The photospheric environment of a solar pore with light bridge

Abstract

Pores are one of the various features forming in the photosphere by the emergence of magnetic field onto the solar surface. They lie at the border between tiny magnetic elements and larger sunspots. Light bridges, in such structures, are bright features separating umbral areas in two or more irregular regions. Commonly, light bridges indicate that a the merging of magnetic regions or, conversely, the breakup of the area is underway. We investigate the velocity structure of a solar pore (AR10812) with light bridge, and of the quiet solar photosphere nearby, analyzing high spatial and spectral resolution images. The pore area has been observed with the Interferometric BI-dimensional Spectrometer (IBIS) at the Dunn Solar Telescope, acquiring monochromatic images in the Ca II 854.2 nm line and in the Fe I 709.0 nm line as well as G-band and broad-band images. We also computed the Line of Sight (LoS) velocity field associated to the Fe I and Fe II photospheric lines. The amplitude of the LoS velocity fluctuations, inside the pore, is smaller than that observed in the quiet granulation near the active region. We computed the azimuthal average LoS velocity and derived its radial profile. The whole pore is characterized by a downward velocity -200 m/s and by an annular downflow structure with an average velocity of -350 m/s with respect to the nearby quiet sun. The light bridge inside the pore, when observed in the broad-band channel of IBIS and in the red wing of Ca II 854.2 nm line, shows an elongated dark structure running along its axis, that we explain with a semi-analytical model. In the highest resolution LoS velocity images the light bridge shows a profile consistent with a convective roll: a weak upflow, 50-100m/s, in correspondence of the dark lane, flanked by a downflow, -(200-300) m/s.