An interpretation is made of rocket data obtained from an electric field mill (EFM) sensor during the international rocket-radar campaign NLC-91. The interpretation is based on a laboratory study of the interaction of a water particle beam with the EFM recovered after the rocket flight. Both rocket and laboratory data show that the field mill is sensitive to ice microparticle impacts and that perturbations in the EFM signal observed in the vicinity of noctilucent clouds and an enhanced radar echo (PMSE) layer are caused not only by atmospheric electric fields but also by the impact of solid particles. The altitude range of the impact signatures suggests the presence of ice particles in the PMSE region above the height of the optically detected noctilucent clouds. The analysis of the rocket data on the basis of the laboratory results allows us to estimate sizes of these PMSE particles to be 13--40 nm.¿ 1997 American Geophysical Union