Improved SOT (Hinode mission) high resolution solar imaging observations: 2—Photometric properties of sunspot umbral dots

Abstract

The origin and evolution of solar sunspots in deep photospheric layers are not yet well understood. The case of a quasi-symmetric single mature sunspot near the solar centre is selected for analysis. We use the best available observations of the partial Sun free of turbulent Earth atmospheric effects from the Solar Optical Telescope (SOT) onboard the Hinode spacecraft, after greatly improving the resolution with an optimum Max-likelihood deconvolution with the Point Spread Function (PSF) deduced in a preceding paper. For several different images both the smearing due to the instrumental diffraction effects (PSF core) and the large angle stray light are removed. The selected iterative processing depends on both the signal/noise ratio and on the desired contrast of the ultimate details under examination. The photometric properties of bright umbral dots (BUDs) are deduced from corrected frames. Calibrated isophote maps are provided to show the intensity variations around each UD across the background umbra and the surrounding photospheric field, including the penumbra. We deduce the typical photometrical properties of bright UDs that populate the whole umbral surface down to sub-pixel scales of 0.05448″. The analysis demonstrates the basic heterogeneous nature of the umbra, similar to a network of minute bright and dark round or elongated cells with a spacing of order of 0.35″. For the first time a complete and detailed map of the color index and temperature deduced from the analysis of deeply corrected continuum images is provided, showing that tiny bright UDs can reach photospheric temperatures and even higher for the peripheral BUDs. In the umbra, there are some very dark small regions with temperatures as low as 3100 K. Close links seemingly exist with bright UDs. Central BUDs and peripheral BUDs are found to have similar properties but significantly different contrast values. Photometric analysis shows a large dispersion that reflects the broad range of dynamical phenomena involved in the umbra and its periphery, suggesting that the small scale activity observed in the deepest measured photospheric layers of the umbra determines the more impressive rapid phenomena observed in higher layers.