A new quasi-electrostatic model of high altitude electric field generation due to the lightning-induced change of a thundercloud electric structure is presented. The key point of the model is the assumption that a highly conducting channel arises due to a cloud-to-ground (CG) discharge, which brings the ground potential to a region near the cloud bottom in some tens of milliseconds after the discharge initiation. Substantial increase in the electric field strength above the thundercloud at this moment is found. A horizontal extension of the lightning channel is taken into account in the framework of the bidirectional model of the channel propagation. This geometry provides a substantially bigger electric field perturbation than the simplest geometry of the vertical lightning channel does. ¿ 2000 American Geophysical Union