The present numerical simulation deals with ion creation by corona at ground level and their evolution between the ground and the thundercloud. The study starts from a previous model in which the external electric field was constant in time. As an extension, the new model includes several types of variation of the external electric field responsible for ion creation. The following phenomena are taken into account: (1) ion creation of both polarities (bipolar medium), (2) capture of small ions by neutral aerosol particles as well as by other ions of opposite polarity, and (3) influence of turbulent diffusion. The model provides the transient response of the space charge to two types of variation of the electric field created by a thundercloud: the rapid variation due to a lightning stroke and the slow recovery which occurs during regeneration. Time and space evolution of the various parameters are deduced from the computation: small and large ion concentrations, space charge density, and electric field. The different results reveal the tremendous influence of the space charge on the electric field variation at ground level, especially after a lightning stroke. Pulses of corona current higher than 20 nA m-2 following 20 kV m-1 field steps can create such a high ion concentration that the surface electric field can be rapidly reduced to a value close to the corona threshold regardless of the intensity aloft. Electric field profiles plotted after a lightning stroke often show opposite polarities of the electric field at ground level and aloft.