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Large-Amplitude Longitudinal Oscillations Triggered by the Merging of Two Solar Filaments: Observations and Magnetic Field Analysis  

Manuel Luna   Submitted: 2017-11-06 03:48

We follow the eruption of two related intermediate filaments observed in Hα (from GONG) and in EUV (from SDO/AIA) and the resulting large-amplitude longitudinal oscillations of the plasma in the filament channels. The events occurred in and around the decayed active region AR12486 on 2016 January 26. Our detailed study of the oscillation reveals that the periods of the oscillations are about one hour. In Hα the period decreases with time and exhibits strong damping. The analysis of 171A images shows that the oscillation has two phases, an initial long period phase and a subsequent oscillation with a shorter period. In this wavelength the damping appears weaker than in Hα. The velocity is the largest ever detected in a prominence oscillation, approximately 100 km s-1. Using SDO/HMI magnetograms we reconstruct the magnetic field of the filaments modeled as flux ropes by using a flux-rope insertion method. Applying seismological techniques we determine that the radii of curvature of the field lines in which cool plasma is condensed are in the range 75-120 Mm, in agreement with the reconstructed field. In addition, we infer a field strength of ≥7 to 30 Gauss, depending on the electron density assumed; that is also in agreement with the values from the reconstruction (8-20 Gauss). The poloidal flux is zero and the axis flux is of the order of 1020 to 1021 Mx, confirming the high shear existing even in a non-active filament.

Authors: M. Luna, Y. Su, B. Schmieder, R. Chandra, T. A. Kucera
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2017-11-06 11:34
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The effects of magnetic-field geometry on longitudinal oscillations of solar prominences: Cross-sectional area variation for thin tubes  

Manuel Luna   Submitted: 2016-08-01 04:17

Solar prominences are subject to both field-aligned (longitudinal) and transverse oscillatory motions, as evidenced by an increasing number of observations. Large-amplitude longitudinal motions provide valuable information on the geometry of the filament-channel magnetic structure that supports the cool prominence plasma against gravity. Our pendulum model, in which the restoring force is the gravity projected along the dipped field lines of the magnetic structure, best explains these oscillations. However, several factors can influence the longitudinal oscillations, potentially invalidating the pendulum model. The aim of this work is to study the influence of large-scale variations in the magnetic field strength along the field lines, i.e., variations of the cross-sectional area along the flux tubes supporting prominence threads. We studied the normal modes of several flux tube configurations, using linear perturbation analysis, to assess the influence of different geometrical parameters on the oscillation properties. We found that the influence of the symmetric and asymmetric expansion factors on longitudinal oscillations is small.}{We conclude that the longitudinal oscillations are not significantly influenced by variations of the cross-section of the flux tubes, validating the pendulum model in this context.

Authors: Luna, M.; Diaz, A. J.; Oliver, R.; Terradas, J.; Karpen, J.
Projects: None

Publication Status: Accepted for publication in Astronomy and Astrophysics
Last Modified: 2016-08-02 13:35
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Damping of prominence longitudinal oscillations due to mass accretion  

