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High resolution spectral observations of the lower solar atmosphere(chromosphere and transition region) during coronal heating events, incombination with predictions from models of impulsively heated loops, providepowerful diagnostics of the properties of the heating in active region cores.Here we analyze the first coordinated observations of such events with theInterface Region Imaging Spectrograph (IRIS) and the CHROMospheric ImagingSpectrometer (CHROMIS), at the Swedish 1-m Solar Telescope (SST), whichprovided extremely high spatial resolution and revealed chromosphericbrightenings with spatial dimensions down to ~150km. We use machine learningmethods (k-means clustering) and find significant coherence in the spatial andtemporal properties of the chromospheric spectra, suggesting, in turn,coherence in the spatial and temporal distribution of the coronal heating. Thecomparison of IRIS and CHROMIS spectra with simulations suggest that bothnon-thermal electrons with low energy (low-energy cutoff ~5keV) and directheating in the corona transported by thermal conduction contribute to theheating of the low atmosphere. This is consistent with growing evidence thatnon-thermal electrons are not uncommon in small heating events (nano- tomicro-flares), and that their properties can be constrained by chromosphericand transition region spectral observations.

High Resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core