We present a model of type II radar echoes associated with Farley-Buneman instability in the auroral E region. The plasma waves responsible for broad-type auroral radar echoes are described as secondary ion acoustic structures driven by the electric fields of the primary Farley-Buneman waves. Numerical solutions of a one-dimensional model of the secondary structures exhibit density irregularities advecting in the background electron flow direction while propagating and evolving in the transverse direction. The velocity of transverse propagation depends on amplitude, a circumstance that leads to nonlinear proper evolution in the form of compression or steepening. On the basis of symmetry, radar Doppler spectra should exhibit spectral shifts and widths proportional to the cosine and sine of the electron flow angle, respectively. This prediction is in line with a number of theoretical and experimental observations.