Waves as the source of apparent twisting motions in sunspot penumbrae

TL;DR

This study combines simulations and observations to demonstrate that apparent twisting motions in sunspot penumbrae are caused by transverse oscillations, or kink waves, propagating along penumbral filaments.

Contribution

The paper provides evidence that dark striations in sunspot penumbrae are due to transverse oscillations, linking observed motions to kink wave phenomena in solar filaments.

Findings

Dark lanes are associated with kink oscillations in filaments.

Oscillations have periods of 5-7 minutes and propagate outward and downward.

Observed and simulated features show strong agreement.

Abstract

The motion of dark striations across bright filaments in a sunspot penumbra has become an important new diagnostic of convective gas flows in penumbral filaments. The nature of these striations has, however, remained unclear. Here we present an analysis of small scale motions in penumbral filaments in both simulations and observations. The simulations, when viewed from above, show fine structure with dark lanes running outwards from the dark core of the penumbral filaments. The dark lanes either occur preferentially on one side or alternate between both sides of the filament. We identify this fine structure with transverse (kink) oscillations of the filament, corresponding to a sideways swaying of the filament. These oscillations have periods in the range of 5-7 min and propagate outward and downward along the filament. Similar features are found in observed G-band intensity time series of penumbral filaments in a sunspot located near disk center obtained by the Broadband Filter Imager (BFI) on board {\it Hinode}. We also find that some filaments show dark striations moving to both sides of the filaments. Based on the agreement between simulations and observations we conclude that the motions of these striations are caused by transverse oscillations of the underlying bright filaments.