85 research outputs found
Twisting solar coronal jet launched at the boundary of an active region
A broad jet was observed in a weak magnetic field area at the edge of active
region NOAA 11106. The peculiar shape and magnetic environment of the broad jet
raised the question of whether it was created by the same physical processes of
previously studied jets with reconnection occurring high in the corona. We
carried out a multi-wavelength analysis using the EUV images from the
Atmospheric Imaging Assembly (AIA) and magnetic fields from the Helioseismic
and Magnetic Imager (HMI) both on-board the SDO satellite. The jet consisted of
many different threads that expanded in around 10 minutes to about 100 Mm in
length, with the bright features in later threads moving faster than in the
early ones, reaching a maximum speed of about 200 km s^{-1}. Time-slice
analysis revealed a striped pattern of dark and bright strands propagating
along the jet, along with apparent damped oscillations across the jet. This is
suggestive of a (un)twisting motion in the jet, possibly an Alfven wave. A
topological analysis of an extrapolated field was performed. Bald patches in
field lines, low-altitude flux ropes, diverging flow patterns, and a null point
were identified at the basis of the jet. Unlike classical lambda or
Eiffel-tower shaped jets that appear to be caused by reconnection in current
sheets containing null points, reconnection in regions containing bald patches
seems to be crucial in triggering the present jet. There is no observational
evidence that the flux ropes detected in the topological analysis were actually
being ejected themselves, as occurs in the violent phase of blowout jets;
instead, the jet itself may have gained the twist of the flux rope(s) through
reconnection. This event may represent a class of jets different from the
classical quiescent or blowout jets, but to reach that conclusion, more
observational and theoretical work is necessary.Comment: 12 pages, 9 figures, accepted for publication in A&
Comparison of the thin flux tube approximation with 3D MHD simulations
The structure and dynamics of small vertical photospheric magnetic flux
concentrations has been often treated in the framework of an approximation
based upon a low-order truncation of the Taylor expansions of all quantities in
the horizontal direction, together with the assumption of instantaneous total
pressure balance at the boundary to the non-magnetic external medium. Formally,
such an approximation is justified if the diameter of the structure (a flux
tube or a flux sheet) is small compared to all other relevant length scales
(scale height, radius of curvature, wavelength, etc.). The advent of realistic
3D radiative MHD simulations opens the possibility of checking the consistency
of the approximation with the properties of the flux concentrations that form
in the course of a simulation.
We carry out a comparative analysis between the thin flux tube/sheet models
and flux concentrations formed in a 3D radiation-MHD simulation. We compare the
distribution of the vertical and horizontal components of the magnetic field in
a 3D MHD simulation with the field distribution in the case of the thin flux
tube/sheet approximation. We also consider the total (gas plus magnetic)
pressure in the MHD simulation box. Flux concentrations with
super-equipartition fields are reasonably well reproduced by the second-order
thin flux tube/sheet approximation. The differences between approximation and
simulation are due to the asymmetry and the dynamics of the simulated
structures
Nonlinear force-free modelling: influence of inaccuracies in the measured magnetic vector
Context: Solar magnetic fields are regularly extrapolated into the corona
starting from photospheric magnetic measurements that can suffer from
significant uncertainties. Aims: Here we study how inaccuracies introduced into
the maps of the photospheric magnetic vector from the inversion of ideal and
noisy Stokes parameters influence the extrapolation of nonlinear force-free
magnetic fields. Methods: We compute nonlinear force-free magnetic fields based
on simulated vector magnetograms, which have been produced by the inversion of
Stokes profiles, computed froma 3-D radiation MHD simulation snapshot. These
extrapolations are compared with extrapolations starting directly from the
field in the MHD simulations, which is our reference. We investigate how line
formation and instrumental effects such as noise, limited spatial resolution
and the effect of employing a filter instrument influence the resulting
magnetic field structure. The comparison is done qualitatively by visual
inspection of the magnetic field distribution and quantitatively by different
metrics. Results: The reconstructed field is most accurate if ideal Stokes data
are inverted and becomes less accurate if instrumental effects and noise are
included. The results demonstrate that the non-linear force-free field
extrapolation method tested here is relatively insensitive to the effects of
noise in measured polarization spectra at levels consistent with present-day
instruments. Conclusions heading: Our results show that we can reconstruct the
coronal magnetic field as a nonlinear force-free field from realistic
