1,015 research outputs found
How useful are monetary policy rules to deal with inflation: The Spanish case.
The role of monetary policy rules to explain the behaviour of central banks has received an increasing attention during the last few years. The Spanish case could be of interest given that, with an inflation above the European average, was able to conduct its monetary policy and to control the inflation in order to join the European monetary union. But after the adoption of the European Central Bank’s monetary policy in January 1999, a higher inflation can be observed. In this paper we explore whether the monetary policy performed by the Bank of Spain would have follow a monetary policy rule, and to which extent monetary rules contribute to achieve the goal of inflation.Monetary policy rules, inflation.
Seismology of Standing Kink Oscillations of Solar Prominence Fine Structures
We investigate standing kink magnetohydrodynamic (MHD) oscillations in a
prominence fine structure modeled as a straight and cylindrical magnetic tube
only partially filled with the prominence material, and with its ends fixed at
two rigid walls representing the solar photosphere. The prominence plasma is
partially ionized and a transverse inhomogeneous transitional layer is included
between the prominence thread and the coronal medium. Thus, ion-neutral
collisions and resonant absorption are the considered damping mechanisms.
Approximate analytical expressions of the period, the damping time, and their
ratio are derived for the fundamental mode in the thin tube and thin boundary
approximations. We find that the dominant damping mechanism is resonant
absorption, which provides damping ratios in agreement with the observations,
whereas ion-neutral collisions are irrelevant for the damping. The values of
the damping ratio are independent of both the prominence thread length and its
position within the magnetic tube, and coincide with the values for a tube
fully filled with the prominence plasma. The implications of our results in the
context of the MHD seismology technique are discussed, pointing out that the
reported short-period (2 - 10 min) and short-wavelength (700 - 8,000 km) thread
oscillations may not be consistent with a standing mode interpretation and
could be related to propagating waves. Finally, we show that the inversion of
some prominence physical parameters, e.g., Alfv\'en speed, magnetic field
strength, transverse inhomogeneity length-scale, etc., is possible using
observationally determined values of the period and damping time of the
oscillations along with the analytical approximations of these quantities.Comment: Accepted for publication in Ap
Magnetohydrodynamic kink waves in two-dimensional non-uniform prominence threads
We analyse the oscillatory properties of resonantly damped transverse kink
oscillations in two-dimensional prominence threads. The fine structures are
modelled as cylindrically symmetric magnetic flux tubes with a dense central
part with prominence plasma properties and an evacuated part, both surrounded
by coronal plasma. The equilibrium density is allowed to vary non-uniformly in
both the transverse and the longitudinal directions.We examine the influence of
longitudinal density structuring on periods, damping times, and damping rates
for transverse kink modes computed by numerically solving the linear resistive
magnetohydrodynamic (MHD) equations. The relevant parameters are the length of
the thread and the density in the evacuated part of the tube, two quantities
that are difficult to directly estimate from observations. We find that both of
them strongly influence the oscillatory periods and damping times, and to a
lesser extent the damping ratios. The analysis of the spatial distribution of
perturbations and of the energy flux into the resonances allows us to explain
the obtained damping times. Implications for prominence seismology, the physics
of resonantly damped kink modes in two-dimensional magnetic flux tubes, and the
heating of prominence plasmas are discussed.Comment: 12 pages, 9 figures, A&A accepte
Kelvin-Helmholtz instability in partially ionized compressible plasmas
The Kelvin-Helmholtz Instability (KHI) has been observed in the solar
atmosphere. Ion-neutral collisions may play a relevant role for the growth rate
and evolution of the KHI in solar partially ionized plasmas as in, e.g., solar
prominences. Here, we investigate the linear phase of the KHI at an interface
between two partially ionized magnetized plasmas in the presence of a shear
flow. The effects of ion-neutral collisions and compressibility are included in
the analysis. We obtain the dispersion relation of the linear modes and perform
parametric studies of the unstable solutions. We find that in the
incompressible case the KHI is present for any velocity shear regardless the
value of the collision frequency. In the compressible case, the domain of
instability depends strongly on the plasma parameters, specially the collision
frequency and the density contrast. For high collision frequencies and low
density contrasts the KHI is present for super-Alfvenic velocity shear only.
