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

¹ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: [rrezaei;bruls;wolfgang;schliche]@kis.uni-freiburg.de
² Instituto de Astrofísica de Canarias (IAC), Via Láctea, E 38 205, La Laguna, Spain e-mail: cbeck@iac.es
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA e-mail: wolf@cfa.harvard.edu

Received: 12 November 2007
Accepted: 11 March 2008

Abstract

Aims. We study chromospheric emission to understand the temperature stratification in the solar chromosphere.

Methods. We observed the intensity profile of the  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  H line profiles do not show a measurable emission peak in and  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.