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 μs) 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 ∼95 km altitude in a single ionization cycle (∼3 μs). With the time constant of heating being 5–10 μs, a number of such ionization cycles can occur during a 50 μs, radiation pulse, leading to even higher density enhancements. This effect can account for previously reported observations of ‘early’ or ‘fast’ subionospheric VLF perturbations.
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