Solar low-lying cool loops and their contribution to the transition region EUV output

Abstract

In the last 30 years, the existence of small and cool magnetic loops (height < 8 Mm, T < 10^5 K) has been proposed and debated to explain the increase of the DEM (differential emission measure) towards the chromosphere. We present hydrodynamic simulations of low-lying cool loops to study their conditions of existence and stability, and their contribution to the transition region EUV output. We find that stable, quasi-static cool loops (with velocities < 1 km/s) can be obtained under different and more realistic assumptions on the radiative losses function with respect to previous works. A mixture of the DEMs of these cool loops plus intermediate loops with temperatures between 10^5 and 10^6 K can reproduce the observed emission of the lower transition region at the critical turn-up temperature point (T ~ 2x10^5 K) and below T = 10^5 K.