  High resolution spectral observations of the lower solar atmosphere
(chromosphere and transition region) during coronal heating events, in
combination with predictions from models of impulsively heated loops, provide
powerful diagnostics of the properties of the heating in active region cores.
Here we analyze the first coordinated observations of such events with the
Interface Region Imaging Spectrograph (IRIS) and the CHROMospheric Imaging
Spectrometer (CHROMIS), at the Swedish 1-m Solar Telescope (SST), which
provided extremely high spatial resolution and revealed chromospheric
brightenings with spatial dimensions down to ~150km. We use machine learning
methods (k-means clustering) and find significant coherence in the spatial and
temporal properties of the chromospheric spectra, suggesting, in turn,
coherence in the spatial and temporal distribution of the coronal heating. The
comparison of IRIS and CHROMIS spectra with simulations suggest that both
non-thermal electrons with low energy (low-energy cutoff ~5keV) and direct
heating in the corona transported by thermal conduction contribute to the
heating of the low atmosphere. This is consistent with growing evidence that
non-thermal electrons are not uncommon in small heating events (nano- to
micro-flares), and that their properties can be constrained by chromospheric
and transition region spectral observations.
