In lieu of an abstract, here is a brief excerpt of the content:

Modern Solar Facilities – Advanced Solar Science, 157–160 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c  Universitätsverlag Göttingen 2007 Internetwork magnetic fields M. J. Martı́nez González1,* , M. Collados2 , and B. Ruiz Cobo2 1 Laboratoire de l’Étude du Rayonnement et de la Matière en Astrophysique, Observatoire de Paris-Section de Meudon, Meudon, France 2 Instituto de Astrofı́sica de Canarias, La Laguna, Spain * Email: Marian.Martinez@obspm.fr Abstract. Spectropolarimetric observations of the internetwork obtained in the 1.56 μm Fe i doublet are used to calculate the distributions of magnetic field strength, filling factor and magnetic flux density at different positions on the solar surface. We go one step further and describe what the observations show about the three-dimensional structure of the magnetic fields at the internetwork quiet Sun. 1 Our knowledge of the magnetic field in the internetwork . . . ... has evolved rapidly in recent years. Nowadays, concerning the Zeeman effect, the most reliable works of internetwork magnetic fields deal with the infrared Fe i doublet situated at 1.56 μm (Khomenko et al. 2003; Sánchez Almeida et al. 2003; Domı́nguez Cerdeña et al. 2006; Martı́nez González et al. 2007) or with spectral lines with hyperfine structure. López Ariste et al. (2006) use the visible Mn i at 553.7 nm and Asensio Ramos et al. (2007) analyze the Mn i line situated at 1.53 μm, in the near infrared part of the spectrum. All these works are based on spectropolarimetric observations at a spatial resolution of the order of 1 . They agree on the sub-kG nature of the magnetic features. They also find that these occupy a very small portion of the resolution element, between 1% and 10%. The rest of the pixel seems to be field free, at least concerning the Zeeman data. However, recently, some works (Faurobert et al. 2001; Trujillo Bueno et al. 2004) present the Hanle effect as a very promising diagnostic technique to study the turbulent component of the photospheric magnetic field that would be undetectable by the Zeeman effect due to cancellations of circular polarization signals. In this contribution, we present the results of a study of the distribution of field strengths and the topology of magnetic features at these low flux internetwork regions. To this aim we use spectropolarimetric observations taken at different positions of the solar disc. On July 2000 we recorded the four Stokes parameters of the Fe i 1.56 μm lines scanning the solar photosphere at disc centre and at positions μ = 0.88, 0.4 and 0.28 (with μ the cosine of the heliocentric angle). The same spectral lines were used for an observing run at disc centre on August 2003. All data sets have a spatial resolution of the order of 1 . The last set has the lowest noise level in polarization: 5 × 10−5 Ic, Ic being the continuum intensity. It was achieved using a Principal Components Analysis (PCA) procedure to filter the data. The noise level in the data from the year 2000, also after denoising with PCA, is 7 × 10−5 Ic for the observation at disc centre, 9 × 10−5 Ic for the set at μ = 0.88 and 10−4 Ic for both scans at μ = 0.4 and μ = 0.28. 158 M. J. Martı́nez González et al.: Internetwork magnetic fields Figure 1. Magnetic field strength distribution at disc centre (left panel) and for granular (solid) and intergranular (dotted) regions (centre panel). Magnetic flux density distribution at disc centre (right panel), average values within each pixel. In order to analyze the data we selected only those points where the Stokes V signal was regular (presented only two lobes) and showed a Stokes V amplitude above 5×10−4 Ic. With these requirements, 29 % of the observed profiles in the 2003 data were selected, while 48%, 47%, 59% and 64% were selected in the 2000 sets at disc centre for μ = 0.88, 0.4 and 0.28, respectively. We inverted the full Stokes vector using the SIR1 code under the hypothesis of a magnetic atmosphere and a field free one mixed in the resolution element. The spatial distribution of the magnetic field strength at the internetwork region observed in 2003 is plotted in the left panel of Fig. 1. In accordance with previous works, the field strengths are in the range of a few hundred Gauss. The histogram reveals two di...

Share