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The Role of Active Region Loop Geometry - II. Symmetry breaking in 3D Active Region: why are vertical kink oscillations observed so rarely?  

Mag Selwa   Submitted: 2010-11-23 05:47

We present numerical results of simulations of kink oscillations of coronal loops in an idealized active region that is initialized as a potential dipole magnetic configuration with gravitationally stratified density. We consider loops, with density higher than the surrounding plasma, embedded into the dipolar AR. We study the excitation of kink oscillations of such loops by velocity pulses at different positions, of a given duration and amplitude. The position of the pulse varies in the parametric studies. For central (symmetric) loop within the AR, we find that the amplitude of vertical kink oscillations is significantly amplified in comparison to horizontal kink oscillations for exciters located centrally (symmetrically) below the loop. For pulses initiated further from such a symmetric loop a combination of vertical and horizontal oscillations is excited. The scenario changes significantly when we study an inclined loop (non-symmetric within a dipole field). In this case we do not see vertical kink oscillations of any significant amplitude being excited, while horizontal ones can be easily detected. These results indicate that the reason why vertical kink oscillations are observed so rarely is that their excitation requires a set of conditions to occur simultaneously: the exciting pulse must be located roughly below the loop apex and the loop itself must be located symmetrically within the group of loops. The new findings of the present study show the importance of not only the position of the pulse, but mainly of the location of the loop within the set of field lines having the same magnetic connectivity. We find that the slow propagating wave is excited in all the studied loops and its excitation does not depend neither on the geometry of the loop not the pulse. We discuss TRACE observations of coronal loops oscillations in view of our findings and find that our results can be used for identifying the polarization of the kink mode based on the location of the loop within the set of field lines of the same connectivity and the position of the flare.

Authors: M. Selwa, S. K. Solanki and L. Ofman
Projects:

Publication Status: ApJ accepted
Last Modified: 2010-11-25 08:36
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The Role of Active Region Loop Geometry - I. How can it affect coronal seismology?  

Mag Selwa   Submitted: 2010-10-28 05:38

We present numerical results of coronal loop oscillation excitation using a three dimensional MHD model of an idealized active region field. The active region is initialized as a potential dipole magnetic configuration with gravitationally stratified density and contains a loop with a higher density than its surroundings. We study different ways of excitation of vertical kink oscillations of this loop by velocity: as an initial condition, and as an impulsive excitation with a pulse of a given position, duration, and amplitude. We vary the geometry of the loop in the 3D MHD model and find that it affects both the period of oscillations and the synthetic observations (difference images) that we get from oscillations. Due to overestimated effective length of the loop in case of loops which have maximum separation between their legs above the footpoints (> 50% of observed loops) magnetic field obtained from coronal seismology can also be overestimated. The 3D MHD model shows how the accuracy of magnetic field strength determined from coronal seismology can be improved. We study the damping mechanism of the oscillations and find that vertical kink waves in 3D stratified geometry are damped mainly due to wave leakage in the horizontal direction.

Authors: M. Selwa, L. Ofman and S. K. Solanki
Projects: None

Publication Status: ApJ accepted
Last Modified: 2010-10-28 05:49
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Excitation of vertical kink waves in a solar coronal arcade loop by a periodic driver  

Mag Selwa   Submitted: 2010-07-13 09:12

Aims: We study an oscillatory driver as a possible excitation mechanism of vertical kink loop oscillations in the ideal MHD regime. Methods: We consider a solar coronal magnetic arcade with a dense photospheric layer. The two-dimensional numerical model that we implement includes the effects of nonlinearity and line curvature on the excitation and attenuation of fast magnetosonic kink waves. We investigate the effects of a driven sinusoidal pressure pulse and compare it with the impulsive excitation by a pressure pulse that impacts the overlying loop. Results: Our numerical simulations reveal wave signatures that are reminiscent of vertical loop oscillations seen in TRACE observational data. Conclusions: We conclude that attenuation of vertical kink oscillations can be reduced to the value observed by adopting an oscillatory instead of an impulsive excitation. An oscillatory driver also naturally explains why only a small subset of all loops is excited to oscillate transversally in an active region.

Authors: Selwa, M.; Murawski, K.; Solanki, S. K.; Ofman, L.
Projects: None

Publication Status: A&A 512 (2010) A76
Last Modified: 2010-07-13 12:29
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The role of active region topology on excitation, trapping and damping of coronal loop oscillations  

Mag Selwa   Submitted: 2009-12-01 04:04

We investigate the role of magnetic field topology on dense coronal loop oscillation by the means of 3D MHD numerical simulations of two models of idealized active regions (AR's). The first model of AR is initialized as a straight cylinder surrounded by the fieldlines of the same length and orientation. The second model consists of a potential dipole magnetic configuration and contains a loop with a higher density than its surroundings. Dipole fieldlines have position dependent length and orientation in contrary to straight ones. We study different ways of excitation of transverse loop oscillations by an external pulse and by a nearly eigenmode excitation implemented inside the loop. We find that perturbation acting directly on a single loop excites oscillations both in cylindrical and dipole loop. However, the leakage of the wave energy is larger in a curved loop compared to straight loop. External excitation of the whole AR is efficient in the excitation of oscillation in the straight field configuration, but results in less efficient excitation in the case of dipole field. We show that excitation of collective motion of straight fieldlines having the same wave-periods and planes of the oscillations requires much less energy than excitation of dipole fieldlines having position-dependent orientation and wave-periods and being excited individually, not having a collective mode of oscillation. We conclude that coherent motion of straight fieldlines is one of the factors that decrease the energy leakage from an oscillating loop, while individual motions of dipole fieldlines require more energy from the source to produce the loop oscillations, and also lead to higher damping rate compared to the straight field case. We discuss TRACE observations of coronal loop oscillations in view of our theoretical findings. We show several examples of time signatures of transversal loop oscillations observed by TRACE that agree with numerical simulations of externally excited oscillations.

