Evidence of convective rolls in a sunspot penumbra

TL;DR

This study provides observational evidence of convective rolls in sunspot penumbral filaments, revealing their magnetic and velocity structure, and suggesting they play a key role in energy transport within the penumbra.

Contribution

The paper presents high-resolution observations confirming the existence of convective rolls with horizontal magnetic fields in sunspot penumbrae, advancing understanding of penumbral dynamics.

Findings

Detection of lateral motions in penumbral filaments consistent with convective rolls.

Measurement of upflows (~1.1 km/s) on limbward sides of filaments.

Identification of weaker, more horizontal magnetic fields in filaments with convective rolls.

Abstract

aims: We study the recently discovered twisting motion of bright penumbral filaments with the aim of constraining their geometry and the associated magnetic field. methods: A large sunspot located $40\degr$ from disk center was observed at high resolution with the 1-m Swedish Solar Telescope. Inversions of multi-wavelength polarimetric data and speckle reconstructed time series of continuum images were used to determine proper motions, as well as the velocity and magnetic structure in penumbral filaments. results: The continuum movie reveals apparent lateral motions of bright and dark structures inside bright filaments oriented parallel to the limb, confirming recent Hinode results. In these filaments we measure upflows of $\approx 1.1 \mathrm{km/s}$ on their limbward side and weak downflows on their centerward side. The magnetic field in them is significantly weaker and more horizontal than in the adjacent dark filaments. conclusions: The data indicate the presence of vigorous convective rolls in filaments with a nearly horizontal magnetic field. These are separated by filaments harbouring stronger, more vertical fields. Because of reduced gas pressure, we see deeper into the latter. When observed near the limb, the disk-centerward side of the horizontal-field filaments appear bright due to the \textit{hot wall} effect known from faculae. We estimate that the convective rolls transport most of the energy needed to explain the penumbral radiative flux.