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Dynamics of coronal rain and descending plasma blobs in solar  

Ramon Oliver   Submitted: 2015-10-28 06:12

Coronal rain clumps and prominence knots are dense condensations with chromospheric to transition region temperatures that fall down in the much hotter corona. Their typical speeds are in the range 30-150 km s-1 and of the order of 10-30 km s-1, respectively, i.e., they are considerably smaller than free fall velocities. These cold blobs contain a mixture of ionized and neutral material that must be dynamically coupled in order to fall together, as observed. We investigate this coupling by means of hydrodynamic simulations in which the coupling arises from the friction between ions and neutrals. The numerical simulations presented here are an extension of those of oliver2014} to the partially ionized case. We find that, although the relative drift speed between the two species is smaller than 1~m s{-1 at the blob center, it is sufficient to produce the forces required to strongly couple charged particles and neutrals. The ionization degree has no discernible effect on the main results of our previous work for a fully ionized plasma: the condensation has an initial acceleration phase followed by a period with roughly constant velocity and, in addition, the maximum descending speed is clearly correlated with the ratio of initial blob to environment density.

Authors: Oliver, R., Soler, R., Terradas, J., Zaqarashvili, T. V.
Projects: None

Publication Status: Submitted
Last Modified: 2015-10-28 15:23
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Propagation and dispersion of sausage wave trains in magnetic flux tubes  

Ramon Oliver   Submitted: 2015-02-04 14:06

A localized perturbation of a magnetic flux tube produces a pair of wave trains that propagate in opposite directions along the tube. These wave packets disperse as they propagate, where the extent of dispersion depends on the physical properties of the magnetic structure, on the length of the initial excitation, and on its nature (e.g., transverse or axisymmetric). In Oliver et al. (2014) we considered a transverse initial perturbation, whereas the temporal evolution of an axisymmetric one is examined here. In both papers we use a method based on Fourier integrals to solve the initial value problem. Previous studies on wave propagation in magnetic wave guides have emphasized that the wave train dispersion is influenced by the particular dependence of the group velocity on the longitudinal wavenumber. Here we also find that long initial perturbations result in low amplitude wave packets and that large values of the magnetic tube to environment density ratio yield longer wave trains. To test the detectability of propagating transverse or axisymmetric wave packets in magnetic tubes of the solar atmosphere (e.g., coronal loops, spicules, or prominence threads) a forward modelling of the perturbations must be carried out. This is left for a future work.

Authors: R. Oliver, M. S. Ruderman, J. Terradas
Projects: None

Publication Status: Submitted
Last Modified: 2015-02-06 11:50
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Propagation and dispersion of transverse wave trains in magnetic flux tubes  

Ramon Oliver   Submitted: 2014-02-18 03:57

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-1 and are accompanied by maximum density variations of the order of 10-15% the unperturbed loop density.

Authors: R. Oliver, M. S. Ruderman, J. Terradas
Projects: None

Publication Status: Submitted
Last Modified: 2014-02-19 09:16
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Dynamics of coronal rain and descending plasma blobs in solar prominences: I. Fully ionised case  

Ramon Oliver   Submitted: 2013-11-04 10:31

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.

Authors: R. Oliver, R. Soler, J. Terradas, T. V. Zaqarashvili, M. L. Khodachenko
Projects: None

Publication Status: ApJ, 784, 21
Last Modified: 2014-02-28 12:58
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Dynamics of coronal rain and descending plasma blobs in solar prominences: I. Fully ionised case  

Ramon Oliver   Submitted: 2013-11-04 10:31

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.

Authors: R. Oliver, R. Soler, J. Terradas, T. V. Zaqarashvili, M. L. Khodachenko
Projects:

Publication Status: ApJ, 784, 21
Last Modified: 2014-03-04 16:08
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Numerical simulations of Magnetoacoustic-Gravity Waves in the Solar Atmosphere  

Ramon Oliver   Submitted: 2012-08-19 10:14

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.

