Investigation of weak solar magnetic fields

New observational results for the SrI 460.7 nm linear polarization and radiative transfer modeling

¹ Département Cassini, UMR 6529, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice, France
² UMR 5572, Observatoire Midi-Pyrénées, 14 avenue Édouard Belin, 31400 Toulouse, France

Corresponding author: M. Faurobert, faurob@obs-nice.fr

Received: 7 June 2001
Accepted: 3 August 2001

Abstract

Scattering polarization measurements were obtained with THEMIS in July 2000, close to the solar south Pole and to the east Equator and in a period of maximum solar activity. Using the THEMIS multi-lines spectro-polarimetric mode (MTR), we observed simultaneously four spectral domains containing the 460.7 nm Sr i line, several molecular lines around 515.9 nm and the Na i D₁ and Na i D₂ lines. This allows us to scan different altitudes in the solar atmosphere at the same time and provides us with a large set of constraints to study the behaviour of the magnetic field. This paper is devoted to the Sr i line which exhibits quite a strong linear polarization peak outside active regions. A detailed radiative transfer modeling is performed in order to interpret the observed center-to-limb variations of the line intensity and polarization. It was shown previously (Faurobert-Scholl [CITE]) that this line, which is sensitive to the Hanle effect, can be used as a diagnostic tool for the presence of weak turbulent magnetic fields in the solar photosphere outside active regions. The line polarization rates that we measured in July 2000 are 25% lower than what has been reported previously, for observations near the minimum, or in the increasing phase, of the activity cycle (Stenflo et al. [CITE]). They are in agreement with other observations performed with a different observational set-up in August 2000 (Bommier & Molodij [CITE]). We show that they are consistent with the presence of a weak turbulent magnetic field with an average strength between 20 G and 30 G in the upper solar photosphere. This is about twice the value which was derived from previous observations. This result raises the possiblity of a long-term variation of the turbulent photospheric magnetic field with the activity cycle.