Manuel Luna   Submitted: 2016-05-11 04:46

We study the damping of longitudinal oscillations of a prominence thread caused by the mass accretion. We suggested a simple model describing this phenomenon. In this model we considered a thin curved magnetic tube filled with the plasma. The prominence thread is in the central part of the tube and it consists of dense cold plasma. The parts of the tube at the two sides of the thread are filled with hot rarefied plasma. We assume that there are flows of rarefied plasma toward the thread caused by the plasma evaporation at the magnetic tube footpoints. Our main assumption is that the hot plasma is instantaneously accommodated by the thread when it arrives at the thread, and its temperature and density become equal to those of the thread. Then we derive the system of ordinary differential equations describing the thread dynamics.
We solve this system of ordinary differential equations in two particular cases. In the first case we assume that the magnetic tube is composed of an arc of a circle with two straight lines attached to its ends such that the whole curve is smooth. A very important property of this model is that the equations describing the thread oscillations are linear for any oscillation amplitude. We obtain the analytical solution of the governing equations. Then we obtain the analytical expressions for the oscillation damping time and periods. We find that the damping time is inversely proportional to the accretion rate. The oscillation periods increase with time. We conclude that the oscillations can damp in a few periods if the inclination angle is sufficiently small, not larger that 10°, and the flow speed is sufficiently large, not less that 30 km s-1.
In the second model we consider the tube with the shape of an arc of a circle. The thread oscillates with the pendulum frequency dependent exclusively of the radius of curvature of the arc. The damping depends on the mass accretion rate and the initial mass of the threads, that is the mass of the thread at the moment when it is perturbed. First we consider small amplitude oscillations and use the linear description. Then we consider nonlinear oscillations and assume that the damping is slow, meaning that the damping time is much larger that the characteristic oscillation time. The thread oscillations are described by the solution of the nonlinear pendulum problem with slowly varying amplitude. The nonlinearity reduces the damping time, however this reduction is small. Again the damping time is inversely proportional to the accretion rate. We also obtain that the oscillation periods decrease with time. However even for the largest initial oscillation amplitude considered in our article the period reduction does not exceed 20%. We conclude that the mass accretion can damp the motion of the threads rapidly. Thus, this mechanism can explain the observed strong damping of large-amplitude longitudinal oscillations. In addition, the damping time can be used to determine the mass accretion rate and indirectly the coronal heating.

Authors: Michael S. Ruderman and Manuel Luna
Projects: None

Publication Status: Accepted for publication in Astronomy and Astrophysics
Last Modified: 2016-05-11 08:53
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On the robustness of the pendulum model for large-amplitude longitudinal oscillations in prominences  

Manuel Luna   Submitted: 2015-12-20 01:47

Large-amplitude longitudinal oscillations (LALOs) in prominences are spectacular manifestations of the solar activity. In such events nearby energetic disturbances induce periodic motions on filaments with displacements comparable to the size of the filaments themselves and with velocities larger than 20 km s-1. The pendulum model, in which the gravity projected along a rigid magnetic field is the restoring force, was proposed to explain these events. However, it can be objected that in a realistic situation where the magnetic field reacts to the mass motion of the heavy prominence, the simplified pendulum model could be no longer valid. We have performed non-linear time-dependent numerical simulations of LALOs considering a dipped magnetic field line structure. In this work we demonstrate that for even relatively weak magnetic fields the pendulum model works very well. We therefore validate the pendulum model and show its robustness, with important implications for prominence seismology purposes. With this model it is possible to infer the geometry of the dipped field lines that support the prominence.

Authors: Luna, M.; Terradas, J.; Khomenko, E.; Collados, M.; de Vicente, A.
Projects: None

Publication Status: Accepted for publication in The Astrophysical Journal
Last Modified: 2015-12-20 12:40
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Are tornado-like magnetic structures able to support solar prominence plasma?  

Manuel Luna   Submitted: 2015-07-02 01:15

Recent high-resolution and high-cadence observations have surprisingly suggested that prominence barbs exhibit apparent rotating motions suggestive of a tornado-like structure. Additional evidence has been provided by Doppler measurements. The observations reveal opposite velocities for both hot and cool plasma on the two sides of a prominence barb. This motion is persistent for several hours and has been interpreted in terms of rotational motion of prominence feet. Several authors suggest that such barb motions are rotating helical structures around a vertical axis similar to tornadoes on Earth. One of the difficulties of such a proposal is how to support cool prominence plasma in almost-vertical structures against gravity. In this work we model analytically a tornado-like structure and try to determine possible mechanisms to support the prominence plasma. We have found that the Lorentz force can indeed support the barb plasma provided the magnetic structure is sufficiently twisted and/or significant poloidal flows are present.

Authors: M. Luna, F. Moreno-Insertis & E. Priest
Projects: None

Publication Status: Accepted for publication in ApJL
Last Modified: 2015-07-04 02:30
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Observations and Implications of Large-Amplitude Longitudinal Oscillations in a Solar Filament  

