5,925 research outputs found
Quiet Sun Magnetic Field Measurements Based on Lines with Hyperfine Structure
The Zeeman pattern of MnI lines is sensitive to hyperfine structure (HFS)
and, they respond to hG magnetic field strengths differently from the lines
used in solar magnetometry. This peculiarity has been employed to measure
magnetic field strengths in quiet Sun regions. However, the methods applied so
far assume the magnetic field to be constant in the resolution element. The
assumption is clearly insufficient to describe the complex quiet Sun magnetic
fields, biasing the results of the measurements. We present the first syntheses
of MnI lines in realistic quiet Sun model atmospheres. The syntheses show how
the MnI lines weaken with increasing field strength. In particular, kG magnetic
concentrations produce NnI 5538 circular polarization signals (Stokes V) which
can be up to two orders of magnitude smaller than the weak magnetic field
approximation prediction. Consequently, (1) the polarization emerging from an
atmosphere having weak and strong fields is biased towards the weak fields, and
(2) HFS features characteristic of weak fields show up even when the magnetic
flux and energy are dominated by kG fields. For the HFS feature of MnI 5538 to
disappear the filling factor of kG fields has to be larger than the filling
factor of sub-kG fields. Stokes V depends on magnetic field inclination
according to the simple consine law. Atmospheres with unresolved velocities
produce asymmetric line profiles, which cannot be reproduced by simple
one-component model atmospheres. The uncertainty of the HFS constants do not
limit the use of MnI lines for magnetometry.Comment: Accepted for publication in ApJ. 10 pages, 14 figure
Plasma turbulence at ion scales: a comparison between PIC and Eulerian hybrid-kinetic approaches
Kinetic-range turbulence in magnetized plasmas and, in particular, in the
context of solar-wind turbulence has been extensively investigated over the
past decades via numerical simulations. Among others, one of the widely adopted
reduced plasma model is the so-called hybrid-kinetic model, where the ions are
fully kinetic and the electrons are treated as a neutralizing (inertial or
massless) fluid. Within the same model, different numerical methods and/or
approaches to turbulence development have been employed. In the present work,
we present a comparison between two-dimensional hybrid-kinetic simulations of
plasma turbulence obtained with two complementary approaches spanning about two
decades in wavenumber - from MHD inertial range to scales well below the ion
gyroradius - with a state-of-the-art accuracy. One approach employs hybrid
particle-in-cell (HPIC) simulations of freely-decaying Alfv\'enic turbulence,
whereas the other consists of Eulerian hybrid Vlasov-Maxwell (HVM) simulations
of turbulence continuously driven with partially-compressible large-scale
fluctuations. Despite the completely different initialization and
injection/drive at large scales, the same properties of turbulent fluctuations
at are observed. The system indeed self-consistently
"reprocesses" the turbulent fluctuations while they are cascading towards
smaller and smaller scales, in a way which actually depends on the plasma beta
parameter. Small-scale turbulence has been found to be mainly populated by
kinetic Alfv\'en wave (KAW) fluctuations for , whereas KAW
fluctuations are only sub-dominant for low-.Comment: 18 pages, 4 figures, accepted for publication in J. Plasma Phys.
(Collection: "The Vlasov equation: from space to laboratory plasma physics"
Differential and Twistor Geometry of the Quantum Hopf Fibration
We study a quantum version of the SU(2) Hopf fibration and its
associated twistor geometry. Our quantum sphere arises as the unit
sphere inside a q-deformed quaternion space . The resulting
four-sphere is a quantum analogue of the quaternionic projective space
. The quantum fibration is endowed with compatible non-universal
differential calculi. By investigating the quantum symmetries of the fibration,
we obtain the geometry of the corresponding twistor space and
use it to study a system of anti-self-duality equations on , for which
we find an `instanton' solution coming from the natural projection defining the
tautological bundle over .Comment: v2: 38 pages; completely rewritten. The crucial difference with
respect to the first version is that in the present one the quantum
four-sphere, the base space of the fibration, is NOT a quantum homogeneous
space. This has important consequences and led to very drastic changes to the
paper. To appear in CM
Using HINODE/Extreme-Ultraviolet Imaging Spectrometer to confirm a seismologically inferred coronal temperature
The Extreme-Ultraviolet Imaging Spectrometer on board the HINODE satellite is used to examine the loop system described in Marsh et al. (2009) by applying spectroscopic diagnostic methods. A simple isothermal mapping algorithm is applied to determine where the assumption of isothermal plasma may be valid, and the emission measure locii technique is used to determine the temperature profile along the base of the loop system. It is found that, along the base, the loop has a uniform temperature profile with a mean temperature of 0.89 +- 0.09 MK which is in agreement with the temperature determined seismologically in Marsh et al. (2009), using observations interpreted as the slow magnetoacoustic mode. The results further strengthen the slow mode interpretation, propagation at a uniform sound speed, and the analysis method applied in Marsh et al. (2009). It is found that it is not possible to discriminate between the slow mode phase speed and the sound speed within the precision of the present observations
