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Recent observations with the HINODE satellite have found abundant horizontalmagnetic fields in the internetwork quiet Sun. We compare the results on thehorizontal fields with ground-based observations. We obtained 30 sec-integrateddata of quiet Sun on disc centre during a period of very good seeing. The datahave a rms noise in polarization of around 2 10^-4 of the continuum intensity.The low noise level allowed for an inversion of the spectra. We compare theinversion results with proxies for the determination of magnetic flux. Weconfirm the presence of the horizontal fields in the quiet Sun internetwork,with voids of some granules extent of nearly zero linear polarization signal.Voids in the circular polarization signal are only of granular scale. More than60 % of the surface show polarization signals above four times the rms noiselevel. We find that the total magnetic flux contained in the more inclined tohorizontal fields (gamma > 45 deg) is smaller by a factor of around 2 than thatof the less inclined fields. The proxies for flux determination are seen tosuffer from a strong influence of the thermodynamic state of the atmosphere. Wesuggest that the difference of the ratio of horizontal to transversal fluxbetween the ground-based infrared data and the satellite-based visible data isdue to the different formation heights of the respective spectral lines. Wecaution that the true amount of magnetic flux cannot be derived directly fromthe spectra. For purely horizontal flux, one would need its vertical extensionthat has to estimated by an explicit modeling with the observed spectra asboundary conditions, or has to be taken from MHD simulations.

The magnetic flux of the quiet Sun internetwork as observed with the Tenerife Infrared Polarimeter