In the polar summer mesopause, negatively charged ice particles probably occur forming clouds with a thickness of a few kilometers. It is assumed that colliding ice particles and neutral molecules behave like rigid elastic spheres. Then calculated Thomson scatter spectra have a narrow particle line in addition to the ion and electron lines. Least-squares fits of theoretical autocorrelation functions to autocorrelation functions measured by the EISCAT 933-MHz radar yield particle radii and concentrations of about 3 nm and 1010 m-3. These results support earlier suggestions that there is a large number of particles producing electron density depletions and a viscous-convective subrange in the turbulence spectrum of the electrons. ¿ American Geophysical Union 1994