Precipitating energetic electrons as an ionization source in the midlatitude nighttimeDregion

Satellite and rocket measurements of precipitating electrons with E ≥40 kev are reviewed for middle latitudes during quiet conditions, and associated ionization rates and ion-pair densities are computed. The precipitating fluxes are highly variable, but even the lowest intensities may represent a significant source for the maintenance of the nighttime undisturbed D region. The energy deposited in the lower ionosphere by these electrons is consistent with the upper limit derived through consideration of the absence of specific night airglow intensities of the first negative system of N2+ at 3914 and 4278 A.