Are there field-free gaps near tau=1 in sunspot penumbrae?

TL;DR

This study uses high-resolution spectropolarimetric data to analyze the magnetic field stratification in sunspot penumbrae, revealing complex depth-dependent behavior and challenging the idea of field-free gaps near tau=1.

Contribution

It provides detailed measurements of magnetic field variation with depth in sunspot penumbrae, supporting the flux-tube model over the field-free gap hypothesis.

Findings

Magnetic field can increase or decrease with depth in penumbral intraspines.

Magnetic field initially decreases then increases below tau=1.

Presence of strong magnetic fields near the continuum contradicts field-free gap models.

Abstract

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\tau_5 = -3 until \log\tau_5 \simeq -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 \tau_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.