A Statistical Study of IRIS Observational Signatures of Nanoflares and Non-thermal Particles

TL;DR

This study analyzes IRIS observations to understand nanoflares and non-thermal particles' effects on the solar atmosphere, revealing their frequency, dynamics, and spectral signatures in active regions.

Contribution

It provides the first statistical analysis of IRIS-detected nanoflares, linking spectral parameters to nanoflare activity and non-thermal particles in active regions.

Findings

Nanoflares are more frequent in flare-productive active regions.

Non-thermal velocities are strong and independent of viewing angle.

Spectral asymmetries indicate highly dynamic motions.

Abstract

Nanoflares are regarded as one of the major mechanisms of magnetic energy release and coronal heating in the solar outer atmosphere. We conduct a statistical study on the response of the chromosphere and transition region to nanoflares, as observed by the Interface Region Imaging Spectrograph (IRIS), by using an algorithm for the automatic detection of these events. The initial atmospheric response to these small heating events is observed, with IRIS, as transient brightening at the footpoints of coronal loops heated to high temperatures (>4 MK). For four active regions, observed over 143 hours, we detected 1082 footpoint brightenings under the IRIS slit, and for those we extracted physical parameters from the IRIS Mg II and Si IV spectra that are formed in the chromosphere and transition region, respectively. We investigate the distribution of the spectral parameters, and the relationship between the parameters, also comparing them with predictions from RADYN numerical simulations of nanoflare-heated loops. We find that these events, and the presence of non-thermal particles, tend to be more frequent in flare productive active regions, and where the hot Atmospheric Imaging Assembly 94 \AA\ emission is higher. We find evidence for highly dynamic motions characterized by strong Si IV non-thermal velocity (not dependent on the heliocentric x coordinate, i.e., on the angle between the magnetic field and the line-of-sight) and asymmetric Mg II spectra. These findings provide tight new constraints on the properties of nanoflares, and non-thermal particles, in active regions, and their effects on the lower atmosphere.