Expansion of magnetic flux concentrations: a comparison of Hinode SOT data and models

Abstract

Context. The expansion of network magnetic fields with height is a fundamental property of flux tube models. A rapid expansion is required to form a magnetic canopy. Aims. We characterize the observed expansion properties of magnetic network elements and compare them with the thin flux tube and sheet approximations, as well as with magnetoconvection simulations. Methods. We used data from the Hinode SOT NFI NaD 1 channel and spectropolarimeter to study the appearance of magnetic flux concentrations seen in circular polarization as a function of position on the solar disk. We compared the observations with synthetic observables from models based on the thin flux tube approximation and magnetoconvection simulations with two different upper boundary conditions for the magnetic field (potential and vertical). Results. The observed circular polarization signal of magnetic flux concentrations changes from unipolar at disk center to bipolar near the limb, which implies an expanding magnetic field. The observed expansion agrees with expansion properties derived from the thin flux sheet and tube approximations. Magnetoconvection simulations with a potential field as the upper boundary condition for the magnetic field also produce bipolar features near the limb while a simulation with a vertical field boundary condition does not. Conclusions. The near-limb apparent bipolar magnetic features seen in high-resolution Hinode observations can be interpreted using a simple flux sheet or tube model. This lends further support to the idea that magnetic features with vastly varying sizes have similar relative expansion rates. The numerical simulations presented here are less useful in interpreting the expansion since the diagnostics we are interested in are strongly influenced by the choice of the upper boundary condition for the magnetic field in the purely photospheric simulations.

Properties of solar plage from a spatially coupled inversion of Hinode SP data

“…The expansion of the selected MFCs and the models agreed well, supporting previous results reported by Pietarila et al (2010). At the highest inverted layers the field did expand somewhat less rapidly than in the model, however, probably due to the interactions (merging) with other magnetic features.…”

Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra

“…This is evident from the increasing size and "fuzzier" appearance of these concentrations at decreasing optical depths. The variations of B LOS with optical depth both near the centers and boundaries of these flux concentrations are thus consistent with canopy fields expanding with height, as demonstrated indirectly from the center-to-limb variations of the LOS magnetic field of network field by Pietarila et al (2010) and from Sunrise data by Martínez González et al (2012).…”

Synoptic Mapping of Chromospheric Magnetic Flux

“…For convenience we use a numerical flux tube model based on the thin flux tube approximation. Previous works (e.g., Grossmann-Doerth et al 1988;Bruls & Solanki 1995;Solanki et al 1999;Yelles Chaouche et al 2009;Pietarila et al 2010) have found that the expansion of magnetic fields with height in the photosphere is consistent with the thin flux tube approximation (Roberts & Webb 1978;Defouw 1976). Here we wish to see if this is also true in the higher layers sampled by the Ca ii 854.2 line core magnetograms.…”

“…Note that the thin flux tube approximation is strictly valid only when the flux tube radius is smaller than the gas pressure or density scale heights, i.e., both of the constructed models are outside the strict validity range of the approximation but that is not significant for this exploration. As shown in Pietarila et al (2010), the choice of r 0 , and to lesser extent B 0 , are the main factors for the expansion, while the choice of the flux tube interior atmospheric model plays in comparison an insignificant role.…”