Diagnostic spectroscopy of G-band brightenings in the photosphere of the sun

¹ Kiepenheuer–Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: wolfgang@kis.uni-freiburg.de
² The Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Centre, 10691 Stockholm, Sweden e-mail: kai@astro.su.se
³ National Solar Observatory, PO Box 62, Sunspot, NM 88349, USA e-mail: rimmele@nso.edu

Received: 16 December 2003
Accepted: 18 May 2004

Abstract

A detailed analysis of high-resolution spectra obtained in three different wavelength regions (at 430 nm, 526 nm and 569 nm) of G-band bright points in the solar photosphere is presented. They show an average intensity contrast of 11% with respect to the “quiet” sun reference. The CH lines are weakened in the bright point interior. The atomic lines, too, e.g. the Fe I line at 569.15 nm, weaken in the bright point interior. In contrast thereto, the absorption line of single ionized iron at 526.48 nm remains almost constant between bright point interior and the immediate surroundings. Line-of-sight velocities show a stronger downflow within bright points than in the close environment. A net downflow relative to the intergranular surroundings of around 80 m/s is measured. Filling factors are calculated from a comparison with synthesized spectra for different flux tube models and are used to estimate the “true” velocity in bright points with respect to their immediate surroundings. We obtain up- and downflows in the order of one km s^-1, in agreement with the magneto-convective picture of the formation and dispersal of magnetic flux tubes. From the different behavior of the metallic lines and the CH lines we conclude that the line-weakening process that leads to the G-band bright points is mainly due to hot-wall radiation. This confirms that these bright points are indeed magnetic flux elements.