Damping of kink waves by mode coupling

Abstract

Context. Recent observations of the corona reveal ubiquitous transverse velocity perturbations that undergo strong damping as they propagate. These can be understood in terms of propagating kink waves that undergo mode coupling in inhomogeneous regions. Aims. The use of these propagating waves as a seismological tool for the investigation of the solar corona depends upon an accurate understanding of how the mode coupling behaviour is determined by local plasma parameters. Our previous work suggests the exponential spatial damping profile provides a poor description of the behaviour of strongly damped kink waves. We aim to investigate the spatial damping profile in detail and provide a guide to the approximations most suitable for performing seismological inversions. Methods. We propose a general spatial damping profile based on analytical results that accounts for the initial Gaussian stage of damped kink waves as well as the asymptotic exponential stage considered by previous authors. The applicability of this profile is demonstrated by a full parametric study of the relevant physical parameters. The implication of this profile for seismological inversions is investigated. Results. The Gaussian damping profile is found to be most suitable for application as a seismological tool for observations of oscillations in loops with a low density contrast. This profile also provides accurate estimates for data in which only a few wavelengths or periods are observed.

Non-stationary quasi-periodic pulsations in solar and stellar flares

“…But, it has been shown that in some cases a Gaussian decay is more appropriate [12,32]. Physical mechanisms responsible for the Gaussian damping of slow magnetoacoustic waves in the corona have not been identified yet, but are known in the case of coronal fast magnetoacoustic waves of the kink symmetry [44].…”

Spatially resolved observation of the fundamental and second harmonic standing kink modes using SDO/AIA

“…Based on numerical (Pascoe et al 2012) and analytical investigations (Hood et al 2013) of mode coupling, Pascoe et al (2013a) described how the damping of kink oscillations by mode coupling can exhibit two different regimes. A Gaussian damping profile applies for early times, and an exponential damping profile at later times.…”

“…Pascoe et al (2016b) examined the damping profile of standing kink oscillations observed by SDO/AIA and found examples of both Gaussian and exponential damping behaviour. Pascoe et al (2013a) proposed that a unique seismological inversion for the transverse density profile would be possible in cases of oscillations for which both regimes were observed. This method was recently applied to three coronal loops by Pascoe et al (2016a).…”

“…4. Pascoe et al (2016b) studied the damping profile of kink oscillations compiled by Goddard et al (2016) to demonstrate the existence of the Gaussian damping regime predicted by numerical (Pascoe et al 2012(Pascoe et al , 2013a and analytical (Hood et al 2013) modelling of mode coupling. One of the requirements for that investigation was for the oscillating loop to have a period of oscillation that remains stable over several cycles so that the damping profile could be accurately examined.…”

Excitation and damping of broadband kink waves in the solar corona

“…The integro-differential equation for the continuous variation in amplitude with height z can be approximated by Gaussian and exponential functions at small and large z, respectively. This was employed by Pascoe et al (2013) who produced a simpler, piecewise spatial damping profile suitable for use for seismological inversions. The initial stage of the damping envelope is described by a Gaussian function,…”