723 research outputs found
Determination of the cross-field density structuring in coronal waveguides using the damping of transverse waves
Time and spatial damping of transverse magnetohydrodynamic (MHD) kink
oscillations is a source of information on the cross-field variation of the
plasma density in coronal waveguides. We show that a probabilistic approach to
the problem of determining the density structuring from the observed damping of
transverse oscillations enables us to obtain information on the two parameters
that characterise the cross-field density profile. The inference is performed
by computing the marginal posterior distributions for density contrast and
transverse inhomo- geneity length-scale using Bayesian analysis and damping
ratios for transverse oscillations under the assumption that damping is
produced by resonant absorption. The obtained distributions show that, for
damping times of a few oscillatory periods, low density contrasts and short
inho- mogeneity length scales are more plausible in explaining observations.
This means that valuable information on the cross-field density profile can be
obtained even if the inversion problem, with two unknowns and one observable,
is a mathematically ill-posed problem.Comment: 5 pages, 3 figures, accepte
Determination of Transverse Density Structuring from Propagating MHD Waves in the Solar Atmosphere
We present a Bayesian seismology inversion technique for propagating
magnetohydrodynamic (MHD) transverse waves observed in coronal waveguides. The
technique uses theoretical predictions for the spatial damping of propagating
kink waves in transversely inhomogeneous coronal waveguides. It combines wave
amplitude damping length scales along the waveguide with theoretical results
for resonantly damped propagating kink waves to infer the plasma density
variation across the oscillating structures. Provided the spatial dependence of
the velocity amplitude along the propagation direction is measured and the
existence of two different damping regimes is identified, the technique would
enable us to fully constrain the transverse density structuring, providing
estimates for the density contrast and its transverse inhomogeneity length
scale
On the nature of transverse coronal waves revealed by wavefront dislocations
Coronal waves are an important aspect of the dynamics of the plasma in the
corona. Wavefront dislocations are topological features of most waves in nature
and also of magnetohydrodynamic waves. Are there dislocations in coronal waves?
The finding and explanation of dislocations may shed light on the nature and
characteristics of the propagating waves, their interaction in the corona and
in general on the plasma dynamics. We positively identify dislocations in
coronal waves observed by the Coronal Multi-channel Polarimeter (CoMP) as
singularities in the Doppler shifts of emission coronal lines. We study the
possible singularities that can be expected in coronal waves and try to
reproduce the observed dislocations in terms of localization and frequency of
appearance. The observed dislocations can only be explained by the interference
of a kink and a sausage wave modes propagating with different frequencies along
the coronal magnetic field. In the plane transverse to the propagation, the
cross-section of the oscillating plasma must be smaller than the spatial
resolution, and the two waves result in net longitudinal and transverse
velocity components that are mixed through projection onto the line of sight.
Alfv\'en waves can be responsible of the kink mode, but a magnetoacoustic
sausage mode is necessary in all cases. Higher (flute) modes are excluded. The
kink mode has a pressure amplitude that is smaller than the pressure amplitude
of the sausage mode, though its observed velocity is larger. This concentrates
dislocations on the top of the loop. To explain dislocations, any model of
coronal waves must include the simultaneous propagation and interference of
kink and sausage wave modes of comparable but different frequencies, with a
sausage wave amplitude much smaller than the kink one.Comment: 11 pages. 5 figures. Accepted for publication in A&
Coronal cooling as a result of mixing by the nonlinear Kelvin–Helmholtz instability
This is the final version. Available from American Astronomical Society via the DOI in this recordRecent observations show cool, oscillating prominence threads fading when observed in cool spectral
lines and appearing in warm spectral lines. A proposed mechanism to explain the observed temperature
evolution is that the threads were heated by turbulence driven by the Kelvin–Helmholtz instability that
developed as a result of wave-driven shear flows on the surface of the thread. As the Kelvin–Helmholtz
instability is an instability that works to mix the two fluids either side of the velocity shear layer, in the
solar corona it can be expected to work by mixing the cool prominence material with that of the hot
corona to form a warm boundary layer. In this paper we develop a simple phenomenological model of
nonlinear Kelvin–Helmholtz mixing, using it to determine the characteristic density and temperature
of the mixing layer, which for the case under study with constant pressure across the two fluids are
ρ_mixed=√ρ_1ρ_2 and T_mixed=√T_1T_2. One result from the model is that it provides an accurate, as
determined by comparison with simulation results, determination of the kinetic energy in the mean
velocity field. A consequence of this is that the magnitude of turbulence, and with it the energy
that can be dissipated on fast time-scales, as driven by this instability can be determined. For the
prominence-corona system, the mean temperature rise possible from turbulent heating is estimated to
be less than 1% of the characteristic temperature (which is found to be
T_mixed=10^5K). These results highlight that mixing, and not heating, are likely to be the cause of the observed transition between
cool to warm material in Okamoto et al. (2015). One consequence of this result is that the mixing
creates a region with higher radiative loss rates on average than either of the original fluids, meaning
that this instability could contribute a net loss of thermal energy from the corona, i.e. coronal cooling.Science and Technology Facilities CouncilSpanish Ministerio de Ciencia, Innovacion y Universidade
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
Legislació i normativa sobre gent gran: una aproximació
Partint de la Declaració Universal dels Drets Humans i passant per la Constitució Espanyola, arribarem a l’Estatut d’Autonomia de les Illes Balears. Enunciat a les normes fonamentals el nou repartiment competencial, una sèrie de reials decrets varen fer efectives les transferències de competències de l’Estat a l’Administració autonòmica. Al mateix temps, es va produint l’entrega de part d’aquestes mateixes competències als consells insulars. Paral·lelament es va promulgant la normativa autonòmica bàsica reguladora de l’acció social . Lleis i decrets n’estableixen el marc legislatiu, enllaçat amb l’estatal i complementant-lo. De la regulació de les condicions d’obertura dels centres de persones majors, a les ajudes específiques per a aquest col·lectiu. De les prestacions econòmiques als programes de vacances. I com a punt d’arribada, el Pla Estratègic de persones majors.Partiendo de la Declaración Universal de Derechos Humanos y pasando por la Constitución Española, llegaremos al Estatuto de Autonomía de las Islas Baleares. Enunciado en las normas fundamentales el nuevo reparto competencial, una serie de reales decretos harán efectivas las transferencias de competencias del Estado a la Administración Autonómica. A la vez, se va produciendo la entrega de parte de esas mismas competencias a los consejos insulares. Paralelamente, se promulga la normativa autonómica básica reguladora de la acción social. Leyes y decretos establecen el marco legislativo propio, engarzado con el estatal y complementándolo. De la regulación de las condiciones de apertura de los centros de personas mayores, a las ayudas específicas para dicho colectivo. De las prestaciones económicas a los programas vacacionales. Y como punto de llegada, el Plan Estratégico de personas mayores
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