A lightning discharge is modeled as an impulse current from cloud to ground which instantaneously removes a quantity of charge from a charge center originally screened by a neutralizing space charge. The resulting time-dependent electric and magnetic fields at the ground and in the upper ionosphere are examined for a time interval that is long in comparison to the duraction of a typical discharge but short in comparison to the relaxation time of the screening space charge. The calculations are carried out for a nominal nighttime ionosphere with an assumed exponential conductivity profile and with anisotropy taken into account. First, analytic approximations are obtained for the various ULF transfer functions of the medium throughout the entire altitude range between the ground and the upper ionosphere. The transfer functions are then used to obtain analytic expressions for the ULF component of the time-dependent fields produced by the assumed source. At the ground the fields result from electric relaxation of the ionosphere and persist for times of a few seconds. The decay time of the electric field at the ground decreases with increasing distance from the source. The time dependence of the fields can be explained on the basis of a simple variable capacitor model of the medium. In the upper ionosphere ther is an outgoing hydromagnetic pulse which arises from a resonant excitation of the ionosphere by the low-frequency components of the discharge itself. The duration of the pulse seems too short to excite geomagnetic pulsations in the magnetosphere.