Excitation and damping of broadband kink waves in the solar corona

Goddard

Nisticò

et al. 2016

A&A

Self Cite

Context. Kink oscillations of solar coronal loops are frequently observed to be strongly damped. The damping can be explained by mode coupling on the condition that loops have a finite inhomogeneous layer between the higher density core and lower density background. The damping rate depends on the loop density contrast ratio and inhomogeneous layer width. Aims. The theoretical description for mode coupling of kink waves has been extended to include the initial Gaussian damping regime in addition to the exponential asymptotic state. Observation of these damping regimes would provide information about the structuring of the coronal loop and so provide a seismological tool. Methods. We consider three examples of standing kink oscillations observed by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO) for which the general damping profile (Gaussian and exponential regimes) can be fitted. Determining the Gaussian and exponential damping times allows us to perform seismological inversions for the loop density contrast ratio and the inhomogeneous layer width normalised to the loop radius. The layer width and loop minor radius are found separately by comparing the observed loop intensity profile with forward modelling based on our seismological results. Results. The seismological method which allows the density contrast ratio and inhomogeneous layer width to be simultaneously determined from the kink mode damping profile has been applied to observational data for the first time. This allows the internal and external Alfvén speeds to be calculated, and estimates for the magnetic field strength can be dramatically improved using the given plasma density. Conclusions. The kink mode damping rate can be used as a powerful diagnostic tool to determine the coronal loop density profile. This information can be used for further calculations such as the magnetic field strength or phase mixing rate.

Section: Introductionmentioning

confidence: 99%

Coronal loop seismology using damping of standing kink oscillations by mode coupling

Goddard

Nisticò

et al. 2016

A&A

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“…When damped by mode coupling, each spectral component has the same signal quality τ{P (or, equivalently, the ratio of damping length to wavelength L d {λ) and so components with larger periods propagate further along waveguides. This frequency filtering effect was considered by Verth et al (2010b) for the exponential damping regime and (Pascoe et al 2015) for the Gaussian and/or exponential damping regimes. In the case of standing modes, the possible frequencies are quantised but the switch between Gaussian and exponential damping regimes is proportional to the period.…”

Section: Discussionmentioning

confidence: 99%

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

Goddard

Nakariakov

2016

A&A

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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.

“…In other words, the process of mode coupling is a very robust (or unavoidable) one and likely to take place in many coronal structures. Again breaking the symmetrical and co-aligned nature of the driver and the flux tube, Pascoe et al 2015 implemented a boundary driver mimicking small-scale turbulent motions. These eddies, with spatial scales smaller than the fluxtube width, still excite propagating kink waves (which will mode-couple to azimuthal Alfvén waves) but were much less efficient than the larger-scale, 2D-dipole driver.…”

Section: Discussionmentioning

confidence: 99%

“…Ruderman & Roberts 2002, Pascoe et al 2011 showed the damping length of the kink waves to be proportional to the wave period (L d ∼ P ), or, in other words, that mode-coupling will have a frequency filtering effect, where the high frequency modes are expected to damp faster and the longer period waves are expected to propagate further. Pascoe et al 2015 describe a method which does not make a priori assumptions for the damping behaviour but allows seismological information to be inferred from measurements of the frequency-dependent damping rate of broadband kink waves. Unfortunately, for the perturbations observed by Tomczyk et al 2007, the signal-to-noise was too low to be able to distinguish between the different forms of frequency-filtering associated with the two different spatial damping profiles (Pascoe et al 2015).…”

Section: Discussionmentioning

confidence: 99%

“…The results presented in this paper are a summary of the mode coupling studies described in detail in the series of papers , Pascoe et al 2011, Pascoe et al 2015. In Section 2, the basic numerical setup of the 3D numerical mode coupling simulations is presented, followed by the results in Section 3 and a discussion of the Gaussian damping with height of the kink waves in Section 4.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Transverse, propagating velocity perturbations in solar coronal loops

Moortel

Plasma Phys. Control. Fusion

Wright

et al. 2015

Abstract. As waves and oscillations carry both energy and information, they have enormous potential as a plasma heating mechanism and, through seismology, to provide estimates of local plasma properties which are hard to obtain from direct measurements. Being sufficiently near to allow high-resolution observations, the atmosphere of the Sun forms a natural plasma laboratory. Recent observations have revealed that an abundance of waves and oscillations is present in the solar atmosphere, leading to a renewed interest in wave heating mechanisms.This short review paper gives an overview of recently observed transverse, propagating velocity perturbations in coronal loops. These ubiquitous perturbations are observed to undergo strong damping as they propagate. Using 3D numerical simulations of footpoint-driven transverse waves propagating in a coronal plasma with a cylindrical density structure, in combination with analytical modelling, it is demonstrated that the observed velocity perturbations can be understood in terms of coupling of different wave modes in the inhomogeneous boundaries of the loops. Mode coupling in the inhomogeneous boundary layers of the loops leads to the coupling of the transversal (kink) mode to the azimuthal (Alfvén) mode, observed as the decay of the transverse kink oscillations. Both the numerical and analytical results show the spatial profile of the damped wave has a Gaussian shape to begin with, before switching to exponential decay at large heights. In addition, recent analysis of CoMP (Coronal Multi-channel Polarimeter) Doppler shift observations of large, offlimb, trans-equatorial loops shows that Fourier power at the apex appears to be higher in the high-frequency part of the spectrum than expected from theoretical models. This excess high-frequency FFT power could be tentative evidence for the onset of a cascade of the low-to-mid frequency waves into (Alfvénic) turbulence.