Horizontal or vertical magnetic fields on the quiet Sun

Angular distributions and their height variations

¹ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
e-mail: jan@stenflo.org
² Istituto Ricerche Solari Locarno, via Patocchi, 6605 Locarno Monti, Switzerland

Received: 30 March 2013
Accepted: 21 May 2013

Abstract

Different analyses of identical Hinode SOT/SP data of quiet-Sun magnetic fields have in the past led to contradictory answers to the question of whether the angular distribution of field vectors is preferentially horizontal or vertical. These answers have been obtained by combining the measured circular and linear polarizations in different ways to derive the field inclinations. A problem with these combinations is that the circular and linear polarizations scale with field strength in profoundly different ways. Here, we avoid these problems by using an entirely different approach that is based exclusively on the fundamental symmetry properties of the transverse Zeeman effect for observations away from the disk center without any dependence on the circular polarization. Systematic errors are suppressed by the application of a doubly differential technique with the 5247–5250 Å line pair for observations with the ZIMPOL-2 imaging polarimeter on the French THEMIS telescope on Tenerife. For the weakest, intranetwork-type magnetic fields, the angular distribution changes sign with the center-to-limb distance, being preferentially horizontal limbwards of μ (cosine of the heliocentric angle) = 0.2, while favoring the vertical direction inside this disk position. Since decreasing μ corresponds to increasing height of line formation, this finding implies that the intranetwork fields are more peaked around the vertical direction in the low to middle photosphere, while they are more horizontal in the upper photosphere. The angular distribution is however also found to become more vertical with increasing flux density. Thus, all facular points that we have observed have a strong preference for the vertical direction for all disk positions, including those all the way to the extreme limb. In terms of spatial averages weighted by the intrinsic magnetic energy density, these results are independent of telescope resolution.