Roberto Soler, Jose Luis Ballester, Susanna Parenti
Context: Magnetohydrodynamic thermal modes may play an important role in the
formation, plasma condensation, and evolution of solar prominences. Unstable
thermal modes due to unbalance between radiative losses and heating can lead to
rapid plasma cooling and condensation. An accurate description of the radiative
loss function is therefore crucial for this process. Aims: We study the
stability of thermal modes in unbounded and uniform plasmas with properties
akin to those in solar prominences. Effects due to partial ionization are taken
into account. Three different parametrizations of the radiative loss function
are used. Methods: By means of a normal mode analysis, we investigate linear
nonadiabatic perturbations superimposed on the equilibrium state. We find an
approximate instability criterion for thermal modes, while the exact linear
growth rate is obtained by numerically solving the general dispersion relation.
The stability of thermal disturbances is compared for the three different loss
functions considered. Results: Using up-to-date computations of radiative
losses derived from the CHIANTI atomic database, we find that thermal modes may
be unstable in prominences for lower temperatures than those predicted with
previously existing loss functions. Thermal instability can take place for
temperatures as low as 15,000 K, approximately. The obtained linear growth
rates indicate that this instability might have an important impact on the
dynamics and evolution of cool prominence condensations.
View original:
http://arxiv.org/abs/1201.4668
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