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

et al. 2018

ApJL

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EUV observations of a multi-thermal coronal loop, taken by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO), which exhibits decay-less kink oscillations are presented. The datacube of the quiet Sun coronal loop was passed through a motion magnification algorithm to accentuate transverse oscillations. Timedistance maps are made from multiple slits evenly spaced along the loop axis and oriented orthogonal to the loop axis. Displacements of the intensity peak are tracked to generate time series of the loop displacement. Fourier analysis on the time series show the presence of two periods within the loop; P 1 = 10.3 +1.5 −1.7 minutes and P 2 = 7.4 +1.1 −1.3 minutes. The longer period component is greatest in amplitude at the apex and remains in phase throughout the loop length. The shorter period component is strongest further down from the apex on both legs, and displays an anti-phase behaviour between the two loop legs. We interpret these results as the coexistence of the fundamental and second harmonics of the standing kink mode within the loop in the decay-less oscillation regime. An illustration of seismological application using the ratio P 1 /2P 2 ∼ 0.7 to estimate the density scale height is presented. The existence of multiple harmonics has implications for understanding the driving and damping mechanisms for decay-less oscillations, and adds credence to their interpretation as standing kink mode oscillations.

Section: Introductionsupporting

confidence: 83%

Detection of the Second Harmonic of Decay-less Kink Oscillations in the Solar Corona

Duckenfield

Anfinogentov

et al. 2018

ApJL

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“…The seismological method is based on the use of the kink oscillation damping profile as described in Pascoe et al (2016bPascoe et al ( , 2017aPascoe et al ( , 2017c. Large-amplitude standing kink oscillations are typically observed for fewer than six cycles (e.g., Figure 2 of .…”

Section: Seismology Using Damped Kink Oscillationsmentioning

confidence: 99%

“…They are commonly used to infer the strength of the coronal magnetic field (e.g., Nakariakov et al 1999;Nakariakov & Ofman 2001;Van Doorsselaere et al 2008;White & Verwichte 2012;Pascoe et al 2016b;Sarkar et al 2016). Additional structuring information may be obtained using higher harmonics, which there is increasing evidence of (e.g., Verwichte et al 2004;De Moortel & Brady 2007;Van Doorsselaere et al 2007;Wang et al 2008;Srivastava et al 2013;Pascoe et al 2016aPascoe et al , 2017aPascoe et al , 2017cLi et al 2017). The strong damping of kink oscillations is attributed to resonant absorption (Sedláček 1971), which requires a smooth transition between the high-density plasma inside coronal loops and the background plasma.…”

Section: Introductionmentioning

confidence: 99%

“…Pascoe et al (2017a) also employed a new method to account for a dynamical background trend and used Bayesian analysis to test the model against the observational data. Pascoe et al (2017c) included an additional term in the background trend to describe loops that experience a rapid shift in the location of the equilibrium, such as the contracting loops previously analyzed by Simões et al (2013) and Russell et al (2015). These applications of the seismological method based on the shape of the damping profile exploit the best cases of kink oscillations that have been observed so far.…”

Section: Introductionmentioning

confidence: 99%

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Spatiotemporal Analysis of Coronal Loops Using Seismology of Damped Kink Oscillations and Forward Modeling of EUV Intensity Profiles

Anfinogentov

Goddard

et al. 2018

ApJ

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The shape of the damping profile of kink oscillations in coronal loops has recently allowed the transverse density profile of the loop to be estimated. This requires accurate measurement of the damping profile that can distinguish the Gaussian and exponential damping regimes, otherwise there are more unknowns than observables. Forward modeling of the transverse intensity profile may also be used to estimate the width of the inhomogeneous layer of a loop, providing an independent estimate of one of these unknowns. We analyze an oscillating loop for which the seismological determination of the transverse structure is inconclusive except when supplemented by additional spatial information from the transverse intensity profile. Our temporal analysis describes the motion of a coronal loop as a kink oscillation damped by resonant absorption, and our spatial analysis is based on forward modeling the transverse EUV intensity profile of the loop under the isothermal and optically thin approximations. We use Bayesian analysis and Markov chain Monte Carlo sampling to apply our spatial and temporal models both individually and simultaneously to our data and compare the results with numerical simulations. Combining the two methods allows both the inhomogeneous layer width and density contrast to be calculated, which is not possible for the same data when each method is applied individually. We demonstrate that the assumption of an exponential damping profile leads to a significantly larger error in the inferred density contrast ratio compared with a Gaussian damping profile.

“…For this reason multi-mode observations would be particularly informative. Observations of standing kink oscillations have recently been used to calculate the density (and Alfvén speed) profiles for coronal loops using their damping profiles by resonant absorption (Pascoe et al 2013a(Pascoe et al , 2016(Pascoe et al , 2017a(Pascoe et al , 2017c. The perpendicular inhomogeneity size has also been independently estimated by forward modeling the EUV intensity (Goddard et al 2017;Pascoe et al 2017b), although so far this has relied on the isothermal approximation (e.g., Aschwanden et al 2007), whereas hot (multi-thermal) flaring loops may require more sophisticated forward modeling (e.g., De Moortel & Bradshaw 2008;Van Doorsselaere et al 2016a).…”

mentioning

confidence: 99%

Dispersive Evolution of Nonlinear Fast Magnetoacoustic Wave Trains

Goddard

Nakariakov

2017

ApJL

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Quasi-periodic rapidly propagating wave trains are frequently observed in extreme ultraviolet observations of the solar corona, or are inferred by the quasi-periodic modulation of radio emission. The dispersive nature of fast magnetohydrodynamic waves in coronal structures provides a robust mechanism to explain the detected quasiperiodic patterns. We perform 2D numerical simulations of impulsively generated wave trains in coronal plasma slabs and investigate how the behavior of the trapped and leaky components depend on the properties of the initial perturbation. For large amplitude compressive perturbations, the geometrical dispersion associated with the waveguide suppresses the nonlinear steepening for the trapped wave train. The wave train formed by the leaky components does not experience dispersion once it leaves the waveguide and so can steepen and form shocks. The mechanism we consider can lead to the formation of multiple shock fronts by a single, large amplitude, impulsive event and so can account for quasi-periodic features observed in radio spectra.