Characteristics of mesospheric optical emissions produced by lightning discharges

A new two-dimensional cylindrically symmetric electromagnetic model of the lightning-ionosphere interaction includes effects of both the lightning radiated electromagnetic pulses (EMP) and the quasi-electrostatic (QE) fields, thus allowing effective studies of lightning-ionosphere interactions on time scales ranging from several microseconds to tens of milliseconds. The temporal and spatial evolution of the electric field, lower ionospheric electron density, and optical emissions calculated with the new model are used to investigate theoretically the effects of the lightning return stroke current waveform (i.e., the current rise and fall timescales) and of the observational geometry on the optical signals observed with a photometer. For typical lightning discharges of similar to 100 mu s duration the ionospheric response is dominated by the EMP-induced heating leading to the highly transient and laterally expanding optical flashes known as elves. The optical signal characteristics are found to be highly sensitive to both the observational geometry and the current waveform. The onset delay with respect to the lightning discharge, the duration, and the peak magnitude of optical emissions are highly dependent on the elevation and azimuth angles of field of view of individual photometric pixels. The shape of the optical signal clearly reflects the source current waveform. For a waveshape with risetime of similar to 50 mu s or longer a double-pulse shape of the photometric signal is observed. For cloud to ground lightning discharges of similar to 1 ms duration removing substantial amount of charge (i.e., similar to 100 C from 10 km altitude), heating and ionization changes induced by the QE field lead to the mesospheric luminous glows with lateral extent < 100 km, referred to as sprites.