Investigating the Response of Loop Plasma to Nanoflare Heating Using RADYN Simulations

Abstract

We present the results of 1D hydrodynamic simulations of coronal loops that are subject to nanoflares, caused by either in situ thermal heating or nonthermal electron (NTE) beams. The synthesized intensity and Doppler shifts can be directly compared with Interface Region Imaging Spectrograph (IRIS) and Atmospheric Imaging Assembly (AIA) observations of rapid variability in the transition region (TR) of coronal loops, associated with transient coronal heating. We find that NTEs with high enough low-energy cutoff (${E}_{{\rm{C}}}$) deposit energy in the lower TR and chromosphere, causing blueshifts (up to ~20 km s−1) in the IRIS Si iv lines, which thermal conduction cannot reproduce. The ${E}_{{\rm{C}}}$ threshold value for the blueshifts depends on the total energy of the events (≈5 keV for 1024 erg, up to 15 keV for 1025 erg). The observed footpoint emission intensity and flows, combined with the simulations, can provide constraints on both the energy of the heating event and ${E}_{{\rm{C}}}$. The response of the loop plasma to nanoflares depends crucially on the electron density: significant Si iv intensity enhancements and flows are observed only for initially low-density loops (<109 cm−3). This provides a possible explanation of the relative scarcity of observations of significant moss variability. While the TR response to single heating episodes can be clearly observed, the predicted coronal emission (AIA 94 Å) for single strands is below current detectability and can only be observed when several strands are heated closely in time. Finally, we show that the analysis of the IRIS Mg ii chromospheric lines can help further constrain the properties of the heating mechanisms.