The photospheric structure of a solar pore with light bridge

Department of Physics, University of Rome “Tor Vergata”, 00133 Rome, Italy e-mail: francesco.berrilli@roma2.infn.it

Received: 22 May 2007
Accepted: 25 June 2008

Abstract

Context. Pores are among the photospheric features that form when the magnetic field emerges onto the solar surface. In pores or sunspots, light bridges are bright features that separate umbral areas into two or more irregular regions.

Aims. We study the structure of a solar pore (AR10812) with a light bridge.

Methods. We analyzed both broad-band and narrow-band images acquired with the Interferometric BI-dimensional Spectrometer at the adaptive optics channel of the NSO/Dunn Solar Telescope. Narrow-band images acquired in the photospheric Fe I 709.04 nm line were used to determine the line-of-sight velocity field.

Results. The roundish shape of the pore allows us to derive the radial profiles of both intensity and vertical velocity. The pore has a downward velocity, of about –200 m s^-1, and is surrounded by an annular downflow structure with an average velocity of about –300 m s^-1 with respect to the nearby quiet sun. The light bridge shows a long narrow dark structure running along its axis. Corresponding to this dark lane, we measure a weak upflow of about 70 m s^-1, flanked by a downflow of about 150 m s^-1 with respect to the pore. The topology of this velocity structure resembles a convective roll. The anticorrelation between continuum intensity and photospheric velocity may be due to the higher gas pressure in a photospheric field-free cusp, above the light bridge, located between two magnetic walls. We present an analytical model capable of reproducing the observations.