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Reversed granulation in Fe I 7090.4
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Modern Solar Facilities – Advanced Solar Science, 135–138 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c Universitätsverlag Göttingen 2007 Reversed granulation in Fe I 7090.4 Å K. Janssen* and G. Cauzzi INAF Osservatorio Astrofisico di Arcetri, Firenze, Italy * Email: kjanssen@arcetri.astro.it Abstract. The reversal of the photospheric continuum intensity pattern is predicted by numerical simulations and has been observed in the wings of chromospheric lines. Here we present reversed granulation extracted from the photospheric Fe i 7090.4 Å line core intensities, after taking into account the velocity effects, in high spatial, spectral and temporal resolution. We study correlations with the underlying continuum in terms of smoothing, time delays and the temporal evolution. The latter appears to be superimposed with an oscillatory signal, that might suggest the presence of gravity waves in the observed mid-photospheric layers. 1 Introduction and data characteristics The phenomenon of “reversed granulation” is referred to as the partial reversal of the granular /intergranular intensity contrast, that occurs in the mid-photosphere as consequence of a temperature decrease of granules rising in the optically thin layers due to adiabatic expansion , that is only partly cancelled by radiative heating, while an overturning element still experiences radiative heating as it begins to descend (Stein & Nordlund 1998; Cheung et al. 2006). The reversed granulation is studied by means of a 45 minutes time series of goodquality 2D intensity maps acquired in the photospheric Fe i 7090.4 Å line, supplemented by some images in the blue Ca ii 8542 Å line wing. We study a very quiet internetwork region, for which no obvious magnetic structures are visible in the Ca ii line wing. The data were taken with IBIS, Interferometric BIdimensional Spectrometer (Cavallini 2006), a tunable, double Fabry-Perot Interferometer at the Dunn Solar Telescope/NSO on June 2nd, 2004. The FOV shows 80 × 80 of the quiet Sun at disc center on 512×512 pixels with the high order AO with 97 subapertures in 16 wavelength positions along the Fe i 7090.4 Å line. A time series of 141 scans was obtained with an interval of 19 s, thus covering a time span of 45 minutes. Dark and flat field corrections are applied, the variation of the transmission wavelength of IBIS throughout the field of view was taken into account in interpolating all spectral profiles to the same wavelength positions, creating thus monochromatic images. The images were tracked in time and a p-mode filter was applied in terms of cut above cs = 6 km/s in the three-dimensional kx − ky − ω space. 136 K. Janssen and G. Cauzzi: Reversed granulation Figure 1. Intensity maps of the Fe i 7090.4 Å line. Spectral positions of -60 mÅ, -30 mÅ , 0 mÅ (upper row), +60 mÅ and +30 mÅ (lower row) off local line core, plus Ca ii 8542 Å at 0.9 Å in the blue wing (lower right). The Fe i line core image (upper right) presents the ’reversed granulation’, comparable to the one seen in the Ca ii line wing (lower right). 2 Reversed granulation The Fe i 7090.4 Å line clearly shows the transition from granular structures in the wings to reversed granulation in the core (Fig. 1). However, to reveal this phenomenon, it is crucial to take the vertical velocity (including p-mode effects) into account. Photospheric lines usually show inverted intensities in the blue wing of the line, as the velocity Doppler shift renders the brighter but upmoving and thus blueshifted granules darker than the average profile: The blueshifted profiles start to be sampled in the line wing while the average profile still shows nearly continuum intensity. The red line wing then shows granulation pattern with an enhanced contrast for the same reason. In order to extract the reversed granulation, we cancel the velocities for all pixels in the FOV, shifting all profile cores to a common zero wavelength. This results in a symmetric behaviour of the spectral positions and reveals the reversed granulation pattern in the line core as seen in Fig. 1c. 3 Anticorrelation Quasi-simultaneous observations in a Ca ii wing position, basically unaffected by velocity, show the same reversed granulation pattern, but reach higher anti-correlation values with the continuum. The iron line core intensity provides a weak anticorrelation with the continuum of about −0.21, while the correlation between calcium wing and continuum reaches −0.32. For a spatial smoothing up to 0.5 the values found for...