On March 9, 1978, a sounding rocket launched from Poker Flat, Alaska, at 2200 LT, made a four-component measurement of a 0.5 Hz hydromagnetic wave as the payload crossed the poleward boundary of a quiet homogeneous auroral arc. An energy flux of ~10-6 W/m2 was observed propagating upward with a left-handed polarization within the arc, and a flux 6 times greater was observed propagating downward with a righthanded polarization on the arc boundary. The waves were identified as shear mode Alfv¿n waves. Various models for the source of the free energy are discussed with the conclusion that the most likely production mechanism was either the electromagnetic or electrostatic Kelvin-Helmholtz instability.