Quiet-Sun inter-network magnetic fields observed in the infrared

Abstract

Abstract. This paper presents the results of an investigation of the quiet Sun's magnetic field based on high-resolution infrared spectropolarimetric observations obtained with the Tenerife Infrared Polarimeter (TIP) at the German VTT of the Observatorio del Teide. We observed two very quiet regions at disc centre. The seeing was exceptionally good during both observing runs, being excellent during one of them. In both cases the network was intentionally avoided to the extent possible, to focus the analysis on the characteristics of the weak polarization signals of the inter-network regions. We find that the Stokes V profile of Fe  15648 Å line in almost 50% of the pixels and Stokes Q and/or U in 20% of the pixels have a signal above 10 −3 (in units of continuum intensity I c ), which is significantly above the noise level of 2−3 × 10 −4 . This implies that we detect fluxes as low as 2 × 10 15 Mx/px. We find evidence that we have detected most of the net flux that is in principle detectable at 1 resolution with the Zeeman effect. The observed linear polarization resulting from the transverse Zeeman effect indicates that the magnetic fields have a broad range of inclinations, although most of the pixels show polarization signatures which imply an inclination of about 20• . Nearly 30% of the selected V-profiles have irregular shapes with 3 or more lobes, suggesting mixed polarities with different LOS velocity within the resolution element. The profiles are classified using a single value decomposition approach. The spatial distribution of the magnetic signal shows that profiles of different classes (having different velocities, splitting, asymmetries) are clustered together and form patches, close to the spatial resolution in size. Most of the field is found to be located in intergranular lanes. The statistical properties of the mainly inter-network field sampled by these observations are presented, showing that most of the observed fields are weak with relatively few kG features. The field strength distribution peaks at 350 G and has a FWHM of 300 G. Other parameters, such as profile asymmetries, filling factors and line-of-sight velocities are also determined and discussed.

Understanding internetwork magnetic fields as determined from visible and infrared spectral lines

“…This is true even if several magnetic fibrils are present in the resolution element, giving strong support to the observational PDFs obtained by Lin (1995), , and Khomenko et al (2003). -Visible lines are very sensitive to noise when dealing with low-amplitude polarization signals such as those found in internetwork regions.…”

“…We take B 0 = 250 G as indicated by the infrared measurements of Collados (2001) and Khomenko et al (2003).…”

“…Cancellation of Stokes V signals due to mixed polarities in the Send offprint requests to: L. R. Bellot Rubio, e-mail: lbellot@kis.uni-freiburg.de resolution element may also be important. Socas-Navarro & Sánchez Almeida (2002) and Khomenko et al (2003), for example, report that 35% and 30% of the spatial points showing magnetic signal in their observed maps require mixed polarities. This explains why internetwork fields remain elusive.…”

“…Analyses based on visible lines suggest that the internetwork contains a sizeable fraction of fields in the kG range (Grossmann-Doerth et al 1996;Sánchez Almeida & Lites 2000;Socas-Navarro & Sánchez Almeida 2002;Domínguez Cerdeña et al 2003; but see Lites 2002). Analyses based on infrared spectral lines, on the contrary, indicate that strong fields are very rare and that weak fields dominate the PDF (Lin 1995;Khomenko et al 2003). The particular scenario favored by each author does not seem to depend on the data employed (spectrograph or narrow-band filter observations), on the way the PDF is obtained (through Gaussian fitting, Stokes V amplitude fitting, or profile inversion), or on the assumptions regarding the model atmosphere (one or more components in the resolution element), but only on the wavelength of the spectral lines utilized.…”

Distribution functions for magnetic fields on the quiet Sun

“…With all the complications arising from processes happening in the domain that is not resolved, it is not surprising that earlier attempts to derive empirical probability density functions or PDFs (Stenflo & Holzreuter 2002, 2003aKhomenko et al 2003;Domínguez Cerdeña et al 2006;Martínez González et al 2008b) have not yet led to a coherent picture. When it comes to the derivation of the angular distribution of field vectors, the results are very contradictory.…”