The quiet Sun magnetic field: statistical description from THEMIS observations^⋆

LESIA, Observatoire de Paris, CNRS-INSU-UMR8109, UPMC Univ. Paris 06, Université Paris Diderot – Paris 7, 5 Place Jules Janssen, 92190 Meudon, France
e-mail: V.Bommier@obspm.fr

Received: 15 September 2010
Accepted: 30 March 2011

Abstract

Context. The pioneer interpretation of Hanle-effect depolarization observed at the limb was the one of a turbulent nature for the solar internetwork photospheric magnetic field. In the present paper, we propose an alternative interpretation of these measurements, which are now complemented with Zeeman-effect observations to allow one to conclude on the properties of weak fields.

Aims. In a previous paper, we presented an internetwork field diagnostic from data obtained with the ZIMPOL II polarimeter mounted on the THEMIS telescope. In the present paper, we present the results obtained with the THEMIS polarimeter with ten times more pixels. The agreement between the results obtained with the two polarimeters confirm the goodness of both the THEMIS polarimeter measurements and our data treatment.

Methods. The Zeeman-effect measurements are interpreted via a 2-component model, which has a field-free component and a magnetized one, filling a fraction of the analyzed pixel equal to 1 − α and α, respectively. We determine this α parameter independently of the magnetic inversion. We already applied these methods to the previous interpretation of the ZIMPOL/THEMIS data.

Results. The magnetic field strength and magnetic field inclination are not independent, because the strongest fields are more vertical and the weakest fields are more horizontal, both in ZIMPOL/THEMIS and THEMIS/THEMIS data.

Conclusions. This suggests that the photospheric internetwork field has the structure of scattered narrow fluxtubes consisting in a vertical field, which weakens in opening – widening with individual field line bending – with height. The weakest fields then have a 2D horizontal structure, instead of the usually admitted 3D turbulent one. This does not contradict the previous Hanle-effect observations, because it is insensitive to vertical fields. The number of fluxtubes inside each pixel (which was 0.21 arcsec for the THEMIS/THEMIS and 0.53 arcsec for the ZIMPOL/THEMIS data) should, however, remain small. We could make a conclusion about this after observing a non-zero spatial correlation of the magnetic field orientation, with an estimated correlation length of 250 km.