A cesium ion cloud was created by an explosive release about 1 km from the instrumented section of the Trigger payload. Band-limited electric field and density waves were observed shortly after the ion cloud was detected. Since the electric field and density components had similar spectral shape and the wave electric field was oriented parallel to the ambient magnetic field, we identify the waves as collisionless ion acoustic waves. Ion-ion streaming between Cs+ and the ambient ions is suggested as a source of free energy for the ion acoustic wave, and this hypothesis is tested using the linear dispersion relation. Ion-ion streaming is found to be a plausible mechanism if Te/Ti is large. Te was measured to be elevated over the ambient value briefly, and modeling indicates that the Cs ion temperature may have been greatly depressed from the ambient value, thereby implying that within the explosive cloud, Te/Ti?10.