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

¹ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
e-mail: csasso@oacn.inaf.it
² INAF-Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy

Received: 28 June 2011
Accepted: 25 November 2011

Abstract

Aims. We aim to investigate the increase of the differential emission measure (DEM) towards the chromosphere. In the past 30 years, small and cool magnetic loops (height  ≲ 8 Mm, T ≲ 10⁵ K) have been proposed as an explanation for this effect.

Methods. We present hydrodynamic simulations of low-lying cool loops in which we studied the loops’ conditions of existence and stability, and their contribution to the transition region EUV output.

Results. We find that stable, quasi-static cool loops (with velocities  <1 km s^-1) can be obtained under different and more realistic assumptions on the radiative loss function with respect to previous works. A mixture of the DEMs of these cool loops plus intermediate loops with temperatures between 10⁵ and 10⁶ K can reproduce the observed emission of the lower transition region at the critical turn-up temperature point (T ~ 2 × 10⁵ K) and below T = 10⁵ K.