During quiet solar and geomagnetic conditions, the observed mean diurnal variation of the reflectivity of polar mesosphere summer echoes is characterized by a distinct maximum near noon and a deep minimum near 20 LT, so that it appears to contain a relatively strong semidiurnal component. Theoretical considerations predict a nonlinear relationship between the reflectivity, the tidal temperature variation, and the electron production by solar radiation and energetic-particle precipitation. Least squares fits based on model predictions of atmospheric tides, on a Chapman function, and on cosmic noise absorption data indicate that the observed diurnal variation of the reflectivity is predominantly determined by the electron production. Its semidiurnal component is essentially caused by the fact that the ionization rates due to the solar radiation and the energetic-particle precipitation have maxima at noon and near midnight, respectively. ¿ 1999 American Geophysical Union