A Statistical Study of Moss Brightening as a Nanoflare Signature Observed by the IRIS.

We investigated the characteristics of the moss brightengings observed by the Interface Region Imaging Spectrograph (IRIS). The moss is believed to present the footpoint of hot coronal loops, and its transient and tiny brightenings are considered as an observational manifestation of nanoflares, which are regarded as one of the main contributors to coronal heating. Within the dataset captured by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we found numerous small-scale moss brightenings in the four active regions. By using an automatic detection algorithm we were able to determine their location and time, and selected 1082 corresponding pixels with the slit position of the IRIS. From the identified dataset, we extracted various environmental properties and spectral properties from the Si IV 1403 Å, Mg II h&k, and Mg II triplet 2798 Å lines. We observed a higher occurrence of moss brightenings within the flare-productive active region exhibiting a complex magnetic field configuration and hosting hot plasma exceeding 4 MK. Most of the spectral properties are consistent with previous works analyzing a limited number of samples. However, our study detected more smaller-scale events and confirmed higher frequency of smaller brightenings. Overall, they generally exhibit good agreement with results from the RADYN numerical simulation with 13 different models, with the exception of the Si IV nonthermal velocity distribution. We also obtained several observational evidences supporting that the moss brightening regions undergo complex dynamics spanning from lower chromosphere to the transition region (e.g. asymmetric Mg II spectra, multiple velocity component of the Si IV spectrum) which may be caused by nonthermal particles. Our study contributes to a better understanding of the nature of the nanoflares and provides constraints for more realistic numerical simulations.