Damping profile of standing kink oscillations observed by SDO/AIA

Anfinogentov

Nisticò

et al. 2016

A&A

Self Cite

Context. Standing transverse oscillations of coronal loops are observed to operate in two regimes: rapidly decaying, large amplitude oscillations and undamped small amplitude oscillations. In the latter regime the damping should be compensated by energy supply, which allows the loop to perform almost monochromatic oscillations with almost constant amplitude and phase. Different loops oscillate with different periods. The oscillation amplitude does not show dependence on the loop length or the oscillation period. Aims. We aim to develop a low-dimensional model explaining the undamped kink oscillations as a self-oscillatory process caused by the effect of negative friction. The source of energy is an external quasi-steady flow, for example, supergranulation motions near the loop footpoints or external flows in the corona. Methods. We demonstrate that the interaction of a quasi-steady flow with a loop can be described by a Rayleigh oscillator equation that is a non-linear ordinary differential equation, with the damping and resonant terms determined empirically. Results. Small-amplitude self-oscillatory solutions to the Rayleigh oscillator equation are harmonic signals of constant amplitude, which is consistent with the observed properties of undamped kink oscillations. The period of self-oscillations is determined by the frequency of the kink mode. The damping by dissipation and mode conversion is compensated by the continuous energy deposition at the frequency of the natural oscillation. Conclusions. We propose that undamped kink oscillations of coronal loops may be caused by the interaction of the loops with quasi-steady flows, and hence are self-oscillations, which is analogous to producing a tune by moving a bow across a violin string.

Section: Introductionmentioning

confidence: 99%

Undamped transverse oscillations of coronal loops as a self-oscillatory process

Anfinogentov

Nisticò

et al. 2016

A&A

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“…McEwan et al (2008) derived an analytical expression for the dependence of the period ratio on the density scale height, which was previously solved numerically by Donnelly et al (2006) and Díaz et al (2007) for loops, and by Donnelly et al (2007) for a magnetic arcade. A spatially resolved observation of the fundamental and second harmonic standing kink modes in a coronal loop was recently reported by Pascoe et al (2016), with a period ratio P 1 {2P 2 " 1.15˘0.22.…”

Section: Introductionmentioning

confidence: 81%

Standing sausage modes in curved coronal slabs

Pascoe

2016

A&A

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Context. Magnetohydrodynamic waveguides such as dense coronal loops can support standing modes. The ratios of the periods of oscillations for different longitudinal harmonics depend on the dispersive nature of the waveguide and so may be used as a seismological tool to determine coronal parameters. Aims. We extend models of standing sausage modes in low β coronal loops to include the effects of loop curvature. The behaviour of standing sausage modes in this geometry is used to explain the properties of observed oscillations which cannot be accounted for using straight loop models. Methods. We perform 2D numerical simulations of an oscillating coronal loop, modelled as a dense slab embedded in a potential magnetic field. The loop is field-aligned and so experiences expansion with height in addition to being curved. Standing sausage modes are excited by compressive perturbations of the loop and their properties are studied. Results. The spatial profiles of standing sausage modes are found to be modified by the expanding loop geometry typical for flaring loops and modelled by a potential magnetic field in our simulations. Longitudinal harmonics of order n ą 1 have antinodes which are shifted towards the loop apex and the amplitude of antinodes near the loop apex is smaller than those near the loop footpoints. Conclusions. We find that the observation of standing sausage modes by Nakariakov et al. (2003) is consistent with interpretation in terms of the global mode (n " 1) and third harmonic (n " 3). This interpretation accounts for the period ratio and spatial structure of the observed oscillations.

“…The time of the switch between damping profiles depends on the loop density contrast ratio, such that the Gaussian damping profile is the dominant one for low contrast loops, and the exponential profile dominant for high contrast loops (see also reviews by Pascoe 2014;De Moortel et al 2016). 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.…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

“…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. The event we examine in this paper was initially chosen for the study by Pascoe et al (2016b) on the basis of the high quality of the data. However, it had to be excluded on the basis that there was evidently two strong periodicities in the time series and so the oscillation could not be easily modelled as a decaying global (fundamental) standing kink mode.…”

Section: Introductionmentioning

confidence: 99%

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

Pascoe

Goddard

2016

A&A

Self Cite

Aims. We consider a coronal loop kink oscillation observed by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO) which demonstrates two strong spectral components. The period of the lower frequency component being approximately twice that of the shorter frequency component suggests the presence of harmonics. Methods. We examine the presence of two longitudinal harmonics by investigating the spatial dependence of the loop oscillation. The time-dependent displacement of the loop is measured at 15 locations along the loop axis. For each position the detrended displacement is fitted as the sum of two damped sinusoids, having periods P 1 and P 2 , and a damping time τ. The shorter period component exhibits anti-phase oscillations in the loop legs. Results. We interpret the observation in terms of the first (global or fundamental) and second longitudinal harmonics of the standing kink mode. The strong excitation of the second harmonic appears connected to the preceding coronal mass ejection (CME) which displaced one of the loop legs. The oscillation parameters found are P 1 " 5.00˘0.62 minutes, P 2 " 2.20˘0.23 minutes, P 1 {2P 2 " 1.15˘0.22, and τ{P " 3.35˘1.45.