Stability, structure, and evolution of cool loops

Low cool loops have been suggested as a possible source of the excess emission observed in lower transition region lines (T≲2 × 10⁵ K) in the solar atmosphere. We examine their stability, both analytically and numerically. It is found that, using the recent Cook et al. radiative loss function Q(T), which includes a Lyα peak, cool loops are certainly unstable if the equilibria exist at all. In fact, they evolve into hot loops in a matter of minutes. The hypothesis that Lyα losses should be neglected (McClymont & Canfield) is discussed and concluded to be unlikely. However, even if it is valid, we point out that the resulting much used Q ∝ T³ for T ≲ 10⁵ K, which is very favorable for cool loops, is based on incorrect Si losses and should be closer to Q ∝ T. Furthermore, the maximum temperature attainable in cool loops even under these artificially favorable conditions is found to be only around 8 × 10⁴ K, a factor 2-3 too low to explain the excess emission. Finally, we present models relating to the formation of cool loops in cases where the radiation and heating functions are specifically chosen to ensure stability. It is found that, starting the evolution from an isothermal state T = 2 × 10⁴ K, loops always evolve to the hot solution when their initial mass is roughly equal to or less than that of the hot equilibrium solution. More massive loops evolve to the cool state.