Comparative Study of a Sunspot at Two Different Instances of Time

Abstract

We investigate the evolutionary effects on the brightness of a sunspot as well as on the properties of its fine-structures using two sets of time series of G-band images of a single sunspot in NOAA 10944 recorded at two symmetric locations on the solar disc by Hinode/SOT (Solar Optical Telescope). The second time series (phase W) was recorded 2.5 d after recording the first time series (phase E). Both time series of images were corrected for instrumental stray light; then the p-mode oscillations were removed. Our analysis demonstrates that the spatially and temporally averaged intensities of the umbra as well as the penumbra in both phases are practically the same. Nevertheless, considering only intensities smaller than 1.0$\mbox{I}_{\mathrm{ph}}$ the penumbra in the phase W is brighter with a contrast of 2 per cent. However, the ratio of the peak intensities of penumbral grains to their background intensities decreases, on average, from 2.9 to 2.6 from phase E to phase W. The penumbra is on average, 6.9 times brighter than the umbra in both phases. The umbra gets smaller in size with a ratio of 0.90 and its magnetic field strength decreases about 200 G after the 2.5 d evolution. Although the central umbral dots (UDs) with lifetimes longer than 2 min are by the factor of 1.6 more frequent in phase E, the most of the physical and kinematic properties of UDs do not show considerable changes after the sunspot evolution at this 2.5 d time interval. The registered UDs have intensities smaller than 0.84$\mbox{I}_{\mathrm{ph}}$ with a mean intensity of about 0.29$\mbox{I}_{\mathrm{ph}}$. The equivalent diameters of the individual UDs have a symmetric distribution around 180 km with a standard deviation about 35 km. The brightest UDs show a constant median diameter. The UDs registered in both phases have a mean lifetime of 7.4 min. Our results suggest that UDs with a certain lifetime have an admissible lower limit for their mean size which grows with lifetime. The proper motions of UDs show velocities less than $1.0~\mbox{km}\,\mbox{s}^{-1}$ with a maximum population around $0.2~\mbox{km}\,\mbox{s}^{-1}$. The UDs formed in the phase W are faster when the umbral area is smaller and the magnetic field is weaker.