The Spatial Structure of the Evershed effect

TL;DR

This study reveals that the Evershed effect's line profile asymmetry abruptly vanishes at the outer sunspot boundary within 500 km, challenging the traditional 'canopy' model and indicating a sharp energy equipartition boundary.

Contribution

It provides new observational evidence that the Evershed effect disappears sharply at the sunspot boundary, suggesting a different magnetic and flow structure than previously thought.

Findings

Line profile asymmetry vanishes at the outer penumbral boundary

Disappearance occurs over less than 500 km

Supports a sharp energy equipartition boundary hypothesis

Abstract

The line profile asymmetry defining the Evershed effect in sunspot penumbrae, disappears abruptly at the outer sunspot boundary over a horizontal distance of less than 500 km immediately at the outermost ends of the dark penumbral continuum structures ('filaments'). This is indicated from the non-magnetic lines Ni I 5435.9 (g=0.5) and Fe I 5434.5 (g=0), which loose their profile asymmetries at the same spatial location although formed at a vertical height distance of 300 km. The widely accepted 'canopy' picture conflicts with this finding which, in contrary, suggests that the gas motion associated with the Evershed effect disappears together with the continuum intensity drop at the outer sunspot (penumbral) border. Corresponding downflows must then be smaller than the spatial resolution presently achieved (i.e., <0.5 arcsec) and located at the immediate outer sunspot border. It is suggested that this location marks the sharp threshold of the equipartition between kinetic and magnetic energy density at the outer penumbral boundary.