14 research outputs found
Stability and mode analysis of solar coronal loops using thermodynamic irreversible energy principles
We study the modes and stability of non - isothermal coronal loop models with
different intensity values of the equilibrium magnetic field. We use an energy
principle obtained via non - equilibrium thermodynamic arguments. The principle
is expressed in terms of Hermitian operators and allow to consider together the
coupled system of equations: the balance of energy equation and the equation of
motion. We determine modes characterized as long - wavelength disturbances that
are present in inhomogeneous media. This character of the system introduces
additional difficulties for the stability analysis because the inhomogeneous
nature of the medium determines the structure of the disturbance, which is no
longer sinusoidal. Moreover, another complication is that we obtain a
continuous spectrum of stable modes in addition to the discrete one. We obtain
a unique unstable mode with a characteristic time that is comparable with the
characteristic life-time observed for loops. The feasibility of wave-based and
flow-based models is examined.Comment: 29 pages 10 figure
Estudio de modos de oscilación
El objeto de este trabajo es estudiar las oscilaciones y la estabilidad en la situación más general posible, es decir abarcando las oscilaciones radial y no radial en los casos adiabáticos y no adiabáticos. Para ello se han preparad códigos computacionales, que se aplican a los modelos más sencillos de pulsaciones estelares descriptos en la literatura, con el fin de ajustarlos para luego extender su aplicación a casos más complejos.Asociación Argentina de Astronomí
Thermal stability analysis of coronal loops
The coronal loops confine a low density (n=10¹⁰ cm⁻³) and hot plasma (T=10⁶ K), whose ends interact with the much denser and hotter photospheric fluid. The linear stability of the dynamical and thermal equilibria of the coronal plasma is analyzed. A formalism based on methods of irreversible thermodynamics was used, which systematically builds up (whenever it is possible) a variational principle for studying the stability. The stability conditions derived in this work are compared with results available in the literature, which were obtained by standard stability methods.Asociación Argentina de Astronomí
Quantum random walk on the line as a markovian process
We analyze in detail the discrete--time quantum walk on the line by
separating the quantum evolution equation into Markovian and interference
terms. As a result of this separation, it is possible to show analytically that
the quadratic increase in the variance of the quantum walker's position with
time is a direct consequence of the coherence of the quantum evolution. If the
evolution is decoherent, as in the classical case, the variance is shown to
increase linearly with time, as expected. Furthermore we show that this system
has an evolution operator analogous to that of a resonant quantum kicked rotor.
As this rotator may be described through a quantum computational algorithm, one
may employ this algorithm to describe the time evolution of the quantum walker.Comment: few typos corrected, 13 pages, 2 figures, to appear in Physica
Markovian Behaviour and Constrained Maximization of the Entropy in Chaotic Quantum Systems
The separation of the Schr\"{o}dinger equation into a Markovian and an
interference term provides a new insight in the quantum dynamics of classically
chaotic systems. The competition between these two terms determines the
localized or diffusive character of the dynamics. In the case of the Kicked
Rotor, we show how the constrained maximization of the entropy implies
exponential localization.Comment: 8 pages, 2 figure
Dynamical Localization in Quasi-Periodic Driven Systems
We investigate how the time dependence of the Hamiltonian determines the
occurrence of Dynamical Localization (DL) in driven quantum systems with two
incommensurate frequencies. If both frequencies are associated to impulsive
terms, DL is permanently destroyed. In this case, we show that the evolution is
similar to a decoherent case. On the other hand, if both frequencies are
associated to smooth driving functions, DL persists although on a time scale
longer than in the periodic case. When the driving function consists of a
series of pulses of duration , we show that the localization time
increases as as the impulsive limit, , is
approached. In the intermediate case, in which only one of the frequencies is
associated to an impulsive term in the Hamiltonian, a transition from a
localized to a delocalized dynamics takes place at a certain critical value of
the strength parameter. We provide an estimate for this critical value, based
on analytical considerations. We show how, in all cases, the frequency spectrum
of the dynamical response can be used to understand the global features of the
motion. All results are numerically checked.Comment: 7 pages, 5 figures included. In this version is that Subsection III.B
and Appendix A on the quasiperiodic Fermi Accelerator has been replaced by a
reference to published wor
Numerical solution of the nonadiabatic model of pulsating stars through the application of a lagrangian formulation
The derivation of a thermodynamical Lagrangian for the non-adiabatical case using the techniques of the irreversible thermodynamic solves existing numerical problems. The usual method consists in obtaining the non-adiabatic solution by solving numerically the equation of motion, using as initial solution that of the adiabatic case. The variational principle associated with the thermodynamical Lagrangian permits to identify the non-adiabatic cases and to adjust the boundary conditions.Asociación Argentina de Astronomí
Stability analysis of quiescent prominences using thermodynamic irreversible energy principles
Using methods of non-equilibrium thermodynamics that
extend and generalize the MHD energy principle of Bernstein et al.
(1958, Proc. Roy. Soc. A, 244, 17) we develop a formalism in order to analyze the stability
properties of prominence models considered as dissipative states
i.e. states far form thermodynamic equilibrium. As an example, the
criterion is applied to the Kippenhahn-Schlüter model
(hereafter K-S) considering the addition of dissipative terms in
the coupled system of equations: the balance of energy equation
and the equation of motion. We show from this application, that
periods corresponding to typical oscillations of the chromosphere
and photosphere (3 and 5 min respectively), that were reported
as observations of the prominence structure, can be explained as
internal modes of the prominence itself. This is an alternative
explanation to the one that supposes that the source of these
perturbations are the cold foot chromospheric and photospheric
basis
Hopf bifurcations in coronal loops. II. Nonlinear evolution of instabilities
In a previous paper, we have modeled the coupling between corona and chromosphere and derived a non-linear set of equations, where the global stability properties of the coronal plasma can be studied. The linear stability analysis indicates that the static equilibrium is stable unless the heating rate falls below a certain critical value. In the present paper, we study the nonlinear evolution of our equations both analytically and numerically. Applying a perturbative technique around the critical point, we find that a subcritical Hopf bifurcation takes place. The numerical integration of the equations agrees satisfactorily with the analytical results when they are compared close to the bifurcation. The nonthermal Doppler widths of EUV lines forming in the transition region can be explained by the existence of relatively low amplitude limit cycles.Fil:Gómez, D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Sicardi Schifino, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Ferro Fontán, C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina