Structure of sunspot light bridges in the chromosphere and transition region

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.

12 pages, 9 figures, accepted in A&A

References in corpus (14)

• On the prevalence of small-scale twist in the solar chromosphere and transition region
• Fluid description of multi-component solar partially ionized plasma
• Two components of the coronal emission revealed by EUV spectroscopic observations
• Vigorous convection in a sunspot granular light bridge
• Relation between photospheric magnetic field and chromospheric emission
• The signature of chromospheric heating in Ca II H spectra
• Three-dimensional structure of a sunspot light bridge
• Small-scale chromospheric jets above a sunspot light bridge
• Oscillating light wall above a sunspot light bridge
• Surge-like oscillations above sunspot light bridges driven by magnetoacoustic shocks
• Active region fine structure observed at 0.08 arcsec resolution
• The photospheric environment of a solar pore with light bridge
• Dynamics in Sunspot Umbra as Seen in New Solar Telescope and Interface Region Imaging Spectrograph Data
• Fast inversion of solar Ca II spectra

Cited by in corpus (4)

• The European Solar Telescope
• On the formation height of low-corona and chromospheric channels of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO)
• Various Activities above Sunspot Light Bridges in IRIS Observations: Classification and Comparison
• Sustained heating of the chromosphere and transition region over a sunspot light bridge