Reversal-free Ca II H profiles: a challenge for solar chromosphere modeling in quiet inter-network

Aims. We study chromospheric emission to understand the temperature stratification in the solar chromosphere. Methods. We observed the intensity profile of the $\ion{Ca}{ii}$ H line in a quiet Sun region close to the disk center at the German Vacuum Tower Telescope. We analyze over 105 line profiles from inter-network regions. For comparison with the observed profiles, we synthesize spectra for a variety of model atmospheres with a non local thermodynamic equilibrium (NLTE) radiative transfer code. Results. A fraction of about 25% of the observed $\ion{Ca}{ii}$ H line profiles do not show a measurable emission peak in $H_{{\rm 2v}}$ and $H_{{\rm 2r}}$ wavelength bands (reversal-free). All of the chosen model atmospheres with a temperature rise fail to reproduce such profiles. On the other hand, the synthetic calcium profile of a model atmosphere that has a monotonic decline of the temperature with height shows a reversal-free profile that has much lower intensities than any observed line profile. Conclusions. The observed reversal-free profiles indicate the existence of cool patches in the interior of chromospheric network cells, at least for short time intervals. Our finding is not only in conflict with a full-time hot chromosphere, but also with a very cool chromosphere as found in some dynamic simulations