Authors: M. Selwa, L. Ofman
Projects: None

Publication Status: Astrophysical Journal, submitted
Last Modified: 2010-01-22 06:17
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3-D numerical simulations of coronal loops oscillations  

Mag Selwa   Submitted: 2009-12-01 04:01

We present numerical results of 3-D MHD model of a dipole active region field containing a loop with a higher density than its surroundings. We study different ways of excitation of vertical kink oscillations by velocity erturbation: as an initial condition, and as an impulsive excitation with a pulse of a given position, duration, and amplitude. These properties are varied in the parametric studies. We find that the amplitude of vertical kink oscillations is significantly amplified in comparison to horizontal kink oscillations for exciters located centrally (symmetrically) below the loop, but not if the exciter is located a significant distance to the side of the loop. This explains why the pure vertical kink mode is so rarely observed in comparison to the horizontally polarized one. We discuss the role of curved magnetic field lines and the pulse overlapping at one of the loop's footpoints in 3-D active regions (AR's) on the excitation and the damping of slow standing waves. We find that footpoint excitation becomes more efficient in 3-D curved loops than in 2-D curved arcades and that slow waves can be excited within an interval of time that is comparable to the observed one wave-period due to the combined effect of the pulse inside and outside the loop. Additionally, we study the effect of AR topology on the excitation and trapping of loop oscillations. We find that a perturbation acting directly on a single loop excites oscillations, but results in an increased leakage compared to excitation of oscillations in an AR field by an external source.

Authors: M. Selwa, L. Ofman
Projects: None

Publication Status: Annales Geophysicae (published)
Last Modified: 2010-01-22 06:17
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Energy leakage as an attenuation mechanism for vertical kink oscillations in solar coronal wave guides  

Mag Selwa   Submitted: 2006-08-09 06:33

AIMS: We study wave leakage as a possible attenuation mechanism of coronal loop oscillations in the ideal MHD regime. METHODS: We consider impulsively generated oscillations in solar coronal magnetic wave guides such as a straight slab and a curved arcade loop. The two-dimensional numerical model we implement includes the effects of nonlinearity and line curvature on attenuation of fast magnetosonic kink waves. RESULTS: We show that these waves are more strongly attenuated in the arcade loop than in the slab and provide evidence that the curvature of magnetic field lines results in excess energy leakage. For parameters appropriate for a coronal loop the kink oscillation is too efficiently attenuated by energy leakage, suggesting that in the solar atmosphere wave leakage must be reduced compared to our simulations. We conclude that energy leakage is an efficient source of attenuation of coronal loop oscillations.

Authors: Selwa, M.; Murawski, K.; Solanki, S.K.; Wang, T.J.
Projects: TRACE

Publication Status: in press.
Last Modified: 2006-08-09 09:28
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Energy leakage as an attenuation mechanism for vertical kink oscillations in solar coronal wave guides  

Mag Selwa   Submitted: 2006-08-09 06:33

AIMS: We study wave leakage as a possible attenuation mechanism of coronal loop oscillations in the ideal MHD regime. METHODS: We consider impulsively generated oscillations in solar coronal magnetic wave guides such as a straight slab and a curved arcade loop. The two-dimensional numerical model we implement includes the effects of nonlinearity and line curvature on attenuation of fast magnetosonic kink waves. RESULTS: We show that these waves are more strongly attenuated in the arcade loop than in the slab and provide evidence that the curvature of magnetic field lines results in excess energy leakage. For parameters appropriate for a coronal loop the kink oscillation is too efficiently attenuated by energy leakage, suggesting that in the solar atmosphere wave leakage must be reduced compared to our simulations. We conclude that energy leakage is an efficient source of attenuation of coronal loop oscillations.

Authors: Selwa, M.; Murawski, K.; Solanki, S.K.; Wang, T.J.
Projects: TRACE

Publication Status: A&A published
Last Modified: 2010-01-22 06:16
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Abstracts by Author
The Role of Active Region Loop Geometry - II. Symmetry breaking in 3D Active Region: why are vertical kink oscillations observed so rarely?
The Role of Active Region Loop Geometry - I. How can it affect coronal seismology?
Excitation of vertical kink waves in a solar coronal arcade loop by a periodic driver
The role of active region topology on excitation, trapping and damping of coronal loop oscillations
3-D numerical simulations of coronal loops oscillations
Energy leakage as an attenuation mechanism for vertical kink oscillations in solar coronal wave guides
Energy leakage as an attenuation mechanism for vertical kink oscillations in solar coronal wave guides

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