Polar Mesospheric Summer Echoes (PMSE) have become a well known characteristic of radar backscatter from the summer mesopause regions in polar regions. The phenomenon has been observed at frequencies from 50 MHz to almost 1 GHz. In this paper we attempt to identify a space-time manifold for the occurrence of PMSE. The resulting manifold, defined only by temperature, agrees well with observations and furthermore indicates when and where we might expect to observe PMSE in the future. ¿ American Geophysical Union 1995