The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter~*

摘要

Context. Observations made with the spectropolarimeter onboard the HINODE satellite have detected abundant horizontal magneticfields in the internetwork quiet Sun.Aims. We compare the results for the horizontal fields obtained at 630 nm with ground-based observations at 1.56 pm, where thesensitivity to magnetic fields is higher than in the visible.Methods. We obtained 30-s integrated spectropolarimetric data of the quiet Sun on disc centre during a period of extremely stableand good seeing. The data have a rms noise in polarization of around 2 x 10-4 of the continuum intensity. The low noise level allowsthe spectra to be inverted with the SIR code. We compare the inversion results with proxies to determine the magnetic flux.Results. We confirm the presence of the horizontal fields in the quiet Sun internetwork as reported for the satellite data, includingvoids without linear polarization signal that extend over an area of a few granules. Voids in the circular polarization signal are onlyof granular scale. More than 60 of the surface show polarization signals of above four times the rms noise level. We find that thetotal magnetic flux contained in the more inclined to horizontal fields (y > 45°) is lower by a factor of around 2 than that of the lessinclined fields. The proxies for flux determination are strongly affected by the thermodynamic state of the atmosphere, and hence,seem to be unreliable.Conclusions. During spells of good seeing conditions, adaptive optics can render ground-based slit-spectrograph observations at a70-cm telescope equivalent to the seeing-free space-based data of half-meter class telescopes. We suggest that the difference in theratio of horizontal to transversal flux between the ground-based infrared data and the satellite-based visible data is due to the differentformation heights of the respective spectral lines. We emphasize that the true amount of magnetic flux cannot be derived directly fromthe spectra. For purely horizontal flux, one would need its vertical extension that has to be estimated by explicit modeling, using theobserved spectra as boundary conditions, or be taken from MHD simulations. Time-series of the evolution of the magnetic flux andchromospheric diagnostics are needed to address its possible contribution to chromospheric heating.