Dynamics of the solar photosphere with IBIS - I. Reversed intensity structure in the mid-photosphere

Context. We investigate the quiet solar photosphere with the new Interferometric BI-dimensional Spectrometer ( IBIS), recently installed at the Dunn Solar Telescope of the National Solar Observatory, Sacramento Peak, NM. Aims. We study the occurrence of reverse granulation in the mid-photosphere, and its properties. Methods. IBIS, a double Fabry-Perot system with a narrow passband of 20m angstrom at 6000 angstrom, was used to acquire a one hour sequence of 2D scans in the photospheric Fe I 7090.4 angstrom and Fe II 7224.5 angstrom lines, and in the chromospheric Ca II 8542 angstrom line. Photospheric velocities are measured for statistical study. To visualize the reversed intensity structure, the influence of velocity on the steep photospheric profiles is taken into account. The correlation between mid-photospheric intensities and continuum is then studied, considering the effects of spatial smoothing and time delay. The temporal evolution of the cross-correlation coefficients is analyzed. Results. The distribution of photospheric velocities is consistent with values in the literature, thus confirming the reliability of the instrument for this kind of study. "Reversed granulation" is clearly visible in the Fe I 7090.4 angstrom line center, formed at about 200 km. It provides a weak anti-correlation with the continuum, as expected from numerical simulations. Images acquired in the far wings of the Ca II line display the same pattern with a slightly higher correlation. In both cases a spatial smoothing larger than 0.5 '' decreases the level of anti- correlation, while a time delay of about 2 min increases it. Together, these facts suggest that the phenomenon is mainly due to a reversal of temperature fluctuations between granules and intergranules, occurring in the mid-photosphere. The temporal evolution of the anti- correlation between the Fe I line center and the continuum reveals a clear oscillatory signal that we consider as highly suggestive of the presence of gravity waves developing from the mid-photosphere outwards.