Vertical flows and mass flux balance of sunspot umbral dots

TL;DR

This study uses a new inversion technique to analyze sunspot umbral dots, revealing temperature increases, magnetic weakening, and mass flux balance through upflows and downflows in deep photospheric layers.

Contribution

It introduces a novel Stokes inversion method that minimizes telescope point spread effects, providing detailed depth-dependent properties of umbral dots.

Findings

Significant temperature enhancements in UD cores

Magnetic field weakenings in deep layers

Detection of systematic downflows balancing upflows

Abstract

A new Stokes inversion technique that greatly reduces the effect of the spatial point spread function of the telescope is used to constrain the physical properties of umbral dots (UDs). The depth-dependent inversion of the Stokes parameters from a sunspot umbra recorded with Hinode SOT/SP revealed significant temperature enhancements and magnetic field weakenings in the core of the UDs in deep photospheric layers. Additionally, we found upflows of around 960 m/s in peripheral UDs (i.e., UDs close to the penumbra) and $\approx$ 600 m/s in central UDs. For the first time, we also detected systematic downflows for distances larger than 200 km from the UD center that balance the upflowing mass flux. In the upper photosphere, we found almost no difference between the UDs and their diffuse umbral background.