IMAGING SPECTROPOLARIMETRY WITH IBIS. II. ON THE FINE STRUCTURE OFG-BAND BRIGHT FEATURES

Abstract

We present new results from first observations of the quiet solar photosphere performed through the Interferometric BIdimensional Spectrometer (IBIS) in spectropolarimetric mode. IBIS allowed us to measure the four Stokes parameters in the Fe I 630.15 nm and Fe I 630.25 nm lines with high spatial and spectral resolutions for 53 minutes; the polarimetric sensitivity achieved by the instrument is 3 × 10 −3 the continuum intensity level.We focus on the correlation which emerges between G-band bright feature brightness and magnetic filling factor of ∼ 10 3 G (kG) fields derived by inverting Stokes I and V profiles. More in detail, we present the correlation first in a pixel-by-pixel study of a ≃ 3 arcsec wide bright feature (a small network patch) and then we show that such a result can be extended to all the bright features found in the dataset at any instant of the time sequence. The higher the kG filling factor associated to a feature the higher the brightness of the feature itself. Filling factors up to ≃ 35% are obtained for the brightest features. Considering the values of the filling factors derived from the inversion analysis of spectropolarimetric data and the brightness variation observed in G-band data we put forward an upper limit for the smallest scale over which magnetic flux concentrations in intergranular lanes produce a G-band brightness enhancement (≃ 0.1 ′′ ). Moreover, the brightness saturation observed for feature sizes comparable to the resolution of the observations is compatible with large G-band bright features being clusters of sub-arcsecond bright points. This conclusion deserves to be confirmed by forthcoming spectropolarimetric observations at higher spatial resolution.

The continuum intensity as a function of magnetic field

“…Lin (1995); Solanki et al (1996); Lites (2002); Khomenko et al (2003) and the ubiquitous horizontal fields detected with Hinode Ishikawa & Tsuneta 2009) are likely to display a much smaller, possibly unimportant contrast (Schnerr & Spruit 2011). A recent study by Viticchié et al (2010) also suggests that the contrast of disk center G-band bright points, which are associated with magnetic elements, mainly depends on their size while the ∼kG field strength is rather constant. Note that the contrast of magnetic elements also increases with the heliocentric angle (with an eventual maximum, see Solanki 1993;Steiner 2007, for reviews), as progressively more of the hot granular wall limbward of the flux tubes becomes visible ("hot wall effect", see e.g.…”

“…To focus solely on magnetic elements, an alternative method is to segment the bright features in images and compare the average contrast and magnetogram value for each feature separately (as performed by e.g. Viticchié et al 2010;Berger et al 2007). However, the segmentation requires the use of joint high-resolution filtergrams in molecular bands in which the feature contrast is enhanced like the G-band or CN band, first used by Sheeley (1969) and Muller & Roudier (1984, see also Zakharov et al 2007Uitenbroek & Tritschler 2006, for comparative studies of both bands based on observations and simulations, respectively).…”

“…Moreover, the segmentation necessarily relies on some arbitrary thresholds, unlike the pixelby-pixel method. Note that the two methods yield distinct results also because the population of magnetic features they study are different: since the segmentation-based method only selects bright magnetic features, the resulting contrast vs. magnetogram signal curves tend to increase monotonically or reach saturation (Viticchié et al 2010;Berger et al 2007). In contrast, due to the inclusion of all pixels, the contrast curves obtained by the second method rather exhibit a peak followed by a decrease due to larger and darker magnetic features (e.g.…”

Magnetic and velocity fields of a solar pore

“…We then applied the standard reduction pipeline (Viticchié et al 2010) on the spectro-polarimetric data, correcting for the instrumental blueshift (Cavallini 2006) and the instrument-and telescope-induced polarisations. The de-stretching applied to the Fe I 617.33 nm spectro-polarimetric images minimised the degradation of the spectro-polarimetric scan caused by image motion and distortion.…”

On the relation between continuum brightness and magnetic field in solar active regions

“…The simulations basically show a monotonic increase of contrast with field strength , while the observations exhibit a maximum contrast (which may even be negative) at intermediate field strength (or magnetogram signal) and a decrease towards stronger field values (e.g., Title et al 1992;Topka et al 1992;Lawrence et al 1993;Narayan & Scharmer 2010;Kobel et al 2011), even if dark pores are explicitely excluded. On the other hand, if only bright features are considered, observers find increasing brightness (or saturation) with growing field strength (Viticchié et al 2010;Berger et al 2007). This suggests that the pixel-based observational results are heavily affected by instrumental effects resulting from finite telescope aperture, optical abberations, straylight, and atmospheric seeing for ground-based instruments (e.g., Title & Berger 1996).…”