R. Oliver

University of the Balearic Islands, Palma, Balearic Islands, Spain

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Publications (151)565.41 Total impact

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    ABSTRACT: The dispersion of small amplitude, impulsively excited wave trains propagating along a magnetic flux tube is investigated. The initial disturbance is a localized transverse displacement of the tube that excites a fast kink wave packet. The spatial and temporal evolution of the perturbed variables (density, plasma displacement, velocity, ...) is given by an analytical expression containing an integral that is computed numerically. We find that the dispersion of fast kink wave trains is more important for shorter initial disturbances (i.e. more concentrated in the longitudinal direction) and for larger density ratios (i.e. for larger contrasts of the tube density with respect to the environment density). This type of excitation generates a wave train whose signature at a fixed position along a coronal loop is a short event (duration ~ 20 s) in which the velocity and density oscillate very rapidly with typical periods of the order of a few seconds. The oscillatory period is not constant but gradually declines during the course of this event. Peak values of the velocity are of the order of 10 km/s and are accompanied by maximum density variations of the order of 10-15% the unperturbed loop density.
    The Astrophysical Journal 02/2014; 789(1). · 6.73 Impact Factor
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    ABSTRACT: Context. Prominence oscillations have been mostly detected using Doppler velocity, although there are also claimed detections by means of periodic variations in half-width or line intensity. However, scarce observational evidence exists about simultaneous detection of oscillations in several spectral indicators. Aims. Our main aim here is to explore the relationship between spectral indicators, such as Doppler shift, line intensity, and line half-width, and the linear perturbations excited in a simple prominence model. Methods. Our equilibrium background model consists of a bounded, homogeneous slab, which is permeated by a transverse magnetic field, having prominence-like physical properties. Assuming linear perturbations, the dispersion relation for fast and slow modes has been derived, as well as the perturbations for the different physical quantities. These perturbations have been used as the input variables in a one-dimensional radiative transfer code, which calculates the full spectral profile of the hydrogen H-alpha and H-beta lines. Results. We have found that different oscillatory modes produce spectral indicator variations in different magnitudes. Detectable variations in the Doppler velocity were found for the fundamental slow mode only. Substantial variations in the H-beta line intensity were found for specific modes. Other modes lead to lower and even undetectable parameter variations. Conclusions. To perform prominence seismology, analysis of the H-alpha and H-beta spectral line parameters could be a good tool to detect and identify oscillatory modes.
    01/2014;
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    ABSTRACT: Prominence oscillations have been mostly detected using Doppler velocity, although there are also claimed detections by means of the periodic variations of half-width or line intensity. Our main aim here is to explore the relationship between spectral indicators such as Doppler shift, line intensity and line half-width and the linear perturbations excited in a simple prominence model.
    Proceedings of the International Astronomical Union 01/2014;
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    ABSTRACT: Transverse oscillations of thin threads in solar prominences are frequently reported in high-resolution observations. The typical periods of the oscillations are in the range of 3 to 20 min. A peculiar feature of the oscillations is that they are damped in time, with short damping times corresponding to few periods. Theoretically, the oscillations are interpreted as kink magnetohydrodynamic waves. However, the mechanism responsible for the damping is not well known. Here we perform a comparative study between different physical mechanisms that may damp kink waves in prominence threads. The considered processes are thermal conduction, cooling by radiation, resonant absorption, and ion-neutral collisions. We find that thermal conduction and radiative cooling are very inefficient for the damping of kink waves. The effect of ion-neutral collisions is minor for waves with periods usually observed. Resonant absorption is the only process that produces an efficient damping. The damping times theoretically predicted by resonant absorption are compatible with those reported in the observations.
