The flux-gap between bright and dark solar magnetic structures

K G Puschmann,E Wiehr

doi:10.1051/0004-6361:20054049

Abstract

The upper size limit of solar small-scale magnetic flux concentrations (G-band bright points, BP) is reconsidered from speckle-reconstructed images taken at the 1-m SST on La Palma. The size-histogram shows a sharp drop towards 250 km diameter, variation of the noise filter threshold diminishes that value due to segmentation of the elongated structures. A further artificial segmentation of still elongated (i.e. not round) BP indicates that the upper limit may well be below 200 km diameter, corresponding to a flux smaller than 2.5 x 10 17 Mx which is more than 40 times smaller than that of smallest dark (mini-) pores. BP with diameters of 130km would already yield to a flux gap of two orders of magnitude. The drop of BP numbers between the histogram maximum and the 90 km resolution limit achieved is found to depend on the low-pass filtering and is thus probably virtual. Higher spatial resolution data will still increase the flux gap between bright and dark solar magnetic flux concentrations which might be a signature of differently deep rooting in the solar atmosphere.

Highlights

Small-scale magnetic flux concentrations play a dominant role among solar structures – even outside active regions and during the minimum of the solar cycle
A further artificial segmentation of still elongated bright points” (BP) indicates that the upper limit may well be below 200 km diameter, corresponding to a flux smaller than 2.5 × 1017 Mx which is more than 40 times smaller than that of smallest dark pores
At high spatial resolution achieved under permanent operation of the adaptive optics at the 1 m Swedish Solar Telescope (SST), we find only few BP of more than 225 km equivalent “round” diameter

Summary

Introduction

Small-scale magnetic flux concentrations play a dominant role among solar structures – even outside active regions and during the minimum of the solar cycle. The spectral lines of the CH molecule near 430 nm (the “G-band”) offer a unique possibility: magnetic structures appear as “G-band bright points” (BP) in filtergrams (cf., Muller & Roudier 1984; Title & Berger 1996) The observation of these BP through an interference filter of few nm width allows to take a large number of short-exposure images for, e.g., post-facto speckle reconstruction yielding images at a spatial resolution near the telescope’s diffraction limit (Fig. 1). A proper determination of noise, important for the reconstruction, was problematic since single-exposed flat field images were not available and the noise had to be estimated directly from the data This could have affected uncertainties at highest spatial frequencies, which these authors removed by post-facto lowpass filtering. It is not clear whether the number decrease of BP toward small sizes might arise from those restrictions

Observations and data reduction

Pattern recognition

Size distribution of inter-granular magnetic features

Conclusions