  We present the first numerical observation of Lyapunov modes (mode structure
of Lyapunov vectors) in a system maintained in a nonequilibrium steady state.
The modes show some similarities and some differences when compared with the
results for equilibrium systems. The breaking of energy conservation removes a
zero exponent and introduces a new mode. The transverse modes are only weakly
altered but there are systematic changes to the longitudinal and momentum
dependent modes.
