Bibcode
Arregui, I.; Soler, R.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 578, id.A130, 9 pp.
Advertised on:
6
2015
Revista
Número de citas
9
Número de citas referidas
9
Descripción
Context. Quiescent solar prominence fine structures are typically
modelled as density enhancements, called threads, which occupy a
fraction of a longer magnetic flux tube. This is justified from the
spatial distribution of the imaged plasma emission or absorption of
prominences at small spatial scales. The profile of the mass density
along the magnetic field is unknown, however, and several arbitrary
alternatives are employed in prominence wave studies. The identification
and measurement of period ratios from multiple harmonics in standing
transverse thread oscillations offer a remote diagnostics method to
probe the density variation of these structures. Aims: We present
a comparison of theoretical models for the field-aligned density along
prominence fine structures. They aim to imitate density distributions in
which the plasma is more or less concentrated around the centre of the
magnetic flux tube. We consider Lorentzian, Gaussian, and parabolic
profiles. We compare theoretical predictions based on these profiles for
the period ratio between the fundamental transverse kink mode and the
first overtone to obtain estimates for the density ratios between the
central part of the tube and its foot-points and to assess which one
would better explain observed period ratio data. Methods:
Bayesian parameter inference and model comparison techniques were
developed and applied. To infer the parameters, we computed the
posterior distribution for the density gradient parameter that depends
on the observable period ratio. The model comparison involved computing
the marginal likelihood as a function of the period ratio to obtain the
plausibility of each density model as a function of the observable. We
also computed the Bayes factors to quantify the relative evidence for
each model, given a period ratio observation. Results: A
Lorentzian density profile, with plasma density concentrated around the
centre of the tube, seems to offer the most plausible inversion result.
A Gaussian profile would require unrealistically high values of the
density gradient parameter, and a parabolic density distribution does
not enable us to obtain well-constrained posterior probability
distributions. However, our model comparison results indicate that the
evidence points to the Gaussian and parabolic profiles for period ratios
in between 2 and 3, while the Lorentzian profile is preferred for higher
period ratio values. The method we present can be used to obtain
information on the plasma structure along threads, provided period ratio
measurements become widely available.
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