The structure of sunspot penumbrae

Abstract

Aims. We study the magnetohydrostatic equilibrium of magnetic flux tubes with circular cross sections embedded in a magnetic surrounding atmosphere. Methods. We solve the static momentum equation in 2.5D to obtain the thermodynamics that are consistent with a prescribed velocity and magnetic fields. Results. We show that force balance is roughly satisfied if the flux tube's magnetic field is aligned with its axis. Equilibrium is guaranteed if this magnetic field possesses a transverse component. Several forms of this transverse field are investigated. The resulting magnetic field configurations are critically reviewed in terms of the results from spectropolarimetric observations. The thermodynamic structure that allows the flux tube to be in mechanical equilibrium is also calculated. We show that the inferred pressure, density, and temperature stratification reproduce intensity features similar to dark core penumbral filaments and penumbral grains.

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

“…Solanki & Montavon 1993;Schlichenmaier & Collados 2002;Müller et al 2002;Bellot Rubio et al 2003, 2004Borrero et al 2005Borrero et al , 2007Borrero 2007;Puschmann et al 2010). The ability of these simplified models to represent observed data (most of which were recorded at low spatial resolution), as well as their apparent consistency with the simulations of Schlichenmaier et al (1998b,a) was interpreted to validate both the theoretical and observationally based models.…”

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.…”

“…The success of the uncombed penumbra model (Solanki & Montavon 1993) in explaining observed Stokes spectra and net circular polarization (NCP) Borrero 2007;Tritschler et al 2007;Bellot Rubio et al 2004, 2003 is unquestionable. Furthermore, the moving tube simulations of Schlichenmaier (2002) make excellent contact with the uncombed penumbra model.…”

Are There Field‐Free Gaps near τ = 1 in Sunspot Penumbrae?

“…One of them possesses a somewhat inclined ('40 Y50 with respect to the vertical direction to the solar surface) and strong ($2000 G) magnetic field, whereas the other is characterized by a weaker and more horizontal one (Lites et al 1993;Rüedi et al 1998;Bellot Rubio et al 2004;Borrero et al 2004Borrero et al , 2005. Traditionally, these two magnetic components have been identified with a horizontal flux tube, that carries the Evershed flow, and is embedded in a more vertical background magnetic field: uncombed model (Solanki & Montavon 1993;Schlichenmaier et al 1998;Borrero 2007). Recently, this view has been challenged by and , who propose instead that the penumbra is formed by magnetic field-free plumes (connected to the underlying convection zone) that pierce the penumbral magnetic field from beneath.…”