting instability disrupt the flux tube into thin,
passively advected flux fragments, thus providing a dynamical disconnection of
the emerged part from its roots. We substantiate this scenario by considering
the quasi-static evolution of a sunspot model under the effects of radiative
cooling, convective energy transport, and pressure buildup by a prescribed
inflow at the bottom of the model. For inflow speeds in the range shown by
simulations of thin flux tubes, we find that the disconnection takes place in a
depth between 2 and 6 Mm for disconnection times up to 3 days.Comment: 11 pages, 5 figures, accepted by A&
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