Manuel Luna   Submitted: 2014-03-05 05:15

On 20 August 2010 an energetic disturbance triggered large-amplitude longitudinal oscillations in a nearby filament. The triggering mechanism appears to be episodic jets connecting the energetic event with the filament threads. In the present work we analyze this periodic motion in a large fraction of the filament to characterize the underlying physics of the oscillation as well as the filament properties. The results support our previous theoretical conclusions that the restoring force of large-amplitude longitudinal oscillations is solar gravity, and the damping mechanism is the ongoing accumulation of mass onto the oscillating threads. Based on our previous work, we used the fitted parameters to determine the magnitude and radius of curvature of the dipped magnetic field along the filament, as well as the mass accretion rate onto the filament threads. These derived properties are nearly uniform along the filament, indicating a remarkable degree of cohesiveness throughout the filament channel. Moreover, the estimated mass accretion rate implies that the footpoint heating responsible for the thread formation, according to the thermal nonequilibrium model, agrees with previous coronal heating estimates. We estimate the magnitude of the energy released in the nearby event by studying the dynamic response of the filament threads, and discuss the implications of our study for filament structure and heating.

Authors: M. Luna, K. Knizhnik, K. Muglach, J. Karpen, H. Gilbert, T.A. Kucera, V. Uritsky
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2014-03-05 12:26
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Propagating Waves Transverse to the Magnetic Field in a Solar Prominence  

Manuel Luna   Submitted: 2013-09-09 04:06

We report an unusual set of observations of waves in a large prominence pillar which consist of pulses propagating perpendicular to the prominence magnetic field. We observe a huge quiescent prominence with the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) in EUV on 2012 October 10 and only a part of it, the pillar, which is a foot or barb of the prominence, with the Hinode Solar Optical Telescope (SOT) (in Ca II and Hα lines), Sac Peak (in Hα, Hβ and Na-D3 lines), THEMIS (?T"lescope H"liographique pour l? Etude du Magn?tisme et des Instabilit"s Solaires") with the MTR (MulTi-Raies) spectropolarimeter (in He D3 line). The THEMIS/MTR data indicates that the magnetic field in the pillar is essentially horizontal and the observations in the optical domain show a large number of horizontally aligned features on a much smaller scale than the pillar as a whole. The data is consistent with a model of cool prominence plasma trapped in the dips of horizontal field lines. The SOT and Sac Peak data over the 4 hour observing period show vertical oscillations appearing as wave pulses. These pulses, which include a Doppler signature, move vertically, perpendicular to the field direction, along thin quasi-vertical columns in the much broader pillar. The pulses have a velocity of propagation of about 10 km s-1, a period about 300 sec, and a wavelength around 2000 km. We interpret these waves in terms of fast magneto-sonic waves and discuss possible wave drivers.

Authors: B. Schmieder, T.A. Kucera, K. Knizhnik, M. Luna, A. Lopez-Ariste, and D.Toot
Projects: Hinode/SOT,SDO-AIA,THEMIS/MTR

Publication Status: Accepted for publication in The Astrophysical Journal
Last Modified: 2013-09-09 10:12
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The effects of magnetic-field geometry on longitudinal oscillations of solar prominences  

Manuel Luna   Submitted: 2012-07-27 02:54

We investigate the influence of the geometry of the solar filament magnetic structure on the large-amplitude longitudinal oscillations. A representative filament flux tube is modeled as composed of a cool thread centered in a dipped part with hot coronal regions on either side. We have found the normal modes of the system, and establish that the observed longitudinal oscillations are well described with the fundamental mode. For small and intermediate curvature radii and moderate to large density contrast between the prominence and the corona, the main restoring force is the solar gravity. In this full wave description of the oscillation a simple expression for the oscillation frequencies is derived in which the pressure-driven term introduces a small correction. We have also found that the normal modes are almost independent of the geometry of the hot regions of the tube. We conclude that observed large-amplitude longitudinal oscillations are driven by the projected gravity along the flux tubes, and are strongly influenced by the curvature of the dips of the magnetic field in which the threads reside.

