Structure of sunspot light bridges in the chromosphere and transition region

TL;DR

This study analyzes the multi-layer structure and dynamics of sunspot light bridges using IRIS spectra, revealing their multi-temperature nature, systematic displacements, and coupled plasma properties across the chromosphere and transition region.

Contribution

It provides a comprehensive spectroscopic analysis of 60 sunspot light bridges, highlighting their multi-temperature structure, displacement patterns, and plasma coupling across layers, which was not previously detailed.

Findings

Light bridges show enhanced emission from photosphere to transition region.

LB displacement increases with height, aligning with chromosphere and TR heights.

Spectral lines from LBs are broader and exhibit reduced redshift compared to umbra and quiet Sun.

Abstract

Light bridges (LBs) are elongated structures with enhanced intensity embedded in sunspot umbra and pores. We studied the properties of a sample of 60 LBs observed with the Interface Region Imaging Spectrograph (IRIS). Using IRIS near- and far-ultraviolet spectra, we measured the line intensity, width, and Doppler shift; followed traces of LBs in the chromosphere and transition region (TR); and compared LB parameters with umbra and quiet Sun. There is a systematic emission enhancement in LBs compared to nearby umbra from the photosphere up to the TR. Light bridges are systematically displaced toward the solar limb at higher layers: the amount of the displacement at one solar radius compares well with the typical height of the chromosphere and TR. The intensity of the LB sample compared to the umbra sample peaks at the middle/upper chromosphere where they are almost permanently bright. Spectral lines emerging from the LBs are broader than the nearby umbra. The systematic redshift of the Si IV line in the LB sample is reduced compared to the quiet Sun sample. We found a significant correlation between the line width of ions arising at temperatures from 3x10^4 to 1.5x10^5 K as there is also a strong spatial correlation among the line and continuum intensities. In addition, the intensity-line width relation holds for all spectral lines in this study. The correlations indicate that the cool and hot plasma in LBs are coupled. Light bridges comprise multi-temperature and multi-disciplinary structures extending up to the TR. Diverse heating sources supply the energy and momentum to different layers, resulting in distinct dynamics in the photosphere, chromosphere, and TR.