472 research outputs found
New analytical and numerical models of solar coronal loop: I. Application to forced vertical kink oscillations
Aims. We construct a new analytical model of a solar coronal loop that is
embedded in a gravitationally stratified and magnetically confined atmosphere.
On the basis of this analytical model, we devise a numerical model of solar
coronal loops. We adopt it to perform the numerical simulations of its vertical
kink oscillations excited by an external driver. Methods. Our model of the
solar atmosphere is constructed by adopting a realistic temperature
distribution and specifying the curved magnetic field lines that constitute a
coronal loop. This loop is described by 2D, ideal magnetohydro- dynamic
equations that are numerically solved by the FLASH code. Results. The vertical
kink oscillations are excited by a periodic driver in the vertical component of
velocity, acting at the top of the photosphere. For this forced driver with its
amplitude 3 km/s, the excited oscillations exhibit about 1.2 km/s amplitude in
their velocity and the loop apex oscillates with its amplitude in displacement
of about 100 km. Conclusions. The newly devised analytical model of the coronal
loops is utilized for the numerical simulations of the vertical kink
oscillations, which match well with the recent observations of decay-less kink
oscillations excited in solar loops. The model will have further implications
on the study of waves and plasma dynamics in coronal loops, revealing physics
of energy and mass transport mechanisms in the localized solar atmosphere.Comment: 6 Pages; 5 Figures; A&
On the Asymmetric Longitudinal Oscillations of a Pikelner's Model Prominence
We present analytical and numerical models of a normal-polarity quiescent
prominence that are based on the model of Pikelner (Solar Phys. 1971, 17, 44 ).
We derive the general analytical expressions for the two-dimensional
equilibrium plasma quantities such as the mass density and a gas pressure, and
we specify magnetic-field components for the prominence, which corresponds to a
dense and cold plasma residing in the dip of curved magnetic-field lines. With
the adaptation of these expressions, we solve numerically the 2D, nonlinear,
ideal MHD equations for a Pikelner's model of a prominence that is initially
perturbed by reducing the gas pressure at the dip of magnetic-field lines. Our
findings reveal that as a result of pressure perturbations the prominence
plasma starts evolving in time and this leads to the antisymmetric
magnetoacoustic--gravity oscillations as well as to the mass-density growth at
the magnetic dip, and the magnetic-field lines subside there. This growth
depends on the depth of magnetic dip. For a shallower dip, less plasma is
condensed and vice-versa. We conjecture that the observed long-period
magnetoacoustic-gravity oscillations in various prominence systems are in
general the consequence of the internal pressure perturbations of the plasma
residing in equilibrium at the prominence dip.Comment: 24 Pages; 16 Figures; Solar Physic
Torsional Alfven Waves in Solar Magnetic Flux Tubes of Axial Symmetry
Aims: Propagation and energy transfer of torsional Alfv\'en waves in solar
magnetic flux tubes of axial symmetry is studied. Methods: An analytical model
of a solar magnetic flux tube of axial symmetry is developed by specifying a
magnetic flux and deriving general analytical formulae for the equilibrium mass
density and a gas pressure. The main advantage of this model is that it can be
easily adopted to any axisymmetric magnetic structure. The model is used to
simulate numerically the propagation of nonlinear Alfv\'en waves in such 2D
flux tubes of axial symmetry embedded in the solar atmosphere. The waves are
excited by a localized pulse in the azimuthal component of velocity and
launched at the top of the solar photosphere, and they propagate through the
solar chromosphere, transition region, and into the solar corona. Results: The
results of our numerical simulations reveal a complex scenario of twisted
magnetic field lines and flows associated with torsional Alfv\'en waves as well
as energy transfer to the magnetoacoustic waves that are triggered by the
Alfv\'en waves and are akin to the vertical jet flows. Alfv\'en waves
experience about 5 % amplitude reflection at the transition region. Magnetic
(velocity) field perturbations experience attenuation (growth) with height is
agreement with analytical findings. Kinetic energy of magnetoacoustic waves
consists of 25 % of the total energy of Alfv\'en waves. The energy transfer may
lead to localized mass transport in the form of vertical jets, as well as to
localized heating as slow magnetoacoustic waves are prone to dissipation in the
inner corona.Comment: 12 pages; 12 Figures, Astron. Astrophys. (A&A); Comment :
High-resolution images will be appeared with the final pape
Eigenvibrations of a bar with load
