Stability of thermal modes in cool prominence plasmas

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. We study the stability of thermal modes in unbounded and uniform plasmas with properties akin to those in solar prominences. Effects of partial ionization are taken into account. Three different parametrizations of the radiative loss function are used. 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 that we consider. 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 about 15 000 K. The obtained linear growth rates indicate that this instability might have a strong impact on the dynamics and evolution of cool prominence condensations.