74 research outputs found
Minimum quadratic helicity states
Building on previous results on the quadratic helicity in
magnetohydrodynamics (MHD) we investigate particular minimum helicity states.
Those are eigenfunctions of the curl operator and are shown to constitute
solutions of the quasi-stationary incompressible ideal MHD equations. We then
show that these states have indeed minimum quadratic helicity.Comment: 13 pages, 2 figure
Two-dimensional infrared spectroscopy of Glycine-L-Alanine-Methylamide
Two-dimensional pump-probe infrared spectroscopy is utilized to study the
structural proprieties of Glycine-L-Alanine-Methylamide in D2O and DMSO solutions.
Experimental results are compared to computational predictions. Preliminary calculations
confirm the presence of a CO-CO coupling stronger in D2O than in DMSO as experimentally
observed.Two-dimensional pump-probe infrared spectroscopy is utilized to study the structural proprieties of Glycine-L-Alanine-Methylamide in D2O and DMSO solutions. Experimental results are compared to computational predictions. Preliminary calculations confirm the presence of a CO-CO coupling stronger in D2O than in DMSO as experimentally observed
Calculations for the Practical Applications of Quadratic Helicity in MHD
For the quadratic helicity Ï(2), we present a generalization of the Arnol'd inequality which relates the magnetic energy to the quadratic helicity, which poses a lower bound. We then introduce the quadratic helicity density using the classical magnetic helicity density and its derivatives along magnetic field lines. For practical purposes, we also compute the flow of the quadratic helicity and show that for an α2-dynamo setting, it coincides with the flow of the square of the classical helicity. We then show how the quadratic helicity can be extended to obtain an invariant even under compressible deformations. Finally, we conclude with the numerical computation of Ï(2) which show cases the practical usage of this higher order topological invariant
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
Introduction
Non peer reviewe
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
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
Effects of fieldline topology on energy propagation in the corona
We study the effect of photospheric footpoint motions on magnetic field
structures containing magnetic nulls. The footpoint motions are prescribed on
the photospheric boundary as a velocity field which entangles the magnetic
field. We investigate the propagation of the injected energy, the conversion of
energy, emergence of current layers and other consequences of the non-trivial
magnetic field topology in this situation. These boundary motions lead
initially to an increase in magnetic and kinetic energy. Following this, the
energy input from the photosphere is partially dissipated and partially
transported out of the domain through the Poynting flux. The presence of
separatrix layers and magnetic null-points fundamentally alters the propagation
behavior of disturbances from the photosphere into the corona. Depending on the
field line topology close to the photosphere, the energy is either trapped or
free to propagate into the corona.Comment: 14 pages, 15 figure
Do current and magnetic helicities have the same sign?
Current helicity, H c , and magnetic helicity, H m , are two main quantities used to characterize magnetic fields. For example, such quantities have been widely used to characterize solar active regions and their ejecta (magnetic clouds). It is commonly assumed that H c and H m have the same sign, but this has not been rigorously addressed beyond the simple case of linear force-free fields. We aim to answer whether H m H c â„ 0 in general, and whether it is true over some useful set of magnetic fields. This question is addressed analytically and with numerical examples. The main focus is on cylindrically symmetric straight flux tubes, referred to as flux ropes (FRs), using the relative magnetic helicity with respect to a straight (untwisted) reference field. Counterexamples with H m H c < 0 have been found for cylindrically symmetric FRs with finite plasma pressure, and for force-free cylindrically symmetric FRs in which the poloidal field component changes direction. Our main result is a proof that H m H c â„ 0 is true for force-free cylindrically symmetric FRs where the toroidal field and poloidal field components are each of a single sign, and the poloidal component does not exceed the toroidal component. We conclude that the conjecture that current and magnetic helicities have the same sign is not true in general, but it is true for a set of FRs of importance to coronal and heliospheric physics
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
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