D region disturbances caused by electromagnetic pulses from lightning

Electromagnetic pulses from weak lightning discharges ( E 100 = 1 V/m, where E 100 is the field strength in the radiation pattern maximum at 100 km) may substantially heat D region electrons, while only pulses with E 100 ≥ 20 V/m may create electron density enhancements ≥10 % of ambient. A E 100 = 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 E 100 = 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.