211 research outputs found
Buoyant magnetic flux ropes in a magnetized stellar envelope: Idealized numerical 2.5-D MHD simulations
Context: The context of this paper is buoyant toroidal magnetic flux ropes,
which is a part of flux tube dynamo theory and the framework of solar-like
magnetic activity. Aims: The aim is to investigate how twisted magnetic flux
ropes interact with a simple magnetized stellar model envelope--a magnetic
"convection zone"--especially to examine how the twisted magnetic field
component of a flux rope interacts with a poloidal magnetic field in the
convection zone. Method: Both the flux ropes and the atmosphere are modelled as
idealized 2.5-dimensional concepts using high resolution numerical
magneto-hydrodynamic (MHD) simulations. Results: It is illustrated that twisted
toroidal magnetic flux ropes can interact with a poloidal magnetic field in the
atmosphere to cause a change in both the buoyant rise dynamics and the flux
rope's geometrical shape. The details of these changes depend primarily on the
polarity and strength of the atmospheric field relative to the field strength
of the flux rope. It is suggested that the effects could be verified
observationally.Comment: 8 pages, 5 figures (9 files), accepted by A&
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&
The 3D structure of an active region filament as extrapolated from photospheric and chromospheric observations
The 3D structure of an active region (AR) filament is studied using nonlinear
force-free field (NLFFF) extrapolations based on simultaneous observations at a
photospheric and a chromospheric height. To that end, we used the Si I 10827
\AA\ line and the He I 10830 \AA\ triplet obtained with the Tenerife Infrared
Polarimeter (TIP) at the VTT (Tenerife). The two extrapolations have been
carried out independently from each other and their respective spatial domains
overlap in a considerable height range. This opens up new possibilities for
diagnostics in addition to the usual ones obtained through a single
extrapolation from, typically, a photospheric layer. Among those possibilities,
this method allows the determination of an average formation height of the He I
10830 \AA\ signal of \approx 2 Mm above the surface of the sun. It allows, as
well, to cross-check the obtained 3D magnetic structures in view of verifying a
possible deviation from the force- free condition especially at the
photosphere. The extrapolations yield a filament formed by a twisted flux rope
whose axis is located at about 1.4 Mm above the solar surface. The twisted
field lines make slightly more than one turn along the filament within our box,
which results in 0.055 turns/Mm. The convex part of the field lines (as seen
from the solar surface) constitute dips where the plasma can naturally be
supported. The obtained 3D magnetic structure of the filament depends on the
choice of the observed horizontal magnetic field as determined from the
180\circ solution of the azimuth. We derive a method to check for the
correctness of the selected 180\circ ambiguity solution.Comment: 31 pages, 13 figures, ApJ Accepte
Sunspot rotation, filament, and flare: The event on 2000 February 10
We find that a sunspot with positive polarity had an obvious
counter-clockwise rotation and resulted in the formation and eruption of an
inverse S-shaped filament in NOAA active region (AR) 08858 from 2000 February 9
to 10. The sunspot had two umbrae which rotated around each other by 195
degrees within about twenty-four hours. The average rotation rate was nearly 8
degrees per hour. The fastest rotation in the photosphere took place during
14:00UT to 22:01UT on February 9, with the rotation rate of nearly 16 degrees
per hour. The fastest rotation in the chromosphere and the corona took place
during 15:28UT to 19:00UT on February 9, with the rotation rate of nearly 20
degrees per hour. Interestingly, the rapid increase of the positive magnetic
flux just occurred during the fastest rotation of the rotating sunspot, the
bright loop-shaped structure and the filament. During the sunspot rotation, the
inverse S-shaped filament gradually formed in the EUV filament channel. The
filament experienced two eruptions. In the first eruption, the filament rose
quickly and then the filament loops carrying the cool and the hot material were
seen to spiral into the sunspot counterclockwise. About ten minutes later, the
filament became active and finally erupted. The filament eruption was
accompanied with a C-class flare and a halo coronal mass ejection (CME). These
results provide evidence that sunspot rotation plays an important role in the
formation and eruption of the sigmoidal active-region filament.Comment: 20 pages, 9 figures, Accepted for publication in Ap
3D MHD Flux Emergence Experiments: Idealized models and coronal interactions
This paper reviews some of the many 3D numerical experiments of the emergence
of magnetic fields from the solar interior and the subsequent interaction with
the pre-existing coronal magnetic field. The models described here are
idealized, in the sense that the internal energy equation only involves the
adiabatic, Ohmic and viscous shock heating terms. However, provided the main
