Electromagnetic pulses from weak lightning discharges (E100=1 V/m, where E100 is the field strength in the radiation pattern maximum at 100 km) may substantially heat D region electrons, while only pulses with E100≥20 V/m may create electron density enhancements ≥10% of ambient. A E100=20 V/m pulse from a horizontal radiator at 5 km altitude (e.g., the cloud discharge at the stepped-leader onset) increases the electron temperature by a factor of ~400 maximum and the electron density (in one ionization cycle) by ~230 cm-3 maximum; the widths at half-maximum of the heated and ionized regions are 200 km and 90 km. A E100=40 V/m pulse from a vertical radiator at 0 km altitude (e.g., the vertical return stroke channel) increases the electron temperature by a factor of ~350 maximum and the electron density (in one ionization cycle) by ~80 cm-3 maximum; the widths at half-maximum of the heated and ionized regions are 440 km and 260 km. Several ionization cycles should occur during a typical lightning pulse. ¿ Amerigan Geophysical Union 1992