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

Structure of sunspot light bridges in the chromosphere and transition region