Observational Tests of Damping by Resonant Absorption in Coronal Loop Oscillations

By Markus J. Aschwanden, R. Nightingale, Jesse Andries, M. Goossens, T. Van Doorsselaere. 2003

171 citations recorded by the source corpus.

One of the proposed damping mechanisms of coronal (transverse) loop oscillations in the kink mode is resonant absorption as a result of the Alfvén speed variation at the outer boundary of coronal loops. Analytical expressions for the period and damping time exist for loop models with thin nonuniform boundaries. They predict a linear dependency of the ratio of the damping time to the period on the thickness of the nonuniform boundary layer. Ruderman and Roberts used a sinusoidal variation of the density in the nonuniform boundary layer and obtained the corresponding analytical expression for the damping time. Here we measure the thickness of the nonuniform layer in oscillating loops for 11 events, by forward-fitting of the cross-sectional density profile ne(r) and line-of-sight integration to the cross-sectional fluxes F(r) observed with TRACE 171 Å. This way we model the internal (ni) and external electron density (ne) of the coronal plasma in oscillating loops. This allows us to test the theoretically predicted damping rates for thin boundaries as a function of the density ratio χ = ne/ni. Since the observations show that the loops have nonuniform density profiles, we also use numerical results for damping rates to determine the value of χ for the loops. We find that the density ratio predicted by the damping time, χLEDA = 0.53 ± 0.12, is a factor of ≈1.2-3.5 higher than the density ratio estimated from the background fluxes, χ = 0.30 ± 0.16. The lower densities modeled from the background fluxes are likely to be a consequence of the neglected hotter plasma that is not detected with the TRACE 171 Å filter. Taking these corrections into account, resonant absorption predicts damping times of kink-mode oscillations that are commensurable with the observed ones and provides a new diagnostic of the density contrast of oscillating loops.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/32f51ee742c9f2c9fd472af0656d7389839b73ae

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Citations in the corpus

Coronal loop oscillations Calculation of resonantly damped MHD quasi-mode kink oscillations of longitudinally stratified loops (2005, result): “We are well aware that this approximation is not a very good representation of reality as we have argued in previous papers (Goossens et al. 2002; Aschwanden et al. 2003).”
Effect of longitudinal magnetic and density inhomogeneity on transversal coronal loop oscillations (2008, background): “…from the work of Hollweg & Yang (1988), it has been shown that varying plasma density in the radial direction results in a change of period and damping of MHD waves (see e.g., Ruderman & Roberts 2002; Goossens et al. 2002; Aschwanden et al. 2003; Van Doorsselaere et al. 2004; Arregui et al. 2007).”
CORONAL LOOP OSCILLATIONS OBSERVED WITH ATMOSPHERIC IMAGING ASSEMBLY—KINK MODE WITH CROSS-SECTIONAL AND DENSITY OSCILLATIONS (2011, methodology): “This density ratio was previously measured to qn = 0.30 ± 0.16, based on loop flux intensities and hydrostatic models of the background corona (Aschwanden et al. 2003), and a skin depth ratio of r0/lskin = 1.5 ± 0.2 was inferred, and hence the typical ratio of the damping time to the oscillation…”
Coronal loop seismology using multiple transverse loop oscillation harmonics (2007, background): “Some attempts to measure the density were undertaken (Aschwanden et al. 2003) and the average value of the magnetic field was estimated by Lin et al. (2000, 2004), but the error bars on the results are large.”
Coronal Seismology by Means of Kink Oscillation Overtones (2009, methodology): “…kink oscillations of coronal loops were first observed very accurately by the TRACE spacecraft (Transition Region And Coronal Explorer) (Schrijver et al. 1999, 2002; Aschwanden et al. 1999; Nakariakov et al. 1999; Aschwanden et al. 2002) (or Aschwanden et al. 2003, for an observational review).”
Multiwavelength Studies of MHD Waves in the Solar Chromosphere (2015, background): “…to be a powerful tool, with vast numbers of high-impact publications produced to date, including those related to the uncovering of magnetic fields (Nakariakov and Ofman 2001), energy transport coefficients (Aschwanden et al. 2003) and sub-resolution fine-scale structuring (King et al. 2003).”
Characteristics of transverse oscillations in a coronal loop arcade (2004, background): “Of course, as Goossens, Andries and Aschwanden (2002) and Aschwanden et al. (2003) argued, this ratio may in fact vary from loop to loop.”
MHD seismology of coronal loops using the period and damping of quasi-mode kink oscillations (2007, methodology): “An attempt to do so was published by Aschwanden et al. (2003).”
Damping of Coronal Loop Oscillations: Calculation of Resonantly Damped Kink Oscillations of One-dimensional Nonuniform Loops (2004, methodology): “As a matter of fact, the results for the oscillations of a simple one-dimensional cylindrical model seem to agree rather well with the observations (Aschwanden et al. 2003).”