HEATING AND IONIZATION OF THE LOWER IONOSPHERE BY LIGHTNING

Nighttime ionospheric electrons at 90-95 km altitude are found to be heated by a factor of 100-500 during the upward passage of short (< 100-mu-s) pulses of intense (5-20 V/m at 100 km distance) electromagnetic radiation from lightning. Heated electrons with average energy of 4-20 eV in turn produce secondary ionization, of up to 400 cm-3 at approximately 95 km altitude in a single ionization cycle (approximately 3-mu-s). With the time constant of heating being 5-10-mu-s, a number of such ionization cycles can occur during a 50-mu-s radiation pulse, leading to even higher density enhancements. This effect can account for previously reported observations of 'early' or 'fast' subionospheric VLF perturbations.