Authors: M. Luna, A. J. D?az, J. Karpen
Projects: None

Publication Status: ApJ accepted
Last Modified: 2012-07-27 08:28
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Large amplitude longitudinal oscillations in a solar filament  

Manuel Luna   Submitted: 2012-03-26 08:51

We have developed the first self-consistent model for the observed large-amplitude oscillations along filament axes that explains the restoring force and damping mechanism. We have investigated the oscillations of multiple threads formed in long, dipped flux tubes through the thermal nonequilibrium process, and found that the oscillation properties predicted by our simulations agree with the observed behavior. We then constructed a model for the large-amplitude longitudinal oscillations that demonstrates that the restoring force is the projected gravity in the tube where the threads oscillate. Although the period is independent of the tube length and the constantly growing mass, the motions are strongly damped by the steady accretion of mass onto the threads by thermal nonequilibrium. The observations and our model suggest that a nearby impulsive event drives the existing prominence threads along their supporting tubes, away from the heating deposition site, without destroying them. The subsequent oscillations occur because the displaced threads reside in magnetic concavities with large radii of curvature. Our model yields a powerful seismological method for constraining the coronal magnetic field and radius of curvature of dips. Furthermore, these results indicate that the magnetic structure is most consistent with the sheared-arcade model for filament channels.

Authors: Manuel Luna, Judith Karpen
Projects:

Publication Status: ApJ (2012), 750, L1
Last Modified: 2012-04-10 11:28
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Formation and evolution of a multi-threaded prominence  

Manuel Luna   Submitted: 2012-01-18 09:22

We investigate the process of formation and subsequent evolution ofprominence plasma in a filament channel and its overlying arcade. We constructa three-dimensional time-dependent model of an intermediate quiescentprominence. We combine the magnetic field structure with one-dimensionalindependent simulations of many flux tubes, of a three-dimensional sheareddouble arcade, in which the thermal nonequilibrium process governs the plasmaevolution. We have found that the condensations in the corona can be dividedinto two populations: threads and blobs. Threads are massive condensations thatlinger in the field line dips. Blobs are ubiquitous small condensations thatare produced throughout the filament and overlying arcade magnetic structure,and rapidly fall to the chromosphere. The threads are the principalcontributors to the total mass. The total prominence mass is in agreement withobservations, assuming a reasonable filling factor. The motion of the threadsis basically horizontal, while blobs move in all directions along the field.The peak velocities for both populations are comparable. We have generatedsynthetic images of the whole structure in an Hα proxy and in two EUVchannels of the AIA instrument aboard SDO, thus showing the plasma at cool,warm, and hot temperatures. The predicted differential emission measure of oursystem agrees very well with observations. We conclude that the sheared-arcademagnetic structure and plasma behavior driven by thermal nonequilibrium fitwell the abundant observational evidence for typical intermediate prominences.

Authors: M. Luna, J. T. Karpen, C. R. DeVore
Projects: None

Publication Status: ApJ, in press (ApJ, 746, 30)
Last Modified: 2012-01-18 10:28
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Transverse oscillations of a multi-stranded loop  

Manuel Luna   Submitted: 2010-01-04 06:34

We investigate the transverse oscillations of a line-tied multi-stranded coronal loop composed of several parallel cylindrical strands. First, the collective fast normal modes of the loop are found with the T-matrix theory. There is a huge quantity of normal modes with very different frequencies and a complex structure of the associated magnetic pressure perturbation and velocity field. The modes can be classified as bottom, middle, and top according to their frequencies and spatial structure. Second, the temporal evolution of the velocity and magnetic pressure perturbation after an initial disturbance are analyzed. We find complex motions of the strands. The frequency analysis reveals that these motions are a combination of low and high frequency modes. The complexity of the strand motions produces a strong modulation of the whole tube movement. We conclude that the presumed internal fine structure of a loop influences its transverse oscillations and so its transverse dynamics cannot be properly described by those of an equivalent monolithic loop.

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

Publication Status: (published) Astrophysical Journal 716 (2010) 1371-1380
Last Modified: 2010-06-02 12:29
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Transverse oscillations of systems of coronal loops  

Manuel Luna   Submitted: 2008-09-24 11:02

We study the collective kinklike normal modes of a system of several cylindrical loops using the T-matrix theory. Loops that have similar kink frequencies oscillate collectively with a frequency which is slightly different from that of the individual kink mode. On the other hand, if the kink frequency of a loop is different from that of the others, it oscillates individually with its own frequency. Since the individual kink frequency depends on the loop density but not on its radius for typical 1 MK coronal loops, a coupling between kink oscillations of neighboring loops take place when they have similar densities. The relevance of these results in the interpretation of the oscillations studied by Schrijver and Brown (2000) and Verwichte et al. (2004), in which transverse collective loop oscillations seem to be detected, is discussed. In the first case, two loops oscillating in antiphase are observed; interpreting this motion as a collective kink mode suggests that their densities are roughly equal. In the second case, there are almost three groups of tubes that oscillate with similar periods and therefore their dynamics can be collective, which again seems to indicate that the loops of each group share a similar density. All the other loops seem to oscillate individually and their densities can be different from the rest.

