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Coronal loop density profile estimated by forward modelling of EUV intensity  

David Pascoe   Submitted: 2017-03-23 14:29

Aims. The transverse density structuring of coronal loops was recently calculated for the first time using the general damping profile for kink oscillations. This seismological method assumes a density profile with a linear transition region.We consider to what extent this density profile accounts for the observed intensity profile of the loop, and how the transverse intensity profile may be used to complement the seismological technique. Methods. We use isothermal and optically transparent approximations for which the intensity of extreme ultraviolet (EUV) emission is proportional to the square of the plasma density integrated along the line of sight. We consider four different models for the transverse density profile; the generalised Epstein profile, the step function, the linear transition region profile, and a Gaussian profile. The effect of the point spread function is included and Bayesian analysis is used for comparison of the models. Results. The two profiles with finite transitions are found to be preferable to the step function profile, which supports the interpretation of kink mode damping as being due to mode coupling. The estimate of the transition layer width using forward modelling is consistent with the seismological estimate. Conclusions. For wide loops, that is those observed with sufficiently high spatial resolution, this method can provide an independent estimate of density profile parameters for comparison with seismological estimates. In the ill-posed case of only one of the Gaussian or exponential damping regimes being observed, it may provide additional information to allow a seismological inversion to be performed. Alternatively, it may be used to obtain structuring information for loops that do not oscillate.

Authors: D. J. Pascoe, C. R. Goddard, S. Anfinogentov, and V. M. Nakariakov
Projects: SDO-AIA

Publication Status: A&A (in press)
Last Modified: 2017-03-23 14:50
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Coupled Alfvén and Kink Oscillations in Coronal Loops  

David Pascoe   Submitted: 2010-02-22 04:51

Observations have revealed ubiquitous transverse velocity perturbation waves propagating in the solar corona. However, there is ongoing discussion regarding their interpretation as kink or Alfvén waves. To investigate the nature of transverse waves propagating in the solar corona and their potential for use as a coronal diagnostic in MHD seismology, we perform three-dimensional numerical simulations of footpoint-driven transverse waves propagating in a low β plasma. We consider the cases of both a uniform medium and one with loop-like density structure and perform a parametric study for our structuring parameters. When density structuring is present, resonant absorption in inhomogeneous layers leads to the coupling of the kink mode to the Alfvén mode. The decay of the propagating kink wave as energy is transferred to the local Alfvén mode is in good agreement with a modified interpretation of the analysis of Ruderman & Roberts for standing kink modes. Numerical simulations support the most general interpretation of the observed loop oscillations as a coupling of the kink and Alfvén modes. This coupling may account for the observed predominance of outward wave power in longer coronal loops since the observed damping length is comparable to our estimate based on an assumption of resonant absorption as the damping mechanism.

Authors: D. J. Pascoe, A. N. Wright and I. De Moortel
Projects: None

Publication Status: ApJ (published)
Last Modified: 2010-02-23 10:07
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Subject will be restored when possible  

David Pascoe   Submitted: 2007-09-20 07:43

The effect of fine multi-layered structuring on the resonant periods of global sausage (m = 0) fast-magnetoacoustic oscillations of coronal loops is studied for the straight magnetic slab model filled with a zero-beta plasma. The slab is considered to have a one-dimensional inhomogeneity of the Alfvén speed in the direction perpendicular to the axis of the slab, which coincides with the equilibrium, straight, magnetic field. This magnetic configuration models an oscillating coronal loop which is formed by a bundle of field-aligned, smooth, concentric, circular shells of variable density and width. The number of the layers (or shells), their density, width, and radial steepness are randomly distributed. It is shown that the resonant properties of long-wavelength sausage standing modes of the slab are not sensitive to the details of fine structuring. It is concluded that the coronal seismological technique for the determination of the magnetic field strength by the period of global sausage oscillations is not sensitive to the fine multi-layered or multi-shell structuring.

Authors: D.J. Pascoe, V.M. Nakariakov, T.D. Arber
Projects: None

Publication Status: Solar Physics (in press)
Last Modified: 2007-09-21 05:29
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Subject will be restored when possible  

David Pascoe   Submitted: 2007-09-20 07:42

The effect of fine multi-layered structuring on the resonant periods of global sausage (m = 0) fast-magnetoacoustic oscillations of coronal loops is studied for the straight magnetic slab model filled with a zero-? plasma. The slab is considered to have a one-dimensional inhomogeneity of the Alfvén speed in the direction perpendicular to the axis of the slab, which coincides with the equilibrium, straight, magnetic field. This magnetic configuration models an oscillating coronal loop which is formed by a bundle of field-aligned, smooth, concentric, circular shells of variable density and width. The number of the layers (or shells), their density, width, and radial steepness are randomly distributed. It is shown that the resonant properties of long-wavelength sausage standing modes of the slab are not sensitive to the details of fine structuring. It is concluded that the coronal seismological technique for the determination of the magnetic field strength by the period of global sausage oscillations is not sensitive to the fine multi-layered or multi-shell structuring.

Authors: D.J. Pascoe, V.M. Nakariakov, T.D. Arber
Projects: None

Publication Status: Accepted
Last Modified: 2007-09-20 07:42
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Sausage oscillations of coronal loops  

David Pascoe   Submitted: 2006-10-24 07:54

Aims. Analytical theory predicts the existence of trapped global (or fundamental) sausage fast magnetoacoustic modes in thick and dense coronal loops only, with the periods estimated as the ratio of double the loop length and the Alfvén speed outside the loop. We extend this study to the leaking regime, considering global sausage modes of long loops with small density contrasts. Methods. Anti-symmetric fast magnetoacoustic perturbations (sausage, or m = 0 modes) of a low beta plasma slab with the symmetric Epstein profile of plasma density are modelled numerically. Results. It was found that long loops with sufficiently small density contrast can support global sausage leaky modes of detectable quality. The periods of the leaky modes are found to be approximately determined by the loop length and the external Alfvén speed. If the loop length can be estimated from imaging observations, the observed period of this mode provides us with the information about the Alfvén speed outside the loop. For typical flaring coronal loops, the estimated periods of the global sausage modes are about 5-60 s.

Authors: D. J. Pascoe, V. M. Nakariakov and T. D. Arber
Projects:

Publication Status: A&A (in press)
Last Modified: 2006-10-26 04:17
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Coronal loop density profile estimated by forward modelling of EUV intensity
Coupled Alfven and Kink Oscillations in Coronal Loops
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Sausage oscillations of coronal loops

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