Solar turbulent magnetic fields: surprisingly homogeneous distribution during the solar minimum

¹ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
e-mail: kleint@astro.phys.ethz.ch
² Istituto Ricerche Solari Locarno (IRSOL), 6605 Locarno Monti, Switzerland
³ Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany
⁴ Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Dorfstrasse 33, 7260 Davos Dorf, Switzerland

Received: 25 June 2010
Accepted: 25 August 2010

Abstract

Context. Small-scale, weak magnetic fields are ubiquitous in the quiet solar atmosphere. Yet their properties and temporal and spatial variations are not well known.

Aims. We have initiated a synoptic program, carried out at the Istituto Ricerche Solari Locarno (IRSOL), to investigate both turbulent, mixed-polarity magnetic fields and nearly horizontal, directed fields and their variation with the solar cycle.

Methods. Through spectropolarimetric observations we monitor linear and circular polarization at the solar limb (5″ on the disk) at five positional angles (N, NW, S, SW, W) with the sensitivity of  ~10^-5. In addition, we analyzed measurements taken at different limb distances. We measure signatures in the 5141 Å region including two C₂ triplets and three Fe i lines. Linear polarization in these lines arises from scattering and can be modified via the Hanle effect in the presence of turbulent magnetic fields. Through the application of the differential Hanle effect to the C₂R-triplet line ratios and the use of a simplified line formation model, we are able to infer a strength of turbulent magnetic fields while using the P-triplet to further restrict it. A Zeeman analysis of Fe i Stokes V/I is used to evaluate flux densities of horizontally directed fields.

Results. We conclude that weak fields were evenly distributed over the Sun during this solar minimum. The turbulent field strength was at least 4.7  ±  0.2 G, and it did not vary during the last two years. This result was complemented with earlier, mainly unpublished measurements in the same region, which extend our set to nearly one decade. A statistical analysis of these all data suggests that there could be a very small variation of the turbulent field strength (3σ-limit) since the solar maximum in 2000. The Zeeman analysis of Fe i Stokes V/I reveals weak horizontal flux densities of 3–8 G.

Conclusions. Our results demonstrate the potential of long-term observations of small-scale magnetic fields, which may vary with the solar cycle in both mean strength and spatial distribution. This provides important constraints on the energy budget of the solar cycle. Extending this synoptic program to many spectral lines would provide a sample of heights in the solar atmosphere.