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 &mgr;s) pulse 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 ~95 km altitude in a single ionization cycle (~3 &mgr;s). With the time constant of heating being 5--10 &mgr;s, a number of such ionization cycles can occur during a 50 &mgr;s radiation pulse, leading to even higher density enhancements. This effect can account for previously reported observations of 'early' or 'fast' subionospheric VLF perturbations. ¿American Geophysical Union 1991