aim is to investigate the dynamical evolution, this is adequate. Many
interesting observational phenomena are explained by these models in a
self-consistent manner.Comment: Review article, accepted for publication in Solar Physic
On Signatures of Twisted Magnetic Flux Tube Emergence
Recent studies of NOAA active region 10953, by Okamoto {\it et al.} ({\it
Astrophys. J. Lett.} {\bf 673}, 215, 2008; {\it Astrophys. J.} {\bf 697}, 913,
2009), have interpreted photospheric observations of changing widths of the
polarities and reversal of the horizontal magnetic field component as
signatures of the emergence of a twisted flux tube within the active region and
along its internal polarity inversion line (PIL). A filament is observed along
the PIL and the active region is assumed to have an arcade structure. To
investigate this scenario, MacTaggart and Hood ({\it Astrophys. J. Lett.} {\bf
716}, 219, 2010) constructed a dynamic flux emergence model of a twisted
cylinder emerging into an overlying arcade. The photospheric signatures
observed by Okamoto {\it et al.} (2008, 2009) are present in the model although
their underlying physical mechanisms differ. The model also produces two
additional signatures that can be verified by the observations. The first is an
increase in the unsigned magnetic flux in the photosphere at either side of the
PIL. The second is the behaviour of characteristic photospheric flow profiles
associated with twisted flux tube emergence. We look for these two signatures
in AR 10953 and find negative results for the emergence of a twisted flux tube
along the PIL. Instead, we interpret the photospheric behaviour along the PIL
to be indicative of photospheric magnetic cancellation driven by flows from the
dominant sunspot. Although we argue against flux emergence within this
particular region, the work demonstrates the important relationship between
theory and observations for the successful discovery and interpretation of
signatures of flux emergence.Comment: 14 pages, 8 figures, accepted for publication in Solar Physic
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
Dysfunction of homeostatic control of dopamine by astrocytes in the developing prefrontal cortex leads to cognitive impairments
Astrocytes orchestrate neural development by powerfully coordinating synapse formation and function and, as such, may be critically involved in the pathogenesis of neurodevelopmental abnormalities and cognitive deficits commonly observed in psychiatric disorders. Here, we report the identification of a subset of cortical astrocytes that are competent for regulating dopamine (DA) homeostasis during postnatal development of the prefrontal cortex (PFC), allowing for optimal DA-mediated maturation of excitatory circuits. Such control of DA homeostasis occurs through the coordinated activity of astroglial vesicular monoamine transporter 2 (VMAT2) together with organic cation transporter 3 and monoamine oxidase type B, two key proteins for DA uptake and metabolism. Conditional deletion of VMAT2 in astrocytes postnatally produces loss of PFC DA homeostasis, leading to defective synaptic transmission and plasticity as well as impaired executive functions. Our findings show a novel role for PFC astrocytes in the DA modulation of cognitive performances with relevance to psychiatric disorders
The APMAP interactome reveals new modulators of APP processing and beta-amyloid production that are altered in Alzheimer's disease.
The adipocyte plasma membrane-associated protein APMAP is expressed in the brain where it associates with γ-secretase, a protease responsible for the generation of the amyloid-β peptides (Aβ) implicated in the pathogenesis of Alzheimer's disease (AD). In this study, behavioral investigations revealed spatial learning and memory deficiencies in our newly generated mouse line lacking the protein APMAP. In a mouse model of AD, the constitutive deletion of APMAP worsened the spatial memory phenotype and led to increased Aβ production and deposition into senile plaques. To investigate at the molecular level the neurobiological functions of APMAP (memory and Aβ formation) and a possible link with the pathological hallmarks of AD (memory impairment and Aβ pathology), we next developed a procedure for the high-grade purification of cellular APMAP protein complexes. The biochemical characterization of these complexes revealed a series of new APMAP interactomers. Among these, the heat shock protein HSPA1A and the cation-dependent mannose-6-phosphate receptor (CD-M6PR) negatively regulated APP processing and Aβ production, while clusterin, calnexin, arginase-1, PTGFRN and the cation-independent mannose-6-phosphate receptor (CI-M6PR/IGF2R) positively regulated APP and Aβ production. Several of the newly identified APMAP interactomers contribute to the autophagy-lysosome system, further supporting an emergent agreement that this pathway can modulate APP metabolism and Aβ generation. Importantly, we have also demonstrated increased alternative splicing of APMAP and lowered levels of the Aβ controllers HSPA1A and CD-M6PR in human brains from neuropathologically verified AD cases
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