Authors: K. Murawski, A. K. Srivastava, J. A. McLaughlin, R. Oliver
Projects: None

Publication Status: Solar Physics (submitted)
Last Modified: 2012-08-21 19:06
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Prominence seismology using small amplitude oscillations  

Ramon Oliver   Submitted: 2009-02-25 05:57

Quiescent prominences are thin slabs of cold, dense plasma embedded in the much hotter and rarer solar corona. Although their global shape is rather irregular, they are often characterised by an internal structure consisting of a large number of thin, parallel threads piled together. Prominences often display periodic disturbances mostly observed in the Doppler displacement of spectral lines and with an amplitude typically of the order of or smaller than 2-3 km s-1, a value which seems to be much smaller than the characteristic speeds of the prominence plasma (namely the Alfvén and sound velocities). Two particular features of these small amplitude prominence oscillations is that they seem to damp in a few periods and that they seem not to affect the whole prominence structure. In addition, in high spatial resolution observations, in which threads can be discerned, small amplitude oscillations appear to be clearly associated to these fine structure constituents. Prominence seismology tries to bring together the results from these observations (e.g. periods, wavelengths, damping times) and their theoretical modeling (by means of the magnetohydrodynamic theory) to gain insight into physical properties of prominences that cannot be derived from direct observation. In this paper we discuss works that have not been described in previous reviews, namely the first seismological application to solar prominences and theoretical advances on the attenuation of prominence oscillations.

Authors: R. Oliver
Projects: None

Publication Status: submitted
Last Modified: 2009-02-25 07:57
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Global shallow water magnetohydrodynamic waves in the solar tachocline  

Ramon Oliver   Submitted: 2008-10-24 01:40

We derive analytical solutions and dispersion relations of global magnetic Poincar'e (magneto-gravity) and magnetic Rossby waves in the approximation of shallow water magnetohydrodynamics. The solutions are obtained in a rotating spherical coordinate system for strongly and weakly stable stratification separately in the presence of toroidal magnetic field. In both cases magnetic Rossby waves split into fast and slow magnetic Rossby modes. In the case of strongly stable stratification (valid in the radiative part of the tachocline) all waves are slightly affected by the layer thickness and the toroidal magnetic field, while in the case of weakly stable stratification (valid in the upper overshoot layer of the tachocline) magnetic Poincar'e and fast magnetic Rossby waves are found to be concentrated near the solar equator, leading to equatorially trapped waves. However, slow magnetic Rossby waves tend to concentrate near the poles, leading to polar trapped waves. The frequencies of all waves are smaller in the upper weakly stable stratification region than in the lower strongly stable stratification one.

Authors: T.V. Zaqarashvili, R. Oliver, J.L. Ballester
Projects: None

Publication Status: ApJL (submitted)
Last Modified: 2008-10-24 06:47
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Global shallow water magnetohydrodynamic waves in the solar tachocline  

Ramon Oliver   Submitted: 2008-10-24 01:40

We derive analytical solutions and dispersion relations of global magnetic Poincar'e (magneto-gravity) and magnetic Rossby waves in the approximation of shallow water magnetohydrodynamics. The solutions are obtained in a rotating spherical coordinate system for strongly and weakly stable stratification separately in the presence of toroidal magnetic field. In both cases magnetic Rossby waves split into fast and slow magnetic Rossby modes. In the case of strongly stable stratification (valid in the radiative part of the tachocline) all waves are slightly affected by the layer thickness and the toroidal magnetic field, while in the case of weakly stable stratification (valid in the upper overshoot layer of the tachocline) magnetic Poincar'e and magnetic Rossby waves are found to be concentrated near the solar equator, leading to equatorially trapped waves. The frequencies of all waves are smaller in the upper weakly stable stratification region than in the lower strongly stable stratification one.

Authors: T.V. Zaqarashvili, R. Oliver, J.L. Ballester
Projects:

Publication Status: ApJL (in press)
Last Modified: 2008-12-09 19:00
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Subject will be restored when possible  

Ramon Oliver   Submitted: 2008-03-19 02:48

Recent observations with the Hinode Solar Optical Telescope display an active region prominence whose fine threads oscillate in the vertical direction as they move along a path parallel to the photosphere. A seismological analysis of this event is carried out by taking advantage of the small radius of these structures compared to the total length of magnetic field lines, i.e. by using the thin tube approximation. This analysis reveals that the oscillatory period is only slightly modified by the existence of the flow and that the difference between the period of a flowing thread and a static one is below the error bars of these observations. Moreover, although it is not possible to obtain values of the physical parameters, a lower bound for the Alfvén speed (ranging between 120 km s-1 and 350 km s-1) is obtained for each of the threads. Such Alfvén speeds agree with the intense magnetic fields and large densities usually found in active region prominences.