photospheric measurements with an accuracy of a few percent, at least in the
absence of sunspots.Comment: A&A, accepted, 9 Pages, 4 Figure
Expansion of magnetic flux concentrations: a comparison of Hinode SOT d ata and models
Context: The expansion of network magnetic fields with height is a
fundamental property of flux tube models. A rapid expansion is required to form
a magnetic canopy. Aims: We characterize the observed expansion properties of
magnetic network elements and compare them with the thin flux tube and sheet
approximations, as well as with magnetoconvection simulations. Methods: We used
data from the Hinode SOT NFI NaD1 channel and spectropolarimeter to study the
appearance of magnetic flux concentrations seen in circular polarization as a
function of position on the solar disk. We compared the observations with
synthetic observables from models based on the thin flux tube approximation and
magnetoconvection simulations with two different upper boundary conditions for
the magnetic field (potential and vertical). Results: The observed circular
polarization signal of magnetic flux concentrations changes from unipolar at
disk center to bipolar near the limb, which implies an expanding magnetic
field. The observed expansion agrees with expansion properties derived from the
thin flux sheet and tube approximations. Magnetoconvection simulations with a
potential field as the upper boundary condition for the magnetic field also
produce bipolar features near the limb while a simulation with a vertical field
boundary condition does not. Conclusions: The near-limb apparent bipolar
magnetic features seen in high-resolution Hinode observations can be
interpreted using a simple flux sheet or tube model. This lends further support
to the idea that magnetic features with vastly varying sizes have similar
relative expansion rates. The numerical simulations presented here are less
useful in interpreting the expansion since the diagnostics we are interested in
are strongly influenced by the choice of the upper boundary condition for the
magnetic field in the purely photospheric simulations.Comment: accepted for publication in A&
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
Participatory analysis for adaptation to climate change in Mediterranean agricultural systems: possible choices in process design (versão Pre Print)
There is an increasing call for local measures to adapt to climate change, based on foresight analyses in collaboration with actors. However, such analyses involve many challenges, particularly because the actors concerned may not consider climate change to be an urgent concern. This paper examines the methodological choices made by three research teams in the design and implementation of participatory foresight analyses to explore agricultural and water management options for adaptation to climate change. Case studies were conducted in coastal areas of France, Morocco, and Portugal where the groundwater is intensively used for irrigation, the aquifers are at risk or are currently overexploited, and a serious agricultural crisis is underway. When designing the participatory processes, the researchers had to address four main issues: whether to avoid or prepare dialogue between actors whose relations may be limited or tense; how to select participants and get them involved; how to facilitate discussion of issues that the actors may not initially consider to be of great concern; and finally, how to design and use scenarios. In each case, most of the invited actors responded and met to discuss and evaluate a series of scenarios. Strategies were discussed at different levels, from farming practices to aquifer management. It was shown that such participatory analyses can be implemented in situations which may initially appear to be unfavourable. This was made possible by the flexibility in the methodological choices, in particular the possibility of framing the climate change issue in a broader agenda for discussion with the actors
Three-dimensional non-LTE radiative transfer effects in Fe I lines I. Flux sheet and flux tube geometries
In network and active region plages, the magnetic field is concentrated into
structures often described as flux tubes (FTs) and sheets (FSs). 3-D radiative
transfer (RT) is important for energy transport in these concentrations. It is
also expected to be important for diagnostic purposes but has rarely been
applied for that purpose. Using true 3-D, non-LTE (NLTE) RT in FT/FS models, we
compute Fe line profiles commonly used to diagnose the Sun's magnetic field by
comparing the results with those obtained from LTE/1-D (1.5-D) NLTE
calculations. Employing a multilevel iron atom, we study the influence of basic
parameters such as Wilson depression, wall thickness, radius/width, thermal
stratification or magnetic field strength on all Stokes parameters in the
thin-tube approximation. The use of different levels of approximations of RT
may lead to considerable differences in profile shapes, intensity contrasts,
equivalent widths, and the determination of magnetic field strengths. In
particular, LTE, which often provides a good approach in planar 1-D
atmospheres, is a poor approximation in our flux sheet model for some of the
most important diagnostic Fe I lines (524.7nm, 525.0nm, 630.1nm, and 630.2nm).
