Up to now, observations had been unable to show conclusively a one-to-one correspondence between perpendicular ion acceleration and a particular type of plasma wave within the O+ source region below 2000 km. In this paper we demonstrate that intense (100--300 mV/m) lower hybrid waves are responsible for transversely accelerating H+ and O+ ions to characteristic energies of up to 6 eV. This wave-particle interaction takes place in thin filamentary density cavities oriented along geomagnetic field lines. The measurements we discuss were conducted in the nightside auroral zone at altitudes between 500 km and 1100 km. Our results are consistent with theories of lower hybrid wave condensation and collapse. ¿ American Geophysical Union 1992