Seismology of contracting and expanding coronal loops using damping of kink oscillations by mode coupling 

David James Pascoe Submitted: 20170822 06:01
We extend recently developed seismological methods to analyse oscillating loops which feature a large initial shift in the equilibrium position and investigate additional observational signatures related to the loop environment and oscillation driver.
We model the motion of coronal loops as a kink oscillation damped by mode coupling, accounting for any change in loop length and the possible presence of parallel harmonics in addition to the fundamental mode. We apply our model to a loop which rapidly contracts due to a postflare implosion (SOL20120309) and a loop with a large lateral displacement (SOL20121020).
The seismological method is used to calculate plasma parameters of the oscillating loops including the transverse density profile, magnetic field strength, and phase mixing timescale. For SOL20120309 the period of oscillation has a linear correlation with the contracting motion and suggests the kink speed remains constant during the oscillation.
The implosion excitation mechanism is found to be associated with an absence of additional parallel harmonics.
The improved Bayesian analysis of the coronal loop motion allows for accurate seismology of plasma parameters and the evolution of the period of oscillation compared with the background trend can be used to distinguish between loop motions in the plane of the loop or perpendicular to it. The seismologically inferred kink speed and density contrast imply subAlfvénic (M_{A} = 0.16 ± 0.03) propagation of the magnetic reconfiguration associated with the implosion, as opposed to triggering by a wave propagating at the Alfvén speed.
Authors: D.J. Pascoe, A.J.B. Russell, S.A. Anfinogentov, P.J.A. Simões, C.R. Goddard, V.M. Nakariakov, L. Fletcher
Projects: SDOAIA

Publication Status: A&A (in press)
Last Modified: 20170823 12:50



Damping of Kink Waves by Mode Coupling I. Analytical Treatment 

David James Pascoe Submitted: 20130118 08:27
Aims. To investigate the spatial damping of propagating kink waves in an inhomogeneous plasma. In the limit of a thin tube surrounded by a thin transition layer, an analytical formulation for kink waves driven in from the bottom boundary of the corona is presented.
Methods. The spatial form for the damping of the kink mode was investigated using various analytical approximations. When the density ratio between the internal density and the external density is not too large, a simple differentialintegral equation was used. Approximate analytical solutions to this equation are presented.
Results. For the first time, the form of the spatial damping of the kink mode is shown analytically to be Gaussian in nature near the driven boundary. For several wavelengths, the amplitude of the kink mode is proportional to (1 + exp(z^2/Lg^2))/2, where Lg^2 = 16/(ε κ^2 k^2). Although the actual value of 16 in Lg depends on the particular form of the driver, this form is very general and its dependence on the other parameters does not change. For large distances, the damping profile appears to be roughly linear exponential decay. This is shown analytically by a series expansion when the inhomogeneous layer width is small enough.
Authors: Hood, A.W., Ruderman, M.S., Pascoe, D.J., De Moortel, I., Terradas, J., and Wright, A.N.
Projects: None

Publication Status: Accepted by A&A
Last Modified: 20130118 12:00




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