A physical picture and quantitative two-dimensional electromagnetic modeling of mesospheric electric field transients produced by cloud-to-ground (CG) lightning discharges with short duration currents (<0.5 ms) are presented. The range of applicability of existing quasi-electrostatic models of sprites and the physical conditions under which relatively weak CG lightning discharges (thundercloud charge moment changes less than 50C¿10 km) may initiate sprites are discussed in the context of recent experimental findings. ¿ 1999 American Geophysical Union