A two-dimensional, isolated-system electrostatic simulation model is used to investigate the coherence properties of single and multiple pulses of electrons injected into the ionosphere from space vehicles. The initial helical structure of the beam is found to be destroyed on a time scale inversely proportional to the beam plasma frequency, resulting in a coherence time of the order of 1 &mgr;s. The subsequent interaction of a beam pulse with the ambient plasma produces an enhanced spectral density in the total parallel current extending up to wave numbers on the order of &ohgr;pe/vxb, where &ohgr;pe is the ambient plasma frequency and vxb is the parallel beam injection speed. The magnitude of the spectrum in this range decreases roughly as k-1/4. Pulsing the beam with a period comparable to the disruption time leads to an enhanced interaction between successive pulses. ¿ American Geophysical Union 1990