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 rad iated electromagnetic pulses (EMP) and the quasi-electrostatic (QE) fields, thus allowing effective studies of lightning-ionosphere interactions on ti me scales ranging from several microseconds to tens of milliseconds. The te mporal and spatial evolution of the electric field, lower ionospheric elect ron density, and optical emissions calculated with the new model are used t o investigate theoretically the effects of the lightning return stroke curr ent waveform (i.e., the current rise and fall timescales) and of the observ ational geometry on the optical signals observed with a photometer. For typ ical lightning discharges of similar to 100 mu s duration the ionospheric r esponse is dominated by the EMP-induced heating leading to the highly trans ient and laterally expanding optical flashes known as elves. The optical si gnal characteristics are found to be highly sensitive to both the observati onal geometry and the current waveform. The onset delay with respect to the lightning discharge, the duration, and the peak magnitude of optical emiss ions are highly dependent on the elevation and azimuth angles of field of v iew of individual photometric pixels. The shape of the optical signal clear ly reflects the source current waveform. For a waveshape with risetime of s imilar 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 du ration removing substantial amount of charge (i.e., similar to 100 C from 1 0 km altitude), heating and ionization changes induced by the QE field lead to the mesospheric luminous glows with lateral extent < 100 km, referred t o as sprites.