© The Authors, published by EDP Sciences, 2017. The differential eigenvalue problem describing eigenvibrations of an elastic bar with load is investigated. The problem has an increasing sequence of positive simple eigenvalues with limit point at infinity. To the sequence of eigenvalues, there corresponds a complete orthonormal system of eigenfunctions. We formulate limit differential eigenvalue problems and prove the convergence of the eigenvalues and eigenfunctions of the initial problem to the corresponding eigenvalues and eigenfunctions of the limit problems as load mass tending to infinity. The original differential eigenvalue problem is approximated by the finite difference method on a uniform grid. Error estimates for approximate eigenvalues and eigenfunctions are established. Investigations of this paper can be generalized for the cases of more complicated and important problems on eigenvibrations of beams, plates and shells with attached loads
Eigenvibrations of a simply supported beam with elastically attached load
© The Authors, published by EDP Sciences, 2018. The nonlinear differential eigenvalue problem describing eigenvibrations of a simply supported beam with elastically attached load is investigated. The existence of an increasing sequence of positive simple eigenvalues with limit point at infinity is established. To the sequence of eigenvalues, there corresponds a system of normalized eigenfunctions. To illustrate the obtained theoretical results, the initial problem is approximated by the finite difference method on a uniform grid. The accuracy of approximate solutions is studied. Investigations of the present paper can be generalized for the cases of more complicated and important problems on eigenvibrations of plates and shells with elastically attached loads
PCT, spin and statistics, and analytic wave front set
A new, more general derivation of the spin-statistics and PCT theorems is
presented. It uses the notion of the analytic wave front set of
(ultra)distributions and, in contrast to the usual approach, covers nonlocal
quantum fields. The fields are defined as generalized functions with test
functions of compact support in momentum space. The vacuum expectation values
are thereby admitted to be arbitrarily singular in their space-time dependence.
The local commutativity condition is replaced by an asymptotic commutativity
condition, which develops generalizations of the microcausality axiom
previously proposed.Comment: LaTeX, 23 pages, no figures. This version is identical to the
original published paper, but with corrected typos and slight improvements in
the exposition. The proof of Theorem 5 stated in the paper has been published
in J. Math. Phys. 45 (2004) 1944-195
Investigation of eigenvibrations of a loaded bar
© The Authors, published by EDP Sciences, 2018. The differential eigenvalue problem describing eigenvibrations of a bar with fixed ends and attached load at an interior point is investigated. This problem has an increasing sequence of positive simple eigenvalues with limit point at infinity. To the sequence of eigenvalues, there corresponds a complete orthonormal system of eigenfunctions. We formulate limit differential eigenvalue problems and prove the convergence of the eigenvalues and eigenfunctions of the initial problem to the corresponding eigenvalues and eigenfunctions of the limit problems as load mass tending to infinity. The original differential eigenvalue problem is approximated by the finite difference method on a uniform grid. Error estimates for approximate eigenvalues and eigenfunctions are established. Theoretical results are illustrated by numerical experiments for a model problem. Investigations of this paper can be generalized for the cases of more complicated and important problems on eigenvibrations of beams, plates and shells with attached loads
Iron based superconductors: magnetism, superconductivity and electronic structure
Angle resolved photoemission spectroscopy (ARPES) reveals the features of the
electronic structure of quasi-two-dimensional crystals, which are crucial for
the formation of spin and charge ordering and determine the mechanisms of
electron-electron interaction, including the superconducting pairing. The newly
discovered iron based superconductors (FeSC) promise interesting physics that
stems, on one hand, from a coexistence of superconductivity and magnetism and,
on the other hand, from complex multi-band electronic structure. In this review
I want to give a simple introduction to the FeSC physics, and to advocate an
opinion that all the complexity of FeSC properties is encapsulated in their
electronic structure. For many compounds, this structure was determined in
numerous ARPES experiments and agrees reasonably well with the results of band
structure calculations. Nevertheless, the existing small differences may help
to understand the mechanisms of the magnetic ordering and superconducting
pairing in FeSC.Comment: Invited Revie
- …