In this paper, we analyze the electric field at ground level during the first stage of triggered lightning experiments, i.e., during the rocket ascent and prior to the lightning initiation. At distances of some tens of meters from the triggering wire, the electric field decreases significantly, while at distances of several hundred meters, there is only a very small decrease of the electric field. Two effects determine the level of the electric field reduction: the corona layer at ground level and the corona sheath around the triggering wire. We present an analytical solution based on the charge simulation method to study the phenomenon. The model is validated by comparing its results to those obtained by numerical simulations using the finite element method. A ground space charge layer and a corona sheath around the rocket-triggered lightning wire are included in the simulation. It is shown that, depending on the charge distribution, the change of the sign of the electric field is correctly predicted by our model. The obtained reductions of the electric field are consistent with simulations and experiments presented in the literature. Moreover, the proposed analytical solution is faster, and it allows studying the influence of several parameters simultaneously, i.e., the radius of the corona sheath and the space charge layer parameters. The described analytical model allows the estimation of the corona sheath radius if the parameters of the space charge layer are known from experiment.