    Proceedings of the International Astronomical Union 01/2014;
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    ABSTRACT: Seismology of coronal loops using observations of damped transverse oscillations in combination with results from theoretical models is a tool to indirectly infer physical parameters in the solar atmospheric plasma. Existing seismology schemes based on approximations to the period and damping time of kink oscillations are often used beyond their theoretical range of applicability. These approximations assume that the variation of density across the loop is confined to a nonuniform layer much thinner than the radius of the loop, but the results of the inversion problem often do not satisfy this preliminary hypothesis. Here, we determine the accuracy of the analytic approximations to the period and damping time, and its impact on seismology estimates, when largely nonuniform loops are considered. We find that the accuracy of the approximations when used beyond their range of applicability is strongly affected by the form of the density profile across the loop, that is observationally unknown and so must be arbitrarily imposed as part of the theoretical model. The error associated with the analytic approximations can be larger than 50% even for relatively thin nonuniform layers. This error directly affects the accuracy of approximate seismology estimates compared to actual numerical inversions. In addition, assuming different density profiles can produce noncoincident intervals of the seismic variables in inversions of the same event. The ignorance about the true shape of density variation across the loop is an important source of error that may dispute the reliability of parameters seismically inferred assuming an ad hoc density profile.
    12/2013; 781(2).
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    ABSTRACT: Observations of active regions and limb prominences often show cold, dense blobs descending with an acceleration smaller than that of free fall. The dynamics of these condensations falling in the solar corona is investigated in this paper using a simple fully ionised plasma model. We find that the presence of a heavy condensation gives rise to a dynamical rearrangement of the coronal pressure that results in the formation of a large pressure gradient that opposes gravity. Eventually this pressure gradient becomes so large that the blob acceleration vanishes or even points upwards. Then, the blob descent is characterised by an initial acceleration phase followed by an essentially constant velocity phase. These two stages can be identified in published time-distance diagrams of coronal rain events. Both the duration of the first stage and the velocity attained by the blob increase for larger values of the ratio of blob to coronal density, for larger blob mass, and for smaller coronal temperature. Dense blobs are characterised by a detectable density growth (up to 60% in our calculations) and by a steepening of the density in their lower part, that could lead to the formation of a shock. They also emit sound waves that could be detected as small intensity changes with periods of the order of 100 s and lasting between a few and about ten periods. Finally, the curvature of the falling path is only relevant when a very dense blob falls along inclined magnetic field lines.
    11/2013; 784(1).
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    ABSTRACT: Solar prominence models used so far in the analysis of MHD waves in such structures are quite elementary. In this work, we calculate numerically magnetohydrostatic models in two-dimensional configurations under the presence of gravity. Our interest is in models that connect the magnetic field to the photosphere and include an overlying arcade. The method used here is based on a relaxation process and requires solving the time-dependent nonlinear ideal MHD equations. Once a prominence model is obtained, we investigate the properties of MHD waves superimposed on the structure. We concentrate on motions purely two-dimensional neglecting propagation in the ignorable direction. We demonstrate how by using different numerical tools we can determine the period of oscillation of stable waves. We find that vertical oscillations, linked to fast MHD waves, are always stable and have periods in the 4-10 min range. Longitudinal oscillations, related to slow magnetoacoustic-gravity waves, have longer periods in the range of 28-40 min. These longitudinal oscillations are strongly influenced by the gravity force and become unstable for short magnetic arcades.
    The Astrophysical Journal 09/2013; 778(1). · 6.73 Impact Factor
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    ABSTRACT: Magnetohydrodynamic (MHD) waves are ubiquitously observed in the solar atmosphere. Kink waves are a type of transverse MHD waves in magnetic flux tubes that are damped due to resonant absorption. The theoretical study of kink MHD waves in solar flux tubes is usually based on the simplification that the transverse variation of density is confined to a nonuniform layer much thinner than the radius of the tube, i.e., the so-called thin boundary approximation. Here, we develop a general analytic method to compute the dispersion relation and the eigenfunctions of ideal MHD waves in pressureless flux tubes with transversely nonuniform layers of arbitrary thickness. Results for kink waves are produced and are compared with fully numerical resistive MHD eigenvalue computations in the limit of small resistivity. We find that the frequency and resonant damping rate are the same in both ideal and resistive cases. The actual results for thick nonuniform layers deviate from the behavior predicted in the thin boundary approximation and strongly depend on the shape of the nonuniform layer. The eigenfunctions in ideal MHD are very different from those in resistive MHD. The ideal eigenfunctions display a global character regardless of the thickness of the nonuniform layer, while the resistive eigenfunctions are localized around the resonance and are indistinguishable from those of ordinary resistive Alfv\'en modes. Consequently, the spatial distribution of wave energy in the ideal and resistive cases is dramatically different. This poses a fundamental theoretical problem with clear observational consequences.
