Chromospheric Response to Nanoflares from IRIS ²⁺ Inversion

Abstract Nanoflares are considered one of the key mechanisms for heating the solar corona. Because they are difficult to observe directly, measurements at footpoints of coronal loops in the lower atmosphere can provide insight into their properties. However, the impact of nanoflares, particularly on the chromosphere, remains poorly understood due to the complexity of interpreting optically thick chromospheric spectral lines formed under nonlocal thermodynamic equilibrium conditions. Recently, IRIS 2+ was developed as an efficient inversion tool for the optically thick spectral lines observed with NASA’s Interface Region Imaging Spectrograph (IRIS) mission. This advancement enables more accessible and reliable analysis of chromospheric thermodynamics from observed spectra. In this study, we investigate chromospheric changes associated with nanoflares using IRIS 2+ . We selected 827 pixels of small-scale loop footpoint brightenings associated with coronal heating in the low-energy tail and performed the IRIS 2+ inversion using the observed C ii and Mg ii spectra. Our analysis reveals that the chromosphere and transition region respond almost simultaneously to the nanoflare. We also find indirect evidence of bidirectional flows in the transition region and chromosphere induced by nanoflare events. Furthermore, the chromospheric thermodynamic models from the IRIS 2+ inversions show a broad distribution, suggesting the occurrence of various types of nanoflare events. Among them, the RADYN simulation with relatively low total energy (10 24 erg) and a high cutoff energy (15 keV) nonthermal electron model shows the closest agreement with the peak of the distribution of thermodynamic parameters derived from IRIS 2+ .