For high density contrasts the threshold velocity shear can be reduced to
sub-Alfvenic values. For the particular case of turbulent plumes in
prominences, we conclude that sub-Alfvenic flow velocities can trigger the KHI
thanks to the ion-neutral coupling.Comment: Accepted for publication in Ap
Numerical simulations of turbulence in prominence threads induced by torsional oscillations
Threads are the main constituents of prominences and are subjected to
oscillations that might be interpreted as MHD waves. Moreover, the
Kelvin-Helmholtz instability (KHI) has been reported in prominences. Both waves
and KHI may affect the thermodynamic state of the threads. We investigate the
triggering of turbulence in a thread caused by the nonlinear evolution of
standing torsional Alfv\'en waves as well as possible observational signatures
of this dynamics and the plasma heating. We modeled the thread as a radially
and longitudinally nonuniform cylindrical flux tube with a uniform axial
magnetic field embedded in a coronal environment. We perturbed the flux tube
with the longitudinally fundamental mode of standing torsional Alfv\'en waves
and numerically solved the 3D MHD equations to study the temporal evolution in
both ideal and dissipative scenarios. We also performed forward modeling to
calculate the synthetic H{\alpha} imaging. Standing torsional Alfv\'en waves
undergo phase-mixing owing to the radially nonuniform density. The phase-mixing
generates azimuthal shear flows that eventually trigger the KHI and, later,
turbulence. If nonideal effects are included, plasma heating is localized in an
annulus region at the thread boundary and does not increase the temperature in
the cool core. Instead, the average temperature in the thread decreases owing
to the mixing of internal and external plasmas. In the synthetic observations,
first we find periodic pulsations in the H{\alpha} intensity caused by the
integration of the phase-mixing flows along the line of sight. Later, we find
fine strands that may be associated with the KHI vortices. Turbulence can be
generated by standing torsional Alfv\'en waves in a thread after the onset of
KHI, but this mechanism is not enough to heat globally the structure. The
dynamics could be seen in high-resolution H{\alpha} observations.Comment: Accepted in Astronomy & Astrophysic
Transition to turbulence in nonuniform coronal loops driven by torsional Alfv\'en waves. II. Extended analysis and effect of magnetic twist
It has been shown in a previous work that torsional Alfv\'en waves can drive
turbulence in nonuniform coronal loops with a purely axial magnetic field. Here
we explore the role of the magnetic twist. We model a coronal loop as a
transversely nonuniform straight flux tube, anchored in the photosphere, and
embedded in a uniform coronal environment. We consider that the magnetic field
is twisted and control the strength of magnetic twist by a free parameter of
the model. We excite the longitudinally fundamental mode of standing torsional
Alfv\'en waves, whose temporal evolution is obtained by means of
high-resolution three-dimensional ideal magnetohydrodynamic numerical
simulations. We find that phase mixing of torsional Alfv\'en waves creates
velocity shear in the direction perpendicular to the magnetic field lines. The
velocity shear eventually triggers the Kelvin-Helmholtz instability (KHi). In
weakly twisted magnetic tubes, the KHi is able to grow nonlinearly and,
subsequently, turbulence is driven in the coronal loop in a similar manner as
in the untwisted case. Provided that magnetic twist remains weak, the effect of
magnetic twist is to delay the onset of the KHi and to slow down the
development of turbulence. In contrast, magnetic tension can suppress the
nonlinear growth of the KHi when magnetic twist is strong enough, even if the
KHi has locally been excited by the phase-mixing shear. Thus, turbulence is not
generated in strongly twisted loopsComment: Published in A&A. 12 pages. 12 figure
Kink oscillations of flowing threads in solar prominences
Recent observations by Hinode/SOT show that MHD waves and mass flows are
simultaneously present in the fine structure of solar prominences. We
investigate standing kink magnetohydrodynamic (MHD) waves in flowing prominence
threads from a theoretical point of view. We model a prominence fine structure
as a cylindrical magnetic tube embedded in the solar corona with its ends
line-tied in the photosphere. The magnetic cylinder is composed of a region
with dense prominence plasma, which is flowing along the magnetic tube, whereas
the rest of the flux tube is occupied by coronal plasma. We use the WKB
approximation to obtain analytical expressions for the period and the amplitude
of the fundamental mode as functions of the flow velocity. In addition, we
solve the full problem numerically by means of time-dependent simulations. We
find that both the period and the amplitude of the standing MHD waves vary in
time as the prominence thread flows along the magnetic structure. The
fundamental kink mode is a good description for the time-dependent evolution of
the oscillations, and the analytical expressions in the WKB approximation are
in agreement with the full numerical results. The presence of flow modifies the
period of the oscillations with respect to the static case. However, for
realistic flow velocities this effect might fall within the error bars of the
observations. The variation of the amplitude due to the flow leads to apparent
damping or amplification of the oscillations, which could modify the real rate
of attenuation caused by an additional damping mechanism.Comment: Accepted for publication in A&
Resonantly Damped Kink Magnetohydrodynamic Waves in a Partially Ionized Filament Thread
Transverse oscillations of solar filament and prominence threads have been
frequently reported. These oscillations have the common features of being of
short period (2-10 min) and being damped after a few periods. Kink
magnetohydrodynamic (MHD) wave modes have been proposed as responsible for the
observed oscillations, whereas resonant absorption in the Alfven continuum and
ion-neutral collisions are the best candidates to be the damping mechanisms.
Here, we study both analytically and numerically the time damping of kink MHD
waves in a cylindrical, partially ionized filament thread embedded in a coronal
environment. The thread model is composed of a straight and thin, homogeneous
filament plasma, with a transverse inhomogeneous transitional layer where the
plasma physical properties vary continuously from filament to coronal
conditions. The magnetic field is homogeneous and parallel to the thread axis.
We find that the kink mode is efficiently damped by resonant absorption for
typical wavelengths of filament oscillations, the damping times being
compatible with the observations. Partial ionization does not affect the
process of resonant absorption, and the filament plasma ionization degree is
only important for the damping for wavelengths much shorter than those
observed. To our knowledge, this is the first time that the phenomenon of
resonant absorption is studied in a partially ionized plasma.Comment: Submitted in Ap
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