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Impulsively generated oscillations in a 3D coronal loop  

Ineke De Moortel   Submitted: 2009-12-04 09:47

Aims. The effect of changing the attack angle for the interaction of a fast MHD wave with a 3D coronal loop is studied, to investigate to what extent the properties of the excited transverse kink mode oscillations of the loop depend on this angle. Methods: 3D numerical simulations are performed of the interaction of a fast MHD wave, generated by a pressure pulse, with a 3D coronal loop. The loop itself is modelled as a density enhancement (with a finite plasma beta) within a magnetic arcade. The initial pressure pulse has a width comparable to the loop diameter and is situated outside of the loop, at the same height as the loop apex. This height is kept fixed but the (horizontal) angle between the pressure pulse and the loop is varied. Results: We find that the global, transverse kink mode is efficiently excited for a range of attack angles and qualitatively in agreement with theoretical expectations. The period and damping time are found to be independent of the attack angle. For larger values of the attack angle, the global (longitudinal) slow wave is excited, whereas for intermediate values the second harmonic kink mode is also present.

Authors: Pascoe, D. J.; de Moortel, I.; McLaughlin, J. A.
Projects: None

Publication Status: A&A, published
Last Modified: 2009-12-07 10:36
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Putting coronal seismology estimates of the magnetic field strength to the test  

Ineke De Moortel   Submitted: 2009-12-04 09:45

The magnetic field strength inside a model coronal loop is `estimated' using coronal seismology, to examine the reliability of magnetic field strengths derived from observed, transverse coronal loop oscillations. 3D numerical simulations of the interaction of an external pressure pulse with a coronal loop (modelled as a 3D density enhancement inside a 2D magnetic arcade) are analysed and the `observed' properties of the excited transverse loop oscillations are used to derive the value of the local magnetic field strength, following the method of Nakariakov & Ofman (2001). Due to the (unexpected) change in periodicity, the magnetic field derived from our `observed' oscillation is substantially different from the actual (input) magnetic field value (approximately 50%). Coronal seismology can derive useful information about the local magnetic field, but the combined effect of the loop curvature, the density ratio and aspect ratio of the loop appears to be more important than previously expected.

Authors: I. De Moortel, D.J.Pascoe
Projects: None

Publication Status: ApJ, published
Last Modified: 2009-12-07 10:36
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Forward Modelling of Coronal Intensity Perturbations  

Ineke De Moortel   Submitted: 2008-06-20 07:35

In this paper, forward modelling is used to investigate the relation between given temperature and density perturbations and the resulting (synthesised) intensity perturbations, as would be observed by e.g. TRACE and EIS (onboard Hinode). Complex and highly non-linear interactions between the components which make up the intensity (density, ionisation balance and emissivity) mean that it is non-trivial to reverse this process, i.e. obtain the density and temperature perturbations associated with observed intensity oscillations. In particular, it is found that the damping rate does not often `survive' the forward modelling process, highlighting the need for a very careful interpretation of observed (intensity) damping rates. With a few examples, it is demonstrated that in some cases even the period of the oscillations can be altered and that it is possible for two different sets of input temperature and density to lead to very similar intensities (the well-known `ill-posed' inversion process).

Authors: I. De Moortel, S.J. Bradshaw
Projects: None

Publication Status: Solar Physics (accepted)
Last Modified: 2008-09-23 20:59
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Observation of higher harmonic coronal loop oscillations  

Ineke De Moortel   Submitted: 2007-04-12 12:25

A sequence of TRACE 171 Å observations taken on 13 May 2001 shows evidence of flare-induced, transverse coronal loop oscillations (Schrijver et al 2002, Aschwanden et al 2002). We revisit this particular data set and present evidence of the presence of spatially resolved higher harmonics in the transverse loop displacements. The oscillations are identified as the second harmonic, fast MHD, kink waves (periods of 577 - 672 sec), with higher harmonics (250 - 346 sec) also present. The apparent absence of the fundamental mode and the fact that it is the second harmonic (P_2) which dominates the oscillatory behaviour of this particular loop may shed more light on either the excitation and/or the damping mechanism(s) of flare-induced, transverse loop oscillations.

Authors: I. De Moortel, C.S. Brady
Projects: None

Publication Status: ApJ (accepted)
Last Modified: 2007-04-12 16:33
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An estimate of p-mode damping by wave leakage.  

Ineke De Moortel   Submitted: 2007-04-12 12:23

High cadence TRACE observations show that outward propagating intensity disturbances are a common feature in large, quiescent coronal loops. Analysis of the frequency distribution of these modes show peaks at both 3 and 5 minute periods, indicating that they may be driven by the solar surface oscillations (pmodes). The energy ux contained within the coronal intensity disturbances is of the order of (1.1 +- 0.4) x 103 ergs/cm^2s. A simple order of magnitude estimate of the damping rate of the relevant p-modes allows us to put an observational constraint on the damping of p-modes and shows that leakage into the overlying coronal atmosphere might be able to account for a significant fraction of p-mode damping.

Authors: I. De Moortel, R. Rosner
Projects: None

Publication Status: Solar Physics (accepted)
Last Modified: 2007-04-12 16:33
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Abstracts by Author
Impulsively generated oscillations in a 3D coronal loop
Putting coronal seismology estimates of the magnetic field strength to the test
Forward Modelling of Coronal Intensity Perturbations
Observation of higher harmonic coronal loop oscillations
An estimate of p-mode damping by wave leakage.

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