The evolution of the vertical electric field profile deduced from simultaneous field measurements at several levels below a thundercloud shows the development of a space charge layer at least up to 600 m. The average charge density in the whole layer from 0 m to 600 m can reach about 1 nC m-3. The ions are generated at the ground by corona effect and the production rate is evaluated with a new method from the comparison of field evolutions at the ground and at altitude after a lightning flash. The modeling of the relevant processes shows that ground corona accounts for the observed field evolutions and that the aerosol particles concentration has a very large effect on the evolution of corona ions. However, with a realistic value for this concentration a large amount of ground corona ions reach the level of 600 m. ¿ American Geophysical Union 1994