  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.
