Coronal loop density profile estimated by forward modelling of EUV intensity

Abstract

Aims. The transverse density structuring of coronal loops was recently calculated for the first time using the general damping profile for kink oscillations. This seismological method assumes a density profile with a linear transition region. We consider to what extent this density profile accounts for the observed intensity profile of the loop, and how the transverse intensity profile may be used to complement the seismological technique. Methods. We use isothermal and optically transparent approximations for which the intensity of extreme ultraviolet (EUV) emission is proportional to the square of the plasma density integrated along the line of sight. We consider four different models for the transverse density profile; the generalised Epstein profile, the step function, the linear transition region profile, and a Gaussian profile. The effect of the point spread function is included and Bayesian analysis is used for comparison of the models. Results. The two profiles with finite transitions are found to be preferable to the step function profile, which supports the interpretation of kink mode damping as being due to mode coupling. The estimate of the transition layer width using forward modelling is consistent with the seismological estimate. Conclusions. For wide loops, that is those observed with sufficiently high spatial resolution, this method can provide an independent estimate of density profile parameters for comparison with seismological estimates. In the ill-posed case of only one of the Gaussian or exponential damping regimes being observed, it may provide additional information to allow a seismological inversion to be performed. Alternatively, it may be used to obtain structuring information for loops that do not oscillate.

A statistical study of the inferred transverse density profile of coronal loop threads observed with SDO/AIA

“…This may mean that the intensity profile we have analysed may include several overlapping threads, which affect the transverse structure we infer, however the number of loops analysed is sufficiently large to avoid this being the case for all intensity profiles. Despite these assumptions and complications, in Pascoe et al (2017b) an agreement was found between the seismologically determined value of and the value inferred from the intensity profile. Additionally, numerical simulations of Magyar & Van Doorsselaere (2016b) suggested that highly multi-threaded, or braided loops could be unstable to transverse oscillations.…”

“…The generalised Epstein profile is not used as the two limits of this profile are well represented by the transition layer profile and the Gaussian profiles. In Pascoe et al (2017b) it was found that the advantage of the Epstein profile, over the layer profile, as reflected in the Bayes factor, was negligible.…”

“…In this study we consider three models for the cross-sectional density profile of the coronal loops; the step function profile, the transition layer profile, and the Gaussian profile as described and motivated in Pascoe et al (2017b). The generalised Epstein profile is not used as the two limits of this profile are well represented by the transition layer profile and the Gaussian profiles.…”

“…We follow the model comparison procedure based on Bayesian inference and Markov chain Monte Carlo (MCMC) sampling described in Pascoe et al (2017a) and applied to a coronal loop intensity profile in Pascoe et al (2017b).…”

“…This can be attributed to the damping profile proposed in Pascoe et al (2012Pascoe et al ( , 2013, which has subsequently been used to perform seismology, including the use of Bayseian model comparison (Pascoe et al 2016a(Pascoe et al , 2017a. In Pascoe et al (2017b) the result of this seismology was compared to density profiles estimated from the EUV intensity for one coronal loop. Arregui et al (2015) considered the dependence of seismological information on the inhomogeneous layer density profile, without the use of the non-exponential section of the damping envelope, meaning inversion curves were obtained and compared for the different density profiles.…”