    The Astrophysical Journal 09/2013; 777(2). · 6.73 Impact Factor
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    ABSTRACT: Magnetohydrodynamic seismology aims to determine difficult to measure physical parameters in the solar corona by a combination of observed and theoretical properties of waves and oscillations. We describe relevant examples of the application of seismology techniques to transversely oscillating coronal loops and prominence fine structures. We also show how the use of statistical techniques, based on Bayesian inference, can be of high value in the determination of physical parameters in these structures, by consistently taking into account the information from observations.
    05/2012;
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    ABSTRACT: We investigate the excitation of magnetoacoustic-gravity waves generated from localized pulses in the gas pressure as well as in vertical component of velocity. These pulses are initially launched at the top of the solar photosphere that is permeated by a weak magnetic field. We investigate three different configurations of the background magnetic field lines: horizontal, vertical and oblique to the gravitational force. We numerically model magnetoacoustic-gravity waves by implementing a realistic (VAL-C) model of solar temperature. We solve two-dimensional ideal magnetohydrodynamic equations numerically with the use of the FLASH code to simulate the dynamics of the lower solar atmosphere. The initial pulses result in shocks at higher altitudes. Our numerical simulations reveal that a small-amplitude initial pulse can produce magnetoacoustic-gravity waves, which are later reflected from the transition region due to the large temperature gradient. The atmospheric cavities in the lower solar atmosphere are found to be the ideal places that may act as a resonator for various oscillations, including their trapping and leakage into the higher atmosphere. Our numerical simulations successfully model the excitation of such wave modes, their reflection and trapping, as well as the associated plasma dynamics.
    Solar Physics 04/2012; 283(2):383. · 3.26 Impact Factor
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    ABSTRACT: Prominences are intriguing, but poorly understood, magnetic structures of the solar corona. The dynamics of solar prominences has been the subject of a large number of studies, and of particular interest is the study of prominence oscillations. Ground- and space-based observations have confirmed the presence of oscillatory motions in prominences and they have been interpreted in terms of magnetohydrodynamic (MHD) waves. This interpretation opens the door to perform prominence seismology, whose main aim is to determine physical parameters in magnetic and plasma structures (prominences) that are difficult to measure by direct means. Here, we review the observational information gathered about prominence oscillations as well as the theoretical models developed to interpret small amplitude oscillations and their temporal and spatial attenuation. Finally, several prominence seismology applications are presented.
    Living Reviews in Solar Physics 04/2012; · 13.83 Impact Factor
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    ABSTRACT: Many solar filaments and prominences show short-lived horizontal threads lying parallel to the photosphere. In this work the possible link between Rayleigh-Taylor instabilities and thread lifetimes is investigated. This is done by calculating the eigenmodes of a thread modelled as a Cartesian slab under the presence of gravity. An analytical dispersion relation is derived using the incompressible assumption for the magnetohydrodynamic (MHD) perturbations. The system allows a mode that is always stable, independently of the value of the Alfv\'en speed in the thread. The character of this mode varies from being localised at the upper interface of the slab when the magnetic field is weak, to having a global nature and resembling the transverse kink mode when the magnetic field is strong. On the contrary, the slab model permits another mode that is unstable and localised at the lower interface when the magnetic field is weak. The growth rates of this mode can be very short, of the order of minutes for typical thread conditions. This Rayleigh-Taylor unstable mode becomes stable when the magnetic field is increased, and in the limit of strong magnetic field it is essentially a sausage magnetic mode. The gravity force might have a strong effect on the modes of oscillation of threads, depending on the value of the Alfv\'en speed. In the case of threads in quiescent filaments, where the Alfv\'en speed is presumably low, very short lifetimes are expected according to the slab model. In active region prominences, the stabilising effect of the magnetic tension might be enough to suppress the Rayleigh-Taylor instability for a wide range of wavelengths.
