Very thin auroral ionization layers of less than 4.5 km extent have been identified in high-resolution EISCAT data during an event of pulsating aurora on December 3, 1986. The auroral forms were recorded by a monostatic TV camera system at the EISCAT site. The structures in the altitude profiles of electron density were found to be thinner than would be expected from collisional energy deposition of a monoenergetic and monodirectional precipitation of electrons. We show that these ionization layers are probably associated with enhanced aurora (see Hallinan et al., 1985). Our measurements suggest that the precipitating electrons lose some of their energy by wave-particle interaction with large-amplitude plasma waves in a confined ionospheric altitude region situated at altitudes around 100--120 km. The ionospheric plasma waves in the electron plasma frequency range might then accelerate ambient ionospheric electrons above the ionization threshold of the neutral gas, leading to enhanced secondary ionization. Such a process would involve high electric field amplitudes. ¿ American Geophysical Union 1989