During several campaigns during the last 10 years, detailed in situ studies of electrons, positive ions, and charged aerosols have been performed by means of rocket borne instruments in the presence of Polar Mesophere Summer Echoes (PMSE) and Noctilucent Clouds (NLC). We have studied the correlation between the amount of charged aerosols present in the mesopause region and the PMSE echo power. We have also correlated the PMSE echo strength with the small-scale structure of electrons at the radar Bragg scale that are responsible for the echoes. We find that PMSE occur for rather small amounts of charged aerosols, with the number of electrons exceeding the number of charged aerosols. This is in contradiction with previous, mainly theoretical, studies predicting that PMSE only occur when the ratio between the aerosol charge number density and the number density of electrons is larger than about 1. We also find that there is a high degree of correlation between the PMSE echo power and the fluctuation intensity of electrons at scales comparable to half the radar wavelength. This confirms that variations in electron number density are responsible for the echoes, but does not explain the mechanism that creates the fluctuations.