Non-equilibrium of Ionization and the Detection of Hot Plasma in Nanoflare-heated Coronal Loops

Abstract

Impulsive nanoflares are expected to transiently heat the plasma confined in coronal loops to temperatures of the order of 10 MK. Such hot plasma is hardly detected in quiet and active regions, outside flares. During rapid and short heat pulses in rarified loops the plasma can be highly out of equilibrium of ionization. Here we investigate the effects of the non-equilibrium of ionization (NEI) on the detection of hot plasma in coronal loops. Time-dependent loop hydrodynamic simulations are specifically devoted to this task, including saturated thermal conduction, and coupled to the detailed solution of the equations of ionization rate for several abundant elements. In our simulations, initially cool and rarified magnetic flux tubes are heated to 10 MK by nanoflares deposited either at the footpoints or at the loop apex. We test for different pulse durations, and find that, due to NEI effects, the loop plasma may never be detected at temperatures above ~5 MK for heat pulses shorter than about 1 min. We discuss some implications in the framework of multi-stranded nanoflare-heated coronal loops.