Ionospheric plasmas containing density gradients are subject to a gradient drift instability under the influence of an externally applied electric field or neutral wind. In this paper we present electric field wave data from the Sudden Atom Layers sounding rocket showing a pure gradient drift instability on the topside of a 1 km thick sporadic E layer. An outer scale of 50 m was determined. Considering the local linear growth rate and the mapping of the electric field structures along magnetic field lines, we conclude that structures with scales larger than 50 m perpendicular to the magnetic field map into regions of linear stability where they are damped. The free energy is localized in wavenumber space, yielding peak fluctuating fields of 6 mV/m, corresponding to electron drifts over 190 m/s and a cascade of energy to meter scales. ¿ 2000 American Geophysical Union