In a recent paper we discussed the penetration of dc thundercloud electric fields into the ionosphere and magnetosphere (Park and Dejnakarintra, 1973). In the present paper we extend the analysis to the time‐varying case and consider how electric field variations due to lightning are transmitted to ionospheric heights. We represent the thundercloud by a vertical dipole whose moment recovers exponentially with time following periodic lightning discharges. Electrical conductivity profiles are approximated by several piecewise exponential functions of altitude, and the anisotropy of the medium is taken into account above 70‐km altitude. The source wave form is Fourier‐analyzed, and electric fields at each harmonic frequency up to 10 Hz are mapped upward separately. The total electric field at any point in space is then obtained by adding all frequency components together. It is found that electric fields are transmitted upward more efficiently as the frequency increases. This compensates for the decrease with frequency in the amplitudes of the source spectrum, so that the electric field spectrum becomes fairly flat at large heights. In the time domain the recovery time following a lightning discharge decreases rapidly with altitude. At the 100‐km level the electric field wave form appears as a sharp spike of ∼100‐ms duration. The peak amplitude of the spike may be several hundred microvolts per meter across the geomagnetic field at night. In the daytime the corresponding amplitude is about an order of magnitude smaller because of larger conductivities. These fields should be investigated as a possible excitation source of geomagnetic pulsations in the magnetosphere.