EVOLUTION OF PHYSICAL CHARACTERISTICS OF UMBRAL DOTS AND PENUMBRAL GRAINS

Abstract

A time series of full-Stokes spectropolarimetric observations of the sunspot NOAA 10944, acquired with HINODE/SOT in 2007 February, is analyzed. The data were inverted using the code SIR into a series of 34 maps covering 3 hr of umbra and penumbra evolution. The retrieved maps of plasma parameters show the spatial distribution of temperature, line-of-sight velocity, magnetic field strength, and inclination in two different ranges of optical depths corresponding to the low and high photosphere. In these maps, the evolution of central and peripheral umbral dots (CUDs and PUDs) and penumbral grains (PGs) was traced. While CUDs do not show any excess of line-of-sight velocity and magnetic field inclination with respect to the surrounding umbra, upflows of 400 m s–1 and a more horizontal magnetic field are detected in the low photospheric layers of PUDs. PGs have even stronger upflows and magnetic field inclination in the low photosphere than PUDs. The absolute values of these parameters decrease when PGs evolve into PUDs. It seems that PGs and PUDs are of a similar physical nature. Both classes of features appear in regions with a weaker and more horizontal magnetic field and their formation height reaches the low photosphere. On the other hand, CUDs appear in regions with a stronger and more vertical magnetic field and they are formed too deep to detect upflows and changes in magnetic field inclination.