Simulations of high-altitude discharges initiated by runaway breakdown

Detailed 2D hydrodynamic and quasi-electrostatic simulations of high-altitude discharges driven by runaway air breakdown are presented for four cases, corresponding to sprites initiated by positive cloud-to-ground lightning strikes in which 200 C of charge is neutralized at an altitude of 11.5 km in 10, 7, 5 and 3 ms. We find that the computed optical emissions agree well with low-light level camera images of sprites, both in terms of the overall intensity and spatial distribution of the emissions. Our results show the presence of blue emissions extending down to 40 km (blue tendrils) and red sprite tops extending from 50 to 77 km. Simulated spectra show that N-2 1st positive emissions dominate in the wavelength range from 550 to 850 nm, in good agreement with observations. Strong radio pulses with durations of similar to 300 mu s and peak electric field amplitudes ranging from 20 to 75 V/m at an altitude of 80 km and an approximate distance from the discharge of 50 km were computed. The magnitude and duration of these pulses is sufficient to cause breakdown and heating of the lower ionosphere (80-95 km) and leads us to suggest that sprites may also launch the EMP responsible for the production of elves. The computed values for the gamma-ray fluxes are in agreement with observations of gamma-ray bursts of atmospheric origin and the peak secondary electron densities which we obtain are in good agreement with recent measurements of HF echoes at mesospheric heights and associated with lightning. (C) 1998 Elsevier Science Ltd. All rights reserved.