A Boltzmann formulation of the electron distribution function and Maxwell's equations for the electromagnetic (EM) fields are used to simulate the interaction of lightning radiated EM pulses with the lower ionosphere. Ionization and dissociative attachment induced by the heated electrons cause significant changes in the local electron density (Ne). Due to 'slow' field changes of typical lightning EM pulses over time scales of tens of &mgr;s, the distribution function follows the quasi-equilibrium solution of the Boltzmann equation in the altitude range of interest (70 to 100 km). The EM pulse is simulated as a planar 100 &mgr;s long single period oscillation of a 10 kHz wave injected at 70 km. Under nighttime conditions, individual pulses of intensity 10--20 V/m (normalized to 100 km horizontal distance) produce changes in Ne of 1--30% while a sequence of pulses leads to strong modification of Ne at altitudes <95 km. The Ne changes produce a 'sharpening' of the lower ionospheric boundary by causing a reduction in electron density at 75--85 km (due to attachment) and a substantial increase at 85--95 km (due to ionization) (e.g., the scale height decreases by a factor of ~2 at ~85 km for a single 20 V/m EM pulse). No substantial Ne changes occur during daytime. ¿ American Geophysical Union 1993