Yuri N. Taranenko

Sprites produced by quasi‐electrostatic heating and ionization in the lower ionosphere

Quasi-electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes lead to ambient electron heating (up to ∼5 eV average energy), ionization of neutrals, and excitation of optical emissions in the mesosphere/lower ionosphere. Model calculations predict the possibility of significant (several orders of magnitude) modification of the lower ionospheric conductivity in the form of depletions of electron density due to dissociative attachment to O2 molecules and/or in the form of enhancements of electron density due to breakdown ionization. Results indicate that the optical emission intensities of the 1st positive band of N2 corresponding to fast (∼ 1 ms) removal of 100–300 C of thundercloud charge from 10 km altitude are in good agreement with observations of the upper part (“head” and “hair” [Sentman et al., 1995]) of the sprites. The typical region of brightest optical emission has horizontal and vertical dimensions ∼10 km, centered at altitudes 70 km and is interpreted as the head of the sprite. The model also shows the formation of low intensity glow (“hair”) above this region due to the excitation of optical emissions at altitudes ∼ 85 km during ∼ 500 μs at the initial stage of the lightning discharge. Comparison of the optical emission intensities of the 1st and 2nd positive bands of N2, Meinel and 1st negative bands of , and 1st negative band of demonstrates that the 1st positive band of N2 is the dominating optical emission in the altitude range around ∼70 km, which accounts for the observed red color of sprites, in excellent agreement with recent spectroscopic observations of sprites. Results indicate that the optical emission levels are predominantly defined by the lightning discharge duration and the conductivity properties of the atmosphere/lower ionosphere (i.e., relaxation time of electric field in the conducting medium). The model demonstrates that for low ambient conductivities the lightning discharge duration can be significantly extended with no loss in production of optical emissions. The peak intensity of optical emissions is determined primarily by the value of the removed thundercloud charge and its altitude. The preexisting inhomogeneities in the mesospheric conductivity and the neutral density may contribute to the formation of a vertically striated fine structure of sprites and explain why sprites often repeatedly occur in the same place in the sky as well as their clustering. Comparison of the model results for different types of lightning discharges indicates that positive cloud to ground discharges lead to the largest electric fields and optical emissions at ionospheric altitudes since they are associated with the removal of larger amounts of charge from higher altitudes.

Heating, ionization and upward discharges in the mesosphere due to intense quasi-electrostatic thundercloud fields

Quasi-electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes are found to significantly heat mesospheric electrons and produce ionization and light. The intensity, spatial extent, duration and spectra of optical emissions produced are consistent with the observed features of the Red Sprite type of upward discharges.

Space‐time structure of optical flashes and ionization changes produced by lighting‐EMP

Intense electromagnetic pulses (EMPs) released by lightning discharges produce bright optical emissions at 80–95 km altitudes emitted in a thin (∼30 km) cylindrical shell expanding to radial distances of up to >150 km, lasting for ∼400 µs, and appearing in limb-view as a thin layer with ∼400 km lateral extent.

High altitude discharges and gamma‐ray flashes: A manifestation of runaway air breakdown

γ-ray flashes of atmospheric origin as well as blue jets and red sprites are naturally explained by high-altitude discharges produced by runaway air breakdown. We present the first detailed model of the development of upward propagating discharges and compute optical and γ-ray emissions that are in excellent agreement with observations. According to our theory, such discharges represent the first known manifestation of runaway air breakdown, a fundamental new process in plasma physics.

SIMULATIONS OF HIGH-ALTITUDE DISCHARGES INITIATED BY RUNAWAY BREAKDOWN

Abstract 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 ∼300 μ 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 γ -ray fluxes are in agreement with observations of γ -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.

Optical characteristics of blue jets produced by runaway air breakdown, simulation results

The results of numerical calculations of the intensity and spectra of optical emissions from blue jets produced by runaway air breakdown in the atmosphere are presented. It is found that a positive runaway streamer develops in the altitude range 20-34 km following an intracloud discharge that possesses a continuing current of ∼ 1.7 kA. The ionization front of the runaway streamer propagates upward with a velocity ∼ 90 km/s and produces optical emissions with a maximum intensity ∼ 400 kR and a duration ∼ 153 ms. The comparison between theory and observation yields good agreement for such important blue jet characteristics as maximum intensity of optical emissions, color, front velocity, duration, maximum radius and vertical dimensions and supports the viability of runaway air breakdown as a driving mechanism for this particular type of high altitude discharge.

Optical characteristics of red sprites produced by runaway air breakdown

The results of numerical calculations of intensity and spectra of optical emissions from red sprites produced by runaway air breakdown in the atmosphere are presented. It is shown that the optical emissions from red sprites consist of two components: (1) short-term (t{approx}0.3{endash}2thinspms) emissions produced as a result of dissipation of an energetic electron beam in air; (2) long-term (t{approx}2{endash}10thinspms) emissions produced by a population of low-energy electrons in an electric field. The long-term optical emissions are calculated for all low-energy electrons, including the secondary low-energy electrons produced by the relativistic electron beam, ambient background electrons, and electrons produced as a result of regular breakdown. The theoretical results are compared with observational data. {copyright} 1998 American Geophysical Union