Self-Similar Approach for Rotating Magnetohydrodynamic Solar and Astrophysical Structures |
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Manuel Luna Submitted: 2018-07-09 16:39
Rotating magnetic structures are common in astrophysics, from vortex tubes and tornados in the Sun all the way to jets in different astrophysical systems. The physics of these objects often combine inertial, magnetic, gas pressure and gravitational terms. Also, they often show approximate symmetries that help simplify the otherwise rather intractable equations governing their morphology and evolution. Here we propose a general formulation of the equations assuming axisymmetry and a self-similar form for all variables: in spherical coordinates (r,θ,φ), the magnetic field and plasma velocity are taken to be of the form: B=f(θ)/rn and
v=g(θ)/rm, with corresponding expressions for the scalar variables like pressure and density. Solutions are obtained for potential, force-free, and non-force-free magnetic configurations. Potential-field solutions can be found for all values of~n. Non-potential force-free solutions possess an azimuthal component Bφ and exist only for n≥2; the resulting structures are twisted and have closed field lines but are not collimated around the system axis. In the non-force free case, including gas pressure, the magnetic field lines acquire an additional curvature to compensate for an outward pointing pressure gradient force. We have also considered a pure rotation situation with no gravity, in the zero-β limit: the solution has cylindrical geometry and twisted magnetic field lines. The latter solutions can be helpful in producing a collimated magnetic field structure; but they exist only when n<0 and m<0: for applications they must be matched to an external system at a finite distance from the origin.
Authors: Manuel Luna, Eric Priest and Fernando Moreno-Insertis
Projects: None
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Publication Status: Accepted for publication in ApJ
Last Modified: 2018-07-11 15:24
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GONG Catalog of Solar Filament Oscillations Near Solar Maximum |
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Manuel Luna Submitted: 2018-04-12 03:35
We have catalogued 196 filament oscillations from the GONG Hα network data during several months near the maximum of solar cycle 24 (January - June 2014). Selected examples from the catalog are described in detail, along with our statistical analyses of all events. Oscillations were classified according to their velocity amplitude: 106 small-amplitude oscillations (SAOs), with velocities <10km s-1, and 90 large-amplitude oscillations (LAOs), with velocities >10km s-1. Both SAOs and LAOs are common, with one event of each class every two days on the visible side of the Sun. For nearly half of the events we identified their apparent trigger. The period distribution has a mean value of 58?15 min for both types of oscillations. The distribution of the damping time per period peaks at τ/P=1.75 and 1.25 for SAOs and LAOs respectively. We confirmed that LAO damping rates depend nonlinearly on the oscillation velocity. The angle between the direction of motion and the filament spine has a distribution centered at 27∘ for all filament types. This angle agrees with the observed direction of filament-channel magnetic fields, indicating that most of the catalogued events are longitudinal (i.e., undergo field-aligned motions). We applied seismology to determine the average radius of curvature in the magnetic dips, R≈89 Mm, and the average minimum magnetic-field strength, B≈16 G. The catalog is available to the community online, and is intended to be expanded to cover at least 1 solar cycle.
Authors: Manuel Luna, Judith Karpen, José Luis Ballester, Karin Muglach, Jaume Terradas, Therese Kucera, Holly Gilbert
Projects: GONG
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Publication Status: Accepted for publication in ApJ Supplement Series
Last Modified: 2018-04-12 08:29
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Large-Amplitude Longitudinal Oscillations Triggered by the Merging of Two Solar Filaments: Observations and Magnetic Field Analysis |
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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
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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 |
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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
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Publication Status: Accepted for publication in Astronomy and Astrophysics
Last Modified: 2016-08-02 13:35
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On the robustness of the pendulum model for large-amplitude longitudinal oscillations in prominences |
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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
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Publication Status: Accepted for publication in The Astrophysical Journal
Last Modified: 2015-12-20 12:40
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Observations and Implications of Large-Amplitude Longitudinal Oscillations in a Solar Filament |
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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
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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 |
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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
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Publication Status: Accepted for publication in The Astrophysical Journal
Last Modified: 2013-09-09 10:12
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Formation and evolution of a multi-threaded prominence |
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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
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Publication Status: ApJ, in press (ApJ, 746, 30)
Last Modified: 2012-01-18 10:28
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Transverse oscillations of systems of coronal loops |
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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
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Publication Status: ApJ (in press)
Last Modified: 2008-11-06 09:01
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Transverse oscillations of systems of coronal loops |
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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
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Publication Status: ApJ, in press
Last Modified: 2008-09-24 11:01
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