Three-dimensional evolution of magnetic and velocity shear driven instabilities in a compressible magnetized jet
The problem of three-dimensional combined magnetic and velocity shear driven
instabilities of a compressible magnetized jet modeled with a plane
neutral/current double vortex sheet in the framework of the resistive
magnetohydrodynamics is addressed. The resulting dynamics given by the
stream+current sheet interaction is analyzed and the effects of a variable
geometry of the basic fields are considered. Depending on the basic asymptotic
magnetic field configuration, a selection rule of the linear instability modes
can be obtained. Hence, the system follows a two-stage path developing either
through a fully three-dimensional dynamics with a rapid evolution of kink modes
leading to a final turbulent state, or rather through a driving two-dimensional
instability pattern that develops on parallel planes on which a
reconnection+coalescence process takes place.Comment: 33 pages, 15 figures, accepted for publication in Physics of Plasma
Gravity from Dirac Eigenvalues
We study a formulation of euclidean general relativity in which the dynamical
variables are given by a sequence of real numbers , representing
the eigenvalues of the Dirac operator on the curved spacetime. These quantities
are diffeomorphism-invariant functions of the metric and they form an infinite
set of ``physical observables'' for general relativity. Recent work of Connes
and Chamseddine suggests that they can be taken as natural variables for an
invariant description of the dynamics of gravity. We compute the Poisson
brackets of the 's, and find that these can be expressed in terms
of the propagator of the linearized Einstein equations and the energy-momentum
of the eigenspinors. We show that the eigenspinors' energy-momentum is the
Jacobian matrix of the change of coordinates from the metric to the
's. We study a variant of the Connes-Chamseddine spectral action
which eliminates a disturbing large cosmological term. We analyze the
corresponding equations of motion and find that these are solved if the energy
momenta of the eigenspinors scale linearly with the mass. Surprisingly, this
scaling law codes Einstein's equations. Finally we study the coupling to a
physical fermion field.Comment: An enlarged and improved version which will be pubblished in Mod.
Phys. Lett.
Bioinformatic characterization of sulfotransferase provides new insights for the exploitation of sulfated polysaccharides in caulerpa
Caulerpa is an unusual algal genus from Caulerpaceae (Chlorophyta, Bryopsidales). Species from this family produce a wide range of metabolites suitable for biotechnology applications. Among these, sulfated polysaccharides (SPs) are often highly desirable for pharmaceutical and nutraceutical applications. Here, we provide a classification of sulfotransferases from Caulerpa; these important enzymes catalyze the nodal step for the biosynthesis of SPs. For this, we performed phylogenetic, genomic, expression analyses and prediction of the protein structure on sulfotransferases from Caulerpa. Sequences, domains and structures of sulfotransferases generally shared common characteristics with other plants and algae. However, we found an extensive duplication of sulfotransferase gene family, which is unique among the green algae. Expression analysis revealed specific transcript abundance in the pinnae and rachis of the alga. The unique genomic features could be utilized for the production of complex SPs, which require multiple and specific sulfation reactions. The expansion of this gene family in Caulerpaceae would have resulted in a number of proteins characterizing the unique SPs found in these algae. We provide a putative biosynthetic pathway of SPs, indicating the unique characteristics of this pathway in Caulerpa species. These data may help in the future selection of Caulerpa species for both commercial applications and genetic studies to improve the synthesis of valuable products from Caulerpa
Sub-structure formation in starless cores
Motivated by recent observational searches of sub-structure in starless
molecular cloud cores, we investigate the evolution of density perturbations on
scales smaller than the Jeans length embedded in contracting isothermal clouds,
adopting the same formalism developed for the expanding Universe and the solar
wind. We find that initially small amplitude, Jeans-stable perturbations
(propagating as sound waves in the absence of a magnetic field), are amplified
adiabatically during the contraction, approximately conserving the wave action
density, until they either become nonlinear and steepen into shocks at a time
, or become gravitationally unstable when the Jeans length
decreases below the scale of the perturbations at a time . We
evaluate analytically the time at which the perturbations enter
the non-linear stage using a Burgers' equation approach, and we verify
numerically that this time marks the beginning of the phase of rapid
dissipation of the kinetic energy of the perturbations. We then show that for
typical values of the rms Mach number in molecular cloud cores, is
smaller than , and therefore density perturbations likely dissipate
before becoming gravitational unstable. Solenoidal modes grow at a faster rate
than compressible modes, and may eventually promote fragmentation through the
formation of vortical structures.Comment: 8 pages, 4 figure
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