In lieu of an abstract, here is a brief excerpt of the content:

Modern Solar Facilities – Advanced Solar Science, 165–168 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c  Universitätsverlag Göttingen 2007 Magnetic properties of G-band bright points C. Beck1,* , K. Mikurda1 , L. R. Bellot Rubio2 , R. Schlichenmaier1 , and P. Sütterlin3 1 Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany 2 Instituto de Astrofı́sica de Andalucı́a (CSIC), Granada, Spain 3 Sterrekundig Instituut, Utrecht, The Netherlands * Email: cbeck@kis.uni-freiburg.de Abstract. Bright points (BPs) visible in the G band at 430 nm are commonly used as tracers of magnetic fields, indicating the location of kG flux concentrations. To study the actual magnetic properties of G-band BPs, we took observations in 2003 and 2005, employing simultaneously a speckle setup in the G band and vector spectropolarimetry to derive the magnetic field vector. From the analysis of the co-aligned polarimetric data we find that the BPs show a broad range of field strengths, magnetic fluxes, and field inclinations. Many G-band BPs are not co-spatial with the central part of the nearby flux concentrations. Even at the small heliocentric angle of only 12◦ , the BPs appear projected on adjacent granules, whereas the magnetic field is concentrated in the intergranular lanes. Our findings support the view that the G-band BPs are a result of the ”hot wall effect”. The downward shift of the optical depth scale in the presence of magnetic fields allows to see deeper and hotter layers in the hot granules next to the field concentrations, where CH dissociates. Thus, information drawn from imaging observations of BPs has limited use to investigate the actual magnetic field structure, when the BPs are not co-spatial with the central part of the flux concentrations. 1 Introduction The solar spectrum around 430 nm shows numerous spectral lines. Many of them are due to absorption by the CH molecule, which forms at low temperature. Images of the photosphere taken with a broadband interference filter in the G band show isolated brightenings located inside and near the intergranular lanes with high contrast to their surroundings. It was found that these bright points (BPs) are caused by the presence of strong magnetic field concentrations . Several authors found the BPs to be co-spatial with the flux concentrations, e.g., Berger & Title (2001). However, BPs are only indirect tracers of the field. The presence of the field leads to a downward shift of the optical depth scale due to the evacuation of the flux concentration . CH dissociates in the deep layers with higher temperature, and hence, the intensity in the G band increases due to the disappearance of the molecular spectral lines. Similar effects take place in the CN band at 388 nm (Zakharov et al. 2005), whereas the BPs visible in chromospheric lines like Hα or Ca II H and K also indicate magnetic fields (Leenaarts et al. 2006), although their origin is probably different. At present, there is an ongoing discussion on which tracer is best suited for the detection of magnetic fields (Zakharov et al. 2005; Leenaarts et al. 2006; Uitenbroek & Tritschler 2006), with Sánchez Almeida (2000) even claiming that indirect tracers are to be preferred over the direct measurements of the field via the Zeeman effect. However, none of the observational studies carried out so far 166 C. Beck et al.: Magnetic properties of G-band bright points Figure 1. Left, counterclockwise: intensity in G-band, intensity at 1.5 μm, mask, polarization degree; 2003. Right, counterclockwise: intensity in G-band, mask, magnetic flux, intensity at 1.5 μm; 2005. Tickmarks are in arcsec. combined simultaneous measurements of G-band intensities (as a proxy of magnetic fields) and vector Stokes polarimetry (for a direct measurement). In this contribution, we present the results of simultaneous G-band proxy magnetometry and vector spectropolarimetry to investigate the relation between G-band BPs and magnetic fields in the solar photosphere. 2 Observations & data analysis For the present study we employ two data sets, taken in 2003 and 2005. On August, 9, 2003, we observed a region close to the sunspot NOAA 10425 from 09:36 to 10:34 UT with the Tenerife Infrared Polarimeter (TIP, Martı́nez Pillet et al. 1999) at 1.5 μm and the POlarimetric LIttrow Spectrograph (POLIS, Beck et al. 2005) at 630 nm attached to the Vacuum Tower Telescope (VTT) on Tenerife. The same area, located at a heliocentric angle of 27◦ , was observed by the Dutch Open Telescope (DOT) on...

Share