Spectral signatures of bright grains determine chromospheric heating

Chromospheric heating is an important ingredient in the energy budget of the solar atmosphere. Quantifying chromospheric heating from observations is a challenging task because spectral observations suffer from non-LTE effects, and heating events may take place at spatial, temporal, and spectral resolutions difficult to achieve by modern telescopes. By using 3D radiative magnetohydrodynamic simulations of the solar atmosphere combined with non-LTE spectral synthesis of several hundred snapshots, we estimated chromospheric heating from synthetic spectra and studied the spectral and temporal signatures of heating events. We performed k-means clustering on the Mg II h, Ca II H, and Ca II 8542 Å lines to identify representative profiles associated with elevated chromospheric heating and studied their atmospheric stratification. We find that locations with the strongest chromospheric heating show spectral signatures with strong emission, typically the so-called chromospheric bright grains. Profiles with strong emission in the blue wing of the lines (blue grains) are created by upward-propagating shock waves and have an order of magnitude higher heating in the chromosphere than the ambient heating. Profiles with strong emission in the red wing (red grains) also display heating that is an order of magnitude stronger than the baseline, but these spectra do not show a characteristic atmospheric stratification. Spectra classified as blue grains have a consistent temporal evolution, which is an oscillating sawtooth pattern in the line core and emission in the blue wing. However, spectra classified as red grains did not show a consistent temporal signature: Red wing emission from the simulations can appear spontaneously or be associated with an oscillation. While red and blue grain profiles account for around 3% of our synthetic spectra, they account for more than 12% of the total chromospheric heating in these simulations. Around half of the chromospheric heating in the simulations came from locations where Ca II H was in emission. By comparing two quiet Sun simulations, we find that the prevalence of bright grains is influenced by the magnetic field configuration, with a unipolar configuration showing fewer bright grains and consequently a lower share of heating from such events.