  Light bridges (LBs) are among the most striking sub-structures in sunspots,
where various activities have been revealed by recent high-resolution
observations from the Interface Region Imaging Spectrograph (IRIS). According
to the variety of physical properties, we classified these activities into four
distinct categories: transient brightening (TB), intermittent jet (IJ), type-I
light wall (LW-I), and type-II light wall (LW-II). In IRIS 1400/1330 {\AA}
observations, TBs are characterized by abrupt emission enhancements, and IJs
appear as collimated plasma ejections with a width of 1-2 Mm at some LB sites.
Most observed TBs are associated with IJs and show superpositions of some
chromosphere absorption lines on enhanced and broadened wings of C II and Si IV
lines, which could be driven by intermittent magnetic reconnection in the lower
atmosphere. LW-I and LW-II are wall-shaped structures with bright fronts above
the whole LB. An LW-I has a continuous oscillating front with a typical height
of several Mm and an almost stationary period of 4-5 minutes. On the contrary,
an LW-II has a indented front with a height of over 10 Mm, which has no stable
period and is accompanied by recurrent TBs in the entire LB. These results
support that LW-IIs are driven by frequent reconnection occurring along the
whole LB due to large-scale magnetic flux emergence or intrusion, rather than
the leakage of waves producing LW-Is. Our observations reveal a highly
dynamical scenario of activities above LBs driven by different basic physical
processes, including magneto-convection, magnetic reconnection, and wave
leakage.
