The signature of chromospheric heating in Ca II H spectra

Abstract

We analyze a 1-hour time series of Ca II H intensity spectra and polarimetric spectra around 630 nm to derive the signature of the chromospheric heating and to investigate its relation to magnetic fields. We derived several characteristic quantities of Ca II H to define the chromospheric atmosphere properties. We study the power of the Fourier transform at different wavelengths and their phase relations. We perform local thermodynamic equilibrium inversions of the data to obtain the magnetic field, once including the Ca spectra. We find that the emission in the Ca II H line core at locations without detectable photospheric polarization signal is due to waves that propagate from low forming continuum layers in the line wing up to the line core. The phase differences of intensity oscillations at different wavelengths indicate standing waves for v < 2 mHz and propagating waves for higher frequencies. The waves steepen into shocks in the chromosphere. On average, shocks are both preceded and followed by intensity reductions. In field-free regions, the profiles show emission about half of the time. The correlation between wavelengths and the decorrelation time is significantly higher in the presence of magnetic fields than for field-free areas. The average Ca II H profile in the presence of magnetic fields contains emission features symmetric to the line core and an asymmetric contribution, where mainly the blue H2V emission peak is increased. We find that acoustic waves steepening into shocks are responsible for the emission in the Ca II H line core for locations without photospheric magnetic fields. We suggest using wavelengths in the line wing of Ca II H, where LTE still applies, to compare theoretical heating models with observations.