Numerical simulations of the ionosphere and protonosphere have been used to investigate diurnal and seasonal variations in magnetospheric plasma density enhancements capable of ducting VLF radio waves. During winter and equinoxes, VLF ducts may extend down to 300-km altitude at night but usually terminate above 1800 km during the day. In summer, ducts terminate above 1000-km altitude at all local times. The daytime flow of thermal plasma from the ionosphere into the protonosphere limits the half-life of a VLF duct to about 2 days.