Within the MHD approximation, the Alfven mode is the only hydromagnetic wave mode in which the field-aligned current can be nonzero. By means of the two-fluid equations, it is shown that the finite Larmor-radius effect is responsible for the field-aligned currents in the fast and slow modes and for the compressibility in the Alfven mode. These results are useful in understanding the time-dependent field aligned current sheets associated with auroral arcs with thickness down to the ion gyroradius scale. ¿American Geophysical Union 1987