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

Publication Status: ApJ (in press)
Last Modified: 2008-11-06 09:01
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Transverse oscillations of systems of coronal loops  

Manuel Luna   Submitted: 2008-09-24 11:01

We study the collective kinklike normal modes of a system of several cylindrical loops using the T-matrix theory. Loops that have similar kink frequencies oscillate collectively with a frequency which is slightly different from that of the individual kink mode. On the other hand, if the kink frequency of a loop is different from that of the others, it oscillates individually with its own frequency. Since the individual kink frequency depends on the loop density but not on its radius for typical 1 MK coronal loops, a coupling between kink oscillations of neighboring loops take place when they have similar densities. The relevance of these results in the interpretation of the oscillations studied by Schrijver and Brown (2000) and Verwichte et al. (2004), in which transverse collective loop oscillations seem to be detected, is discussed. In the first case, two loops oscillating in antiphase are observed; interpreting this motion as a collective kink mode suggests that their densities are roughly equal. In the second case, there are almost three groups of tubes that oscillate with similar periods and therefore their dynamics can be collective, which again seems to indicate that the loops of each group share a similar density. All the other loops seem to oscillate individually and their densities can be different from the rest.

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

Publication Status: ApJ, in press
Last Modified: 2008-09-24 11:01
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Fast magnetohydrodynamic waves in a two-slab coronal structure: collective behaviour  

Manuel Luna   Submitted: 2006-09-27 05:14

Aims.We study fast magnetohydrodynamic waves in a system of two coronal loops modeled as smoothed, dense plasma slabs in a uniform magnetic field. This allows us to analyse in a simple configuration the collective behaviour of the structure due to the interaction between the slabs. Methods.We first calculate the normal modes of the system and find analytical expressions for the dispersion relation of the two-slab configuration. Next, we study the time-dependent problem of the excitation of slab oscillations by numerically solving the initial value problem. We investigate the behaviour of the system for several shapes of the initial disturbances. Results.The symmetric mode respect to the centre of the structure is the only trapped mode for all distances between the slabs while the antisymmetric mode is leaky for small slab separations. Nevertheless, there is a wide range of slab separations for which the fundamental symmetric and antisymmetric trapped modes are allowed and have very close frequencies. These modes are excited according to the parity of the initial perturbation. Conclusions.We find that for any initial disturbance the slabs oscillate with the normal modes of the coupled slab system, which are different from the modes of the individual slabs. We show that it is possible to excite the symmetric and antisymmetric trapped modes at the same time. This kind of excitation can produce the beating phenomenon, characterised by a continuous exchange of energy between the individual slabs.

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

Publication Status: published
Last Modified: 2006-09-27 09:14
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Abstracts by Author
Large-Amplitude Longitudinal Oscillations Triggered by the Merging of Two Solar Filaments: Observations and Magnetic Field Analysis
The effects of magnetic-field geometry on longitudinal oscillations of solar prominences: Cross-sectional area variation for thin tubes
Damping of prominence longitudinal oscillations due to mass accretion
On the robustness of the pendulum model for large-amplitude longitudinal oscillations in prominences
Are tornado-like magnetic structures able to support solar prominence plasma?
Observations and Implications of Large-Amplitude Longitudinal Oscillations in a Solar Filament
Propagating Waves Transverse to the Magnetic Field in a Solar Prominence
The effects of magnetic-field geometry on longitudinal oscillations of solar prominences
Large amplitude longitudinal oscillations in a solar filament
Formation and evolution of a multi-threaded prominence
Transverse oscillations of a multi-stranded loop
Transverse oscillations of systems of coronal loops
Transverse oscillations of systems of coronal loops
Fast magnetohydrodynamic waves in a two-slab coronal structure: collective behaviour

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