The evolution of the electric field underneath a thunderstorm is measured and modeled, taking into account ion production by corona at ground level. Measurements of electric field and calculations of charge densities were made at various levels above the surface during two summer experiments. The collected data show that although the corona effect drastically influences the surface electrical parameters, some information about thunderstorm electrification can be obtained from consideration of the Maxwell current density. Thus the rate of change of electric field as well as the rate of generation of corona ions are obtained from the evolution of the surface electric field. Average charge densities ranging up to 6 nC/m3 are commonly found over intervals of several minutes within the lowest 15 m, especially during periods of intense lightning activity. Modeling reveals the influence of field changes due to lightning flashes and of the corona threshold on the surface electric field variation. The orders of magnitude computed by the model agree reasonably well with the experimental data. ¿American Geophysical Union 1987