    Astronomy and Astrophysics 03/2012; · 5.08 Impact Factor
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    ABSTRACT: Given the difficulty in directly determining prominence physical parameters from observations, prominence seismology stands as an alternative method to probe the nature of these structures. We show recent examples of the application of magnetohydrodynamic (MHD) seismology techniques to infer physical parameters in prominence plasmas. They are based on the application of inversion techniques using observed periods, damping times, and plasma flow speeds of prominence thread oscillations. The contribution of Hinode to the subject has been of central importance. We show an example based on data obtained with Hinode's Solar Optical Telescope. Observations show an active region limb prominence, composed by a myriad of thin horizontal threads that flow following a path parallel to the photosphere and display synchronous vertical oscillations. The coexistence of waves and flows can be firmly established. By making use of an interpretation based on transverse MHD kink oscillations, a seismological analysis of this event is performed. It is shown that the combination of high quality Hinode observations and proper theoretical models allows flows and waves to become two useful characteristics for our understanding of the nature of solar prominences.
    01/2012;
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    ABSTRACT: We investigate the temporal evolution of impulsively generated perturbations in a potential coronal arcade with an embedded loop. As the initial configuration we consider a coronal loop, represented by a density enhancement, which is unbounded in the ignorable direction of the arcade. The linearized time-dependent magnetohydrodynamic equations have been numerically solved in field-aligned coordinates and the time evolution of the initial perturbations has been studied in the zero-beta approximation. For propagation constrained to the plane of the arcade, the considered initial perturbations do not excite trapped modes of the system. This weakness of the model is overcome by the inclusion of wave propagation in the ignorable direction. The inclusion of perpendicular propagation produces two main results. First, damping by wave leakage is less efficient because the loop is able to act as a wave trap of vertical oscillations. Second, the consideration of an inhomogeneous corona enables the resonant damping of vertical oscillations and the energy transfer from the interior of the loop to the external coronal medium.
    The Astrophysical Journal 01/2012; 763(1). · 6.73 Impact Factor
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    ABSTRACT: We aim to study large-scale shallow water waves in the tachoclines of rapidly rotating stars and their connection to the periodicity and the formation of starspots at high latitudes. Shallow water magnetohydrodynamic equations are used to study the dynamics of largescale waves at the rapidly rotating stellar tachoclines in the presence of toroidal magnetic field. We found that low frequency magnetic Rossby waves tend to locate at poles, but high frequency magnetic Poincaré waves are concentrated near the equator in rapidly rotating stars. Unstable magnetic Rossby waves may lead to the local enhancement of magnetic flux at high latitudes of tachoclines in rapidly rotating stars. The enhanced magnetic flux may rise upwards due to the magnetic buoyancy in the form of tubes and appear as starspots at polar regions. Magnetic Rossby waves may also cause observed short term periodicity in the stellar magnetic activity. These results have important implications for the evolution of the stellar wind and exoplanets in young Sun-like stars.
    10/2011;
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    ABSTRACT: Rapidly rotating stars show short-period oscillations in magnetic activity and polar appearance of starspots. The aim of this paper is to study large-scale shallow water waves in the tachoclines of rapidly rotating stars and their connection to the periodicity and the formation of starspots at high latitudes. Shallow-water magnetohydrodynamic equations were used to study the dynamics of large-scale waves at the rapidly rotating stellar tachoclines in the presence of toroidal magnetic field. Dispersion relations and latitudinal distribution of wave modes were derived. We found that low-frequency magnetic Rossby waves tend to be located at poles, but high-frequency magnetic Poincare waves are concentrated near the equator in rapidly rotating stars. These results have important implications for the evolution of the stellar wind in young Sun-like stars. Unstable magnetic Rossby waves may lead to the local enhancement of magnetic flux at high latitudes of tachoclines in rapidly rotating stars. The enhanced magnetic flux may rise upwards owing to the magnetic buoyancy in the form of tubes and appear as starspots at polar regions. Magnetic Rossby waves may also cause observed short-term periodicity in the stellar magnetic activity.
