On the fine structure of sunspot penumbrae

Abstract

In this paper we study the fine structure of the penumbra as inferred from the uncombed model (flux tube embedded in a magnetic surrounding) when applied to penumbral spectropolarimetric data from the neutral iron lines at 6300 Å. The inversion infers very similar radial dependences in the physical quantities (LOS velocity, magnetic field strength etc.) as those obtained from the inversion of the Fe I 1.56 µm lines. In addition, the large Stokes V area asymmetry exhibited by the visible lines helps to constrain the size of the penumbral flux tubes. As we demonstrate here, the uncombed model is able to reproduce the area asymmetry with striking accuracy, returning flux tubes as thick as 100−300 kilometers in the vertical direction, in good agreement with previous investigations.

Modeling the Subsurface Structure of Sunspots

“…Assuming more than one magnetic component or gradients along the line of sight, high-resolution polarimetric studies present evidence for magnetic flux that returns to the surface in the outer penumbra (e.g. Westendorp Plaza et al, 2001;Bellot Rubio, Balthasar, and Collados, 2004;Borrero et al, 2004Borrero et al, , 2006Langhans et al, 2005;Ichimoto et al, 2007Ichimoto et al, , 2009Beck, 2008;Jurčák and Bellot Rubio, 2008), i.e. there is evidence that a fraction of the sunspot magnetic field does not extend into the chromosphere, but submerges beneath the photosphere in the outer penumbra.…”

“…• Bellot Rubio (2003) and Borrero et al (2004Borrero et al ( , 2005Borrero et al ( , 2006: Inverted the polarization profiles from the penumbra and considered the uncombed penumbral model with two magnetic components, where the component with a more horizontal and weaker field harbors the Evershed flow.…”

Recent Evidence for Convection in Sunspot Penumbrae

“…These and other recent high-resolution Stokes data do not provide evidence for flow channels and flux tubes elevated above the photosphere, as discussed in some papers, e.g., Solanki & Montavon (1993); Rimmele (1995); Borrero et al (2006). Based on observations of penumbra magnetic fields, interpretations in terms of convective gaps or flux tubes partly buried below the τ = 1 surface thus are inherently ambiguous.…”

“…These calculations demonstrate consistency between the calculated azimuthal variation of NCP and measurements made at low spatial resolution in visible and near infrared spectral lines. Two-component inversions interpreted within the context of embedded flux tube models and applied to low spatial resolution (0.6-1 arcsec) Stokes data by Borrero et al (2004Borrero et al ( , 2005Borrero et al ( , 2006 similarly were shown to be largely consistent with the assumed inversion (flux tube) model.…”

“…To compensate for this and in order to test the validity of the embedded flux tube (uncombed penumbra) model, various observers, e.g., (Borrero et al 2004(Borrero et al , 2006Bellot Rubio et al 2003 have implemented two-component inversion techniques to interpret Stokes spectra. These investigations show that it is indeed possible to fit the data with the assumed (highly idealized) representations of (2007) with dashed curves showing (top to bottom) optical depths τ of 0.1,1 and 10.…”