Striation and convection in penumbral filaments

TL;DR

This paper presents high-resolution observations of penumbral filaments revealing overturning motions and striations aligned with magnetic fields, providing insights into the magnetic and convective interactions in sunspot penumbrae.

Contribution

It offers the first detailed observational analysis linking filament striations to boundary corrugations caused by overturning convection and magnetic field interactions.

Findings

Striations are aligned with local magnetic fields.

Overturning motions are similar to those in small magnetic structures.

Striation wavelength suggests weak horizontal magnetic fields.

Abstract

Observations with the 1-m Swedish Solar Telescope of the flows seen in penumbral filaments are presented. Time sequences of bright filaments show overturning motions strikingly similar to those seen along the walls of small isolated structures in the active regions. The filaments show outward propagating striations with inclination angles suggesting that they are aligned with the local magnetic field. We interpret it as the equivalent of the striations seen in the walls of small isolated magnetic structures. Their origin is then a corrugation of the boundary between an overturning convective flow inside the filament and the magnetic field wrapping around it. The outward propagation is a combination of a pattern motion due to the downflow observed along the sides of bright filaments, and the Evershed flow. The observed short wavelength of the striation argues against the existence of a dynamically significant horizontal field inside the bright filaments. Its intensity contrast is explained by the same physical effect that causes the dark cores of filaments, light bridges and `canals'. In this way striation represents an important clue to the physics of penumbral structure and its relation with other magnetic structures on the solar surface. We put this in perspective with results from the recent 3-D radiative hydrodynamic simulations.