Field-aligned Evershed flows in the photosphere of a sunspot penumbra

Abstract

Abstract.We determine the inclinations of the vector magnetic field and flow velocity in a sunspot penumbra by interpreting full Stokes profiles of three infrared lines observed with the Tenerife Infrared Polarimeter. It is shown that analyses based on one-component atmospheres deliver flow velocities which are more horizontal than the average magnetic field by up to 10 deg. This apparent violation of the concept of frozen-in magnetic fields is solved as soon as two magnetic atmospheres are allowed to coexist in the resolution element. The magnetic field and velocity in the atmospheric component carrying the Evershed flow are found to be aligned to within ±2 deg all the way from the inner to the outer penumbra. This is the first observational confirmation of magnetic fields being frozen into the plasma in sunspots. Our results indicate that sunspot penumbrae can be understood in terms of inclined flux tubes embedded in a more vertical background field. The flux tubes carry most of the Evershed flows and return to the solar surface in the middle penumbra and beyond. The background atmosphere is essentially at rest in the inner penumbra, and harbors small flows in the outer penumbra.

Recent Evidence for Convection in Sunspot Penumbrae

“…Martínez Pillet (2000); Schlichenmaier & Collados (2002); Borrero et al (2007); Borrero (2007); Tritschler et al (2007); Bellot Rubio et al ( , 2003. 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.…”

“…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.…”

Modeling the Subsurface Structure of Sunspots

“…The strongest field within a sunspot is usually associated with the darkest part of the umbra and is generally close to vertical. At the visible outer sunspot boundary it is inclined by 70 -80 • to the vertical (Bellot Rubio et al, 2003a;Mathew et al, 2004) if one applies inversions that assume a constant field inclination in the atmosphere. 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.…”

“…• Observed velocities of the flow typically exceed 6 km s −1 (Rouppe van der Voort, 2002;Bellot Rubio et al, 2003a) in the outer penumbra, but can also exceed 10 km s −1 in localized patches (e.g. Bellot Rubio, Balthasar, and Collados, 2004).…”

“…This has become the dominant theme in interpretations of penumbral structure (e.g. Bellot Rubio et al, 2003b;Bellot Rubio, Balthasar, and Collados, 2004;Borrero, 2007).…”