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Astron. Astrophys. 331, 771-781 (1998)

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Infrared lines as probes of solar magnetic features

XIII. The relative flux in weak and strong quiet-sun magnetic fields

N. Meunier 1, S.K. Solanki 2 and W.C. Livingston 3

1 URA 2080 CNRS, DASOP, Observatoire de Paris, 5 Place Janssen, F-92195 Meudon, France
2 Institute of Astronomy, ETH-Zentrum, CH-8092 Zürich, Switzerland
3 National Solar Observatory, NOAO, P.O. Box 26732, Tucson, AZ 85726, USA

Received 29 May 1997 / Accepted 11 August 1997

Abstract

An estimate of the fraction of magnetic flux in intrinsically weak-field form, i.e. fields with less than 1 kG intrinsic strength, in the quiet Sun is presented. We find that on average approximately 2/3 of the flux is in weak-field form, although our data allow a range of values between 25% and 85%. These estimates have been derived with the help of radiative transfer model calculations from low-noise scans through the quiet Sun at fixed wavelengths within a Zeeman sensitive spectral line at 1.56 µm. They represent the first rapidly modulated polarimetric observations of solar 1.56 µm radiation. These scans show that the polarimetric signature near the core of the line exhibits a very different spatial structure than in the outer line wings. Since the outer part of the line profile is only sensitive to intrinsically strong magnetic fields, while the central part reacts also to intrinsically weak fields, this implies that intrinsically weak fields are distributed differently on the surface than strong fields are (weak-field features lie closer together). The spatial distribution suggests that the strong fields we are observing are network elements, whereas the weak fields are intranetwork features.

Further results, such as the average distance between weak and strong field features, flux distribution, spatial power spectra and the Stokes asymmetry of intrinsically weak fields are also presented. In particular, we find that the flux per magnetic feature is distributed lognormally, in agreement with a similar finding for sunspot umbral areas by Bogdan et al. (1988).

Key words: Sun: magnetic fields – Sun: infrared – polarization

Send offprint requests to: N. Meunier

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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