    Astronomy and Astrophysics 07/2011; 532. · 5.08 Impact Factor
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    ABSTRACT: Aims: We study the equilibrium and stability of twisted magnetic flux tubes with mass flows along the field lines. Then, we focus on the stability and oscillatory modes of magnetic tubes with uniform twist B0 = B0(r/p evarphi + ez) in a zero-beta plasma, surrounded by a uniform, purely longitudinal field. Methods: First we investigate the possible equilibriums, and then consider the linearised MHD equations and obtain a system of two first-order differential equations. These are solved numerically, while analytical approximations involving confluent hypergeometric functions are found in the thin tube limit. Finally, new appropriate boundary conditions are deduced and the outer solution considered (with the apparition of cut-off frequencies). We use this to derive a dispersion relation, from which the frequencies of the normal modes can be obtained. Results: Regarding the equilibrium, the only value of the flow that satisfies the equations for this magnetic field configuration is a super-Alfvénic one. Then, we consider the normal modes of this configuration. The thin-tube approximation proves accurate for typical values, and it is used to prove that the equilibrium is unstable, unless the pitch is large. The stability criteria for twisted tubes are significantly lowered. Conclusions: The twisted tube is subject to the kink instability unless the pitch is very high, since the Lundquist criterion is significantly lowered. This is caused by the requirement of having a magnetic Mach number greater than 1, so the magnetic pressure balances the magnetic tension and fluid inertia. This type of instability might be observed in some solar atmospheric structures, like surges. Appendix is available in electronic form at http://www.aanda.org
    Astronomy and Astrophysics 01/2011; 533. · 5.08 Impact Factor
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    ABSTRACT: Prominence threads are expected to be cold plasma condensations in a long magnetic tube. Because of this density inhomogeneity along the magnetic field, the ratio of the fundamental transverse mode period to twice that of its first overtone, P{sub 1}/2P{sub 2}, must differ from 1. We investigate the dependence of this ratio on the equilibrium parameters of prominence threads and its possible use as a diagnostic tool for prominence seismology. Using the low-beta plasma approximation, we follow the procedure of previous works to obtain the frequencies and spatial distribution of the modes. We also check the thin tube approximation and find it reasonably accurate. The period ratio P{sub 1}/2P{sub 2} is found to be greater than unity, in contrast with coronal loops, for which the effect of inhomogeneities is to make this ratio smaller than 1. The ratio is very sensitive to the thread length, while the dependence on other parameters is less important for threads than for coronal loops. Hence, the period ratio can be used to obtain an estimation of the length of the supporting magnetic tube, since the thread length is known from observations. The obtained value of the tube length does not depend on other parameters, so their potential for prominence seismology may be great.
    The Astrophysical Journal 12/2010; 725(2). · 6.73 Impact Factor
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    ABSTRACT: Quasi-biennial oscillations (QBO) are frequently observed in the solar activity indices. However, no clear physical mechanism for the observed variations has been suggested so far. Here we study the stability of magnetic Rossby waves in the solar tachocline using the shallow water magnetohydrodynamic approximation. Our analysis shows that the combination of typical differential rotation and a toroidal magnetic field with a strength > 10^5 G triggers the instability of the m=1 magnetic Rossby wave harmonic with a period of 2 years. This harmonic is antisymmetric with respect to the equator and its period (and growth rate) depends on the differential rotation parameters and the magnetic field strength. The oscillations may cause a periodic magnetic flux emergence at the solar surface and consequently may lead to the observed QBO in the solar activity features. The period of QBO may change throughout the cycle, and from cycle to cycle, due to variations of the mean magnetic field and differential rotation in the tachocline. Comment: 13 pages, 4 figures, ApJ (in press)
    The Astrophysical Journal Letters 11/2010; · 6.35 Impact Factor
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    ABSTRACT: 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 damping mechanisms considered. 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 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 minutes) and short-wavelength (700-8000 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., Alfvén 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.
    The Astrophysical Journal 10/2010; 722(2):1778. · 6.73 Impact Factor

Publication Stats

1k Citations
565.41 Total Impact Points

Institutions

  • 1970–2014
    • University of the Balearic Islands
      • Department of Physics
      Palma, Balearic Islands, Spain
  • 2007–2008
    • University of Leuven
      • Section of Plasma-Astrophysics
      Louvain, Flanders, Belgium
  • 1992–1993
    • University of St Andrews
      • School of Mathematics and Statistics
      Saint Andrews, Scotland, United Kingdom