The observed effects depend on parameters such as the height of line formation,
field strength, and internal temperature stratification. Differences between
the profile shapes may lead to errors in the determination of magnetic fields
on the order of 10 to 20%, while errors in the determined temperature can reach
300-400K. The empirical FT models NET and PLA turn out to minimize the effects
of 3D RT, so that results obtained with these models by applying LTE may also
remain valid for 3-D NLTE calculations. Finally, horizontal RT is found to only
insignificantly smear out structures such as the optically thick walls of FTs
and FSs, allowing features as narrow as 10km to remain visible.Comment: 20 pages, 21 figures, accepted for publication to "Astronomy and
Astrophysics
Power spectrum of turbulent convection in the solar photosphere
The solar photosphere provides us with a laboratory for understanding
turbulence in a layer where the fundamental processes of transport vary rapidly
and a strongly superadiabatic region lies very closely to a subadiabatic layer.
Our tools for probing the turbulence are high-resolution spectropolarimetric
observations such as have recently been obtained with the two sunrise missions,
and numerical simulations. Our aim is to study photospheric turbulence with the
help of Fourier power spectra that we compute from observations and
simulations. We also attempt to explain some properties of the photospheric
overshooting flow with the help of its governing equations and simulations. We
find that quiet-Sun observations and smeared simulations exhibit a power-law
behavior in the subgranular range of their Doppler velocity power spectra with
an index of. The unsmeared simulations exhibit a power-law index
of. The smearing considerably reduces the extent of the
power-law-like portion of the spectra. Therefore, the limited spatial
resolution in some observations might eventually result in larger uncertainties
in the estimation of the power-law indices.
The simulated vertical velocity power spectra as a function of height show a
rapid change in the power-law index from the solar surface to ~km above
it. A scale-dependent transport of the vertical momentum occurs. At smaller
scales, the vertical momentum is more efficiently transported sideways than at
larger scales. This results in less vertical velocity power transported upward
at small scales than at larger scales and produces a progressively steeper
vertical velocity power law below km. Above this height, the gravity work
progressively gains importance at all scales, making the atmosphere
progressively more hydrostatic and resulting in a gradually less steep power
law.Comment: 10 pages, 7 figures, Accepted in A and
Helicobacter pylori Infection of Gastrointestinal Epithelial Cells in vitro Induces Mesenchymal Stem Cell Migration through an NF-κB-Dependent Pathway
The role of bone marrow-derived mesenchymal stem cells (MSC) in the physiology of the gastrointestinal tract epithelium is currently not well established. These cells can be recruited in response to inflammation due to epithelial damage, home, and participate in tissue repair. In addition, in the case of tissue repair failure, these cells could transform and be at the origin of carcinomas. However, the chemoattractant molecules responsible for MSC recruitment and migration in response to epithelial damage, and particularly to Helicobacter pylori infection, remain unknown although the role of some chemokines has been suggested. This work aimed to get insight into the mechanisms of mouse MSC migration during in vitro infection of mouse gastrointestinal epithelial cells by H. pylori. Using a cell culture insert system, we showed that infection of gastrointestinal epithelial cells by different H. pylori strains is able to stimulate the migration of MSC. This mechanism involves the secretion by infected epithelial cells of multiple cytokines, with a major role of TNFα, mainly via a Nuclear Factor-kappa B-dependent pathway. This study provides the first evidence of the role of H. pylori infection in MSC migration and paves the way to a better understanding of the role of bone marrow-derived stem cells in gastric pathophysiology and carcinogenesis
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