Spectropolarimetry of Sunspot Penumbrae -- A Comprehensive Study of the Evershed Effect Using High Resolution Data from the Space-Borne Solar Observatory HINODE

TL;DR

This study uses high-resolution HINODE spectropolarimetric data to analyze the small-scale magnetic and velocity structures in sunspot penumbrae, revealing detailed flow patterns, magnetic field configurations, and their relation to the Evershed effect.

Contribution

It provides a comprehensive analysis of penumbral magnetic fields and flows, confirming flux-tube models and revealing magnetic polarity in Evershed sinks using high-resolution observations.

Findings

Penumbral velocity fields differ significantly from quiet Sun.

Evershed flows originate from elongated upflow channels and sink in round downflows.

Magnetic fields in Evershed sinks have opposite polarity.

Abstract

High resolution spectropolarimetric data obtained by HINODE is used to characterize the small-scale penumbral magnetic field as well as the vertical and horizontal component of the Evershed effect (EE). The results demonstrate that the penumbral velocity field differs significantly from that of the quiet Sun. Morphological studies yield elongated upflow channels in the inner penumbra and round downflows in the outer penumbra. These flows are identified as the sources and the sinks of the EE. Remarkable is the high plasma velocity in these sinks, which is much larger when compared to the quiet Sun. Furthermore, an extraordinary high zenith angle was found for the penumbral downflows. The small-scale velocity field within penumbral filaments is investigated both by statistics and case studies. The outcome of these surveys confirms the predictions of penumbral flux-tube models. Additionally, bright penumbral downflows as well as two families of penumbral filaments are studied. Observations of sunspots close to the solar limb are used to review the horizontal component of the EE. Asymmetries of Stokes profiles prove that the penumbral plasma flow is concentrated in the deep photosphere and that its amplitude diminishes much faster with height when compared to the quiet Sun. Furthermore, the magnetized character of the horizontal and the vertical EE is unequivocally confirmed. In contrast to previous studies, this analysis proves that the sinks of the EE are filled with a magnetic field of opposite polarity. Finally, the influence of atmospheric parameters on the asymmetries of Stokes profiles is explored within the framework of a two-layer model atmosphere and by means of spectral inversion. Interestingly, it is only the polarity of the gradients with height of the magnetic field strength that causes the sign of the total net circular polarization in the center side penumbra.