101 research outputs found
Magnetic field line braiding in the solar atmosphere
AbstractUsing a magnetic carpet as model for the near surface solar magnetic field we study its effects on the propagation of energy injectected by photospheric footpoint motions. Such a magnetic carpet structure is topologically highly non-trivial and with its magnetic nulls exhibits qualitatively different behavior than simpler magnetic fields. We show that the presence of magnetic fields connecting back to the photosphere inhibits the propagation of energy into higher layers of the solar atmosphere, like the solar corona. By applying certain types of footpoint motions the magnetic field topology is is greatly reduced through magnetic field reconnection which facilitates the propagation of energy and disturbances from the photosphere.</jats:p
Topological constraints in the reconnection of vortex braids
We study the relaxation of a topologically nontrivial vortex braid with zero net helicity in a barotropic fluid. The aim is to investigate the extent to which the topology of the vorticity field—characterized by braided vorticity field lines—determines the dynamics, particularly the asymptotic behavior under vortex reconnection in evolution at high Reynolds numbers (25 000). Analogous to the evolution of braided magnetic fields in plasma, we find that the relaxation of our vortex braid leads to a simplification of the topology into large-scale regions of opposite swirl, consistent with an inverse cascade of the helicity. The change of topology is facilitated by a cascade of vortex reconnection events. During this process, the existence of regions of positive and negative kinetic helicities imposes a lower bound for the kinetic energy. For the enstrophy, we derive analytically a lower bound given by the presence of unsigned kinetic helicity, which we confirm in our numerical experiments
Braided magnetic fields:equilibria, relaxation and heating
We examine the dynamics of magnetic flux tubes containing non-trivial field
line braiding (or linkage), using mathematical and computational modelling, in
the context of testable predictions for the laboratory and their significance
for solar coronal heating. We investigate the existence of braided force-free
equilibria, and demonstrate that for a field anchored at perfectly-conducting
plates, these equilibria exist and contain current sheets whose thickness
scales inversely with the braid complexity - as measured for example by the
topological entropy. By contrast, for a periodic domain braided exact
equilibria typically do not exist, while approximate equilibria contain thin
current sheets. In the presence of resistivity, reconnection is triggered at
the current sheets and a turbulent relaxation ensues. We finish by discussing
the properties of the turbulent relaxation and the existence of constraints
that may mean that the final state is not the linear force-free field predicted
by Taylor's hypothesis.Comment: To appear in Plasma Physics and Controlled Fusio
Estimating the Rate of Field Line Braiding in the Solar Corona by Photospheric Flows
In this paper, we seek to understand the timescale in which the photospheric motions on the Sun braid coronal magnetic field lines. This is a crucial ingredient for determining the viability of the braiding mechanism for explaining the high temperatures observed in the corona. We study the topological complexity induced in the coronal magnetic field, primarily using plasma motions extracted from magneto-convection simulations. This topological complexity is quantified using the field line winding, finite time topological entropy (FTTE), and passive scalar mixing. With these measures, we contrast mixing efficiencies of the magneto-convection simulation, a benchmark flow known as a "blinking vortex", and finally photospheric flows inferred from sequences of observed magnetograms using local correlation tracking. While the highly resolved magneto-convection simulations induce a strong degree of field line winding and FTTE, the values obtained from the observations from the plage region are around an order of magnitude smaller. This behavior is carried over to the FTTE. Nevertheless, the results suggest that the photospheric motions induce complex tangling of the coronal field on a timescale of hours
Quantifying the tangling of trajectories using the topological entropy
We present a simple method to efficiently compute a lower limit of the
topological entropy and its spatial distribution for two-dimensional mappings.
These mappings could represent either two-dimensional time-periodic fluid flows
or three-dimensional magnetic fields, which are periodic in one direction. This
method is based on measuring the length of a material line in the flow.
Depending on the nature of the flow, the fluid can be mixed very efficiently
which causes the line to stretch. Here we study a method that adaptively
increases the resolution at locations along the line where folds lead to high
curvature. This reduces the computational cost greatly which allows us to study
unprecedented parameter regimes. We demonstrate how this efficient
implementation allows the computation of the variation of the finite-time
topological entropy in the mapping. This measure quantifies spatial variations
of the braiding efficiency, important in many practical applications.Comment: 11 pages, 9 figure
Turbulent dynamo with advective magnetic helicity flux
Many astrophysical bodies harbor magnetic fields that are thought to be
sustained by a dynamo process. However, it has been argued that the production
of large-scale magnetic fields by mean-field dynamo action is strongly
suppressed at large magnetic Reynolds numbers owing to the conservation of
magnetic helicity. This phenomenon is known as {\it catastrophic quenching}.
Advection of magnetic fields by stellar and galactic winds toward the outer
boundaries and away from the dynamo is expected to alleviate such quenching.
