T. F. Bell

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.

Spatial structure of sprites

A theory of the electrical breakdown (EB) above thunderstorms is developed. The streamer type of the EB is proposed for the explanation of recent observations of fine spatial structures and bursts of blue optical emissions associated with sprites.

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.

Heating and ionization of the lower ionosphere by lightning

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.

Monte Carlo simulation of runaway MeV electron breakdown with application to red sprites and terrestrial gamma ray flashes

A three-dimensional Monte Carlo model of the uniform relativistic runaway electron breakdown in air in the presence of static electric and magnetic fields is used to calculate electron distribution functions, avalanche rates, and the direction and velocity of avalanche propagation. We also derive the conditions required for an electron with a given momentum to start an avalanche in the absence of a magnetic field. The results are compared to previously developed kinetic and analytical models and our own analytical estimates, and it is concluded that the rates used in many early models [e.g., Lehtinen et al., 1997; Taranenko and Roussel-Dupre, 1996; Yukhimuk et al., 1998; Roussel-Dupre et al., 1998] are overestimated by a factor of ∼10. The Monte Carlo simulation results are applied to a fluid model of runaway electron beams in the middle atmosphere accelerated by quasi-electrostatic fields following a positive lightning stroke. In particular, we consider the case of lightning discharges which drain positive charge from remote regions of a laterally extensive (> 100 km) thundercloud, using a Cartesian two-dimensional model. The resulting optical emission intensities in red sprites associated with the runaway electrons are found to be negligible compared to the emissions from thermal electrons heated in the conventional type of breakdown. The calculated gamma ray flux is of the same order as the terrestrial gamma ray flashes observed by the Burst and Transient Source Experiment detector on the Compton Gamma Ray Observatory.

Ionospheric D region remote sensing using VLF radio atmospherics

Lightning discharges radiate the bulk of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands. This energy, contained in impulse-like signals called radio atmospherics or sferics, is guided for long distances by multiple reflections from the ground and lower ionosphere. This suggests that observed sferic waveforms radiated from lightning and received at long distances (>1000 km) from the source stroke contain information about the state of the ionosphere along the propagation path. The focus of this work is on the extraction of nighttime D region electron densities (in the altitude range of ∼70–95 km) from observed VLF sferics. In order to accurately interpret observed sferic characteristics, we develop a model of sferic propagation which is based on an existing frequency domain subionospheric VLF propagation code. The model shows that the spectral characteristics of VLF sferics depend primarily on the propagation path averaged ionospheric D region electron density profile, covering the range of electron densities from ∼100 to 103 cm−3. To infer the D region density from observed VLF sferics, we find the electron density profile that produces a modeled sferic spectrum that most closely matches an observed sferic spectrum. In most nighttime cases the quality of the agreement and the uncertainties involved allow the height of an exponentially varying electron density profile to be inferred with a precision of ∼0.2 km.

ELF radiation produced by electrical currents in sprites

Measurements of ELF-radiating currents associated with sprite-producing lightning discharges exhibit a second current peak simultaneous in time with sprite luminosity, suggesting that the observed ELF radiation is produced by intense electrical currents flowing in the body of the sprite.

Interaction with the lower ionosphere of electromagnetic pulses from lightning: Heating, attachment, and ionization

A Boltzmann formulation of the electron distribution function and Maxwells equations for the electromagnetic (EM) fields are used to simulate the interaction of lightning radiated EM pulses with the lower ionosphere. Ionization and dissociative attachment induced by the heated electrons cause significant changes in the local electron density (Ne). Due to ‘slow’ field changes of typical lightning EM pulses over time scales of tens of µs, the distribution function follows the quasi-equilibrium solution of the Boltzmann equation in the altitude range of interest (70 to 100 km). The EM pulse is simulated as a planar 100 µs long single period oscillation of a 10 kHz wave injected at 70 km. Under nighttime conditions, individual pulses of intensity 10–20 V/m (normalized to 100 km horizontal distance) produce changes in Ne of 1–30% while a sequence of pulses leads to strong modification of Ne at altitudes <95 km. The Ne changes produce a ‘sharpening’ of the lower ionospheric boundary by causing a reduction in electron density at 75–85 km (due to attachment) and a substantial increase at 85–95 km (due to ionization) (e.g., the scale height decreases by a factor of ∼2 at ∼85 km for a single 20 V/m EM pulse). No substantial Ne changes occur during daytime.

Runaway electrons as a source of red sprites in the mesosphere

Large quasi-electrostatic (QE) fields above thunderclouds [Pasko et al., 1995] produce an upward traveling beam of ∼1 MeV runaway electrons which may contribute to the production of optical emissions above thunderclouds referred to as Red Sprites. Results of a one dimensional computer simulation model suggest that the runaway electrons can produce optical emissions similar in intensity and spectra to those observed in Red Sprites, but only for large QE fields produced by positive CG discharges lowering 250 C or more to ground from an altitude of at least 10 km. Differences in predicted optical spectra from that of other mechanisms suggest that the runaway electron mechanism can be readily tested by high resolution spectral measurements of Red Sprites.

VLF signatures of ionospheric disturbances associated with sprites

VLF perturbations on signals propagating along great-circle-paths (GCP) through electrically active midwest thunderstorms are associated with luminous high altitude glows (referred to as sprites) observed from aircraft or ground. The data constitutes the first evidence that the physical processes leading to sprites also alter the conductivity of the lower ionosphere.