Field - Free Gaps near Τ = 1 in Sunspot Penumbrae ?

The vertical stratification of the magnetic field strength in sunspot penumbrae is investigated by means of spectropolarimetric observations at high spatial resolution from the Hinode spacecraft. Assuming that the magnetic field changes linearly with optical depth we find that, in those regions where the magnetic field is more inclined and the Evershed flow is strongest (penumbral intraspines), the magnetic field can either increase or decrease with depth. Allowing more degrees of freedom to the magnetic field stratification reveals that the magnetic field initially decreases from log τ 5 = −3 until log τ 5 ≃ −1.0, but increases again below that. The presence of strong magnetic fields near the continuum is at odds with the existence of regions void of magnetic fields at, or right below, the τ 5 = 1 level in the penumbra. However, they are compatible with the presence of a horizontal flux-tube-like field embedded in a magnetic atmosphere. 1. introduction It is now widely accepted that the horizontal structure of the sunspot penumbra is composed of two magnetic components (Solanki 2003; Bellot Rubio 2003). One of them possesses a somewhat inclined (≃ 40 − 50 • 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. 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: un-who propose instead that the penum-bra is formed by magnetic field-free plumes (connected to the underlying convection zone) that pierce the penumbral magnetic field from beneath. This is the so-called gappy penumbral model. So long as these two different magnetic structures (weak/horizontal and strong/vertical) have remained spatially (horizontally) unresolved, distinguishing between the uncombed and gappy penumbral scenarios has not been possible. However, with the new spectropolarime-ter on board of the Japanese spacecraft Hinode (Kosugi et al. 2007; Shimizu et al. 2007) it is now possible to obtain high spatial resolution (≃ 0.32 ") observations of the sunspot penumbra. This could be sufficient to distinguish between the uncombed and gappy models, since they both postulate the existence of flux tubes or field-free gaps that are about 200-300 km in diameter (Martínez Pillet 2000; Spruit & Scharmer 2006). This feature is particularly interesting , because the uncombed and gappy models predict very …