Here we explore the relative roles played by advective and turbulent--diffusive
fluxes of magnetic helicity in the dynamo. In particular, we study how the
dynamo is affected by advection. We do this by performing direct numerical
simulations of a turbulent dynamo of type driven by forced
turbulence in a Cartesian domain in the presence of a flow away from the
equator where helicity changes sign. Our results indicate that in the presence
of advection, the dynamo, otherwise stationary, becomes oscillatory. We confirm
an earlier result for turbulent--diffusive magnetic helicity fluxes that for
small magnetic Reynolds numbers (\Rm\lesssim 100...200, based on the
wavenumber of the energy-carrying eddies) the magnetic helicity flux scales
less strongly with magnetic Reynolds number (\Rm^{-1/2}) than the term
describing magnetic helicity destruction by resistivity (\Rm^{-1}). Our new
results now suggest that for larger \Rm the former becomes approximately
independent of \Rm, while the latter falls off more slowly. We show for the
first time that both for weak and stronger winds, the magnetic helicity flux
term becomes comparable to the resistive term for \Rm\gtrsim 1000, which is
necessary for alleviating catastrophic quenching.Comment: 9 pages, 9 figures, accepted for publication in MNRA
The kinetic helicity needed to drive large-scale dynamos
Magnetic field generation on scales large compared with the scale of the
turbulent eddies is known to be possible via the so-called effect when
the turbulence is helical and if the domain is large enough for the
effect to dominate over turbulent diffusion. Using three-dimensional turbulence
simulations, we show that the energy of the resulting mean magnetic field of
the saturated state increases linearly with the product of normalized helicity
and the ratio of domain scale to eddy scale, provided this product exceeds a
critical value of around unity. This implies that large-scale dynamo action
commences when the normalized helicity is larger than the inverse scale ratio.
Our results show that the emergence of small-scale dynamo action does not have
any noticeable effect on the large-scale dynamo. Recent findings by Pietarila
Graham et al. (2012, Phys. Rev. E85, 066406) of a smaller minimal helicity may
be an artifact due to the onset of small-scale dynamo action at large magnetic
Reynolds numbers. However, the onset of large-scale dynamo action is difficult
to establish when the kinetic helicity is small. Instead of random forcing,
they used an ABC-flow with time-dependent phases. We show that such dynamos
saturate prematurely in a way that is reminiscent of inhomogeneous dynamos with
internal magnetic helicity fluxes. Furthermore, even for very low fractional
helicities, such dynamos display large-scale fields that change direction,
which is uncharacteristic of turbulent dynamos.Comment: 10 pages, 13 figure
HCC development is associated to peripheral insulin resistance in a mouse model of NASH
NAFLD is the most common liver disease worldwide but it is the potential evolution to NASH and eventually to hepatocellular carcinoma (HCC), even in the absence of cirrhosis, that makes NAFLD of such clinical importance. Aim: we aimed to create a mouse model reproducing the pathological spectrum of NAFLD and to investigate the role of possible co-factors in promoting HCC. Methods: mice were treated with a choline-deficient L-amino-acid-defined-diet (CDAA) or its control (CSAA diet) and subjected to a low-dose i.p. injection of CCl 4 or vehicle. Insulin resistance was measured by the euglycemic-hyperinsulinemic clamp method. Steatosis, fibrosis and HCC were evaluated by histological and molecular analysis. Results: CDAA-treated mice showed peripheral insulin resistance at 1 month. At 1-3 months, extensive steatosis and fibrosis were observed in CDAA and CDAA+CCl4 groups. At 6 months, equal increase in steatosis and fibrosis was observed between the two groups, together with the appearance of tumor. At 9 months of treatment, the 100% of CDAA+ CCl4 treated mice revealed tumor versus 40% of CDAA mice. Insulin-like Growth Factor-2 (IGF-2) and Osteopontin (SPP-1) were increased in CDAA mice versus CSAA. Furthermore, Immunostaining for p-AKT, p-c-Myc and Glypican-3 revealed increased positivity in the tumors. Conclusions: the CDAA model promotes the development of HCC from NAFLD-NASH in the presence of insulin resistance but in the absence of cirrhosis. Since this condition is increasingly recognized in humans, our study provides a model that may help understanding mechanisms of carcinogenesis in NAFLD. © 2014 De Minicis et al
Ideal relaxation of the Hopf fibration
Ideal MHD relaxation is the topology-conserving reconfiguration of a magnetic
field into a lower energy state where the net force is zero. This is achieved
by modeling the plasma as perfectly conducting viscous fluid. It is an
important tool for investigating plasma equilibria and is often used to study
the magnetic configurations in fusion devices and astrophysical plasmas. We
study the equilibrium reached by a localized magnetic field through the
topology conserving relaxation of a magnetic field based on the Hopf fibration
in which magnetic field lines are closed circles that are all linked with one
another. Magnetic fields with this topology have recently been shown to occur
in non-ideal numerical simulations. Our results show that any localized field
can only attain equilibrium if there is a finite external pressure, and that
for such a field a Taylor state is unattainable. We find an equilibrium plasma
configuration that is characterized by a lowered pressure in a toroidal region,
with field lines lying on surfaces of constant pressure. Therefore, the field
is in a Grad-Shafranov equilibrium. Localized helical magnetic fields are found
when plasma is ejected from astrophysical bodies and subsequently relaxes
against the background plasma, as well as on earth in plasmoids generated by
e.g.\ a Marshall gun. This work shows under which conditions an equilibrium can
be reached and identifies a toroidal depression as the characteristic feature
of such a configuration
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