Authors: J. Terradas, I. Arregui, R. Oliver, J. L. Ballester
Projects: Hinode/SOT

Publication Status: ApJ (in press)
Last Modified: 2008-09-23 21:13
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Transverse oscillations of two coronal loops  

Ramon Oliver   Submitted: 2007-07-06 03:57

We study transverse fast magnetohydrodynamic waves in a system of two coronal loops modeled as smoothed, dense plasma cylinders in a uniform magnetic field. The collective oscillatory properties of the system due to the interaction between the individual loops are investigated from two points of view. Firstly, the frequency and spatial structure of the normal modes are studied. The system supports four trapped normal modes in which the loops move rigidly in the transverse direction. The direction of the motions is either parallel or perpendicular to the plane containing the axes of the loops. Two of these modes correspond to oscillations of the loops in phase, while in the other two they move in antiphase. Thus, these solutions are the generalization of the kink mode of a single cylinder to the double cylinder case. Secondly, we analyze the time-dependent problem of the excitation of the pair of tubes. We find that depending on the shape and location of the initial disturbance, different normal modes can be excited. The frequencies of normal modes are accurately recovered from the numerical simulations. In some cases, because of the simultaneous excitation of several eigenmodes, the system shows beating and the phase lag between the loops is pi/2.

Authors: M. Luna, J. Terradas, R. Oliver, J.L. Ballester
Projects: None

Publication Status: Submitted
Last Modified: 2007-07-06 12:01
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Rossby waves in ''shallow water'' magnetohydrodynamics  

Ramon Oliver   Submitted: 2007-03-07 01:58

The influence of a toroidal magnetic field on the dynamics of Rossby waves in a thin layer of ideal conductive fluid on a rotating sphere is studied in the ''shallow water'' magnetohydrodynamic approximation for the first time. Dispersion relations for magnetic Rossby waves are derived analytically in Cartesian and spherical coordinates. It is shown that the magnetic field causes the splitting of low order (long wavelength) Rossby waves into two different modes, here denoted fast and slow {em magnetic Rossby waves}. The high frequency mode (the fast magnetic Rossby mode) corresponds to an ordinary hydrodynamic Rossby wave slightly modified by the magnetic field, while the low frequency mode (the slow magnetic Rossby mode) has new and interesting properties since its frequency is significantly smaller than that of the same harmonics of pure Rossby and Alfvén waves.

Authors: Zaqarashvili, T. V., Oliver, R., Ballester, J. L., Shergelashvili, B. M.
Projects: None

Publication Status: A&A (submitted)
Last Modified: 2007-03-07 09:45
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Rossby waves in ''shallow water'' magnetohydrodynamics  

Ramon Oliver   Submitted: 2007-03-07 01:58

The influence of a toroidal magnetic field on the dynamics of Rossby waves in a thin layer of ideal conductive fluid on a rotating sphere is studied in the ''shallow water'' magnetohydrodynamic approximation for the first time. Dispersion relations for magnetic Rossby waves are derived analytically in Cartesian and spherical coordinates. It is shown that the magnetic field causes the splitting of low order (long wavelength) Rossby waves into two different modes, here denoted fast and slow {em magnetic Rossby waves}. The high frequency mode (the fast magnetic Rossby mode) corresponds to an ordinary hydrodynamic Rossby wave slightly modified by the magnetic field, while the low frequency mode (the slow magnetic Rossby mode) has new and interesting properties since its frequency is significantly smaller than that of the same harmonics of pure Rossby and Alfvén waves.

Authors: Zaqarashvili, T. V., Oliver, R., Ballester, J. L., Shergelashvili, B. M.
Projects:

Publication Status: A&A (in press)
Last Modified: 2007-06-13 02:52
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Abstracts by Author
Dynamics of coronal rain and descending plasma blobs in solar
Propagation and dispersion of sausage wave trains in magnetic flux tubes
Propagation and dispersion of transverse wave trains in magnetic flux tubes
Dynamics of coronal rain and descending plasma blobs in solar prominences: I. Fully ionised case
Dynamics of coronal rain and descending plasma blobs in solar prominences: I. Fully ionised case
Numerical simulations of Magnetoacoustic-Gravity Waves in the Solar Atmosphere
Prominence seismology using small amplitude oscillations
Global shallow water magnetohydrodynamic waves in the solar tachocline
Global shallow water magnetohydrodynamic waves in the solar tachocline
Subject will be restored when possible
Transverse oscillations of two coronal loops
Rossby waves in ''shallow water'' magnetohydrodynamics
Rossby waves in ''shallow water'' magnetohydrodynamics

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