Eugene Symbalisty

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.

Gamma‐ray emissions observed in a thunderstorm anvil

Balloon-borne gamma-ray and electric-field-change instruments were launched into a daytime summer thunderstorm to evaluate a new experimental design to test hypotheses for the production of transient luminous events (TLE) (eg. sprites, and blue jets) in the mesosphere. While ascending, the instrument triggered many times on the signals from the electric-field-change instrument, recording the gamma-ray background at those times. A greater than three-fold increase in the gamma-ray flux was observed as the balloon descended through a thunderstorm anvil where a strong electric field was suspected to be present. These observations suggest that gamma-ray production in thunderstorms may not be as uncommon as previously believed.

Finite volume solution of the relativistic Boltzmann equation for electron avalanche studies

The Boltzmann equation for a population of relativistic electrons in a constant external electric field in air is solved numerically in order to determine parameters of the resulting runaway electron beam. The numerical solution takes advantage of algorithms from computational fluid dynamics. The work revises the results from the original work where the numerical solution was found to be unstable.

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

On the temporal evolution of red sprites: Runaway theory versus data

The results of numerical simulations of red sprite discharges, namely the temporal evolutions of optical emissions, are presented and compared with observations. The simulations are done using the recently recalculated runaway avalanche rates. The temporal evolution of these simulations is in good agreement with ground-based photometer and CCD TV camera observations of red sprites. Our model naturally explains the ‘hairline’ of red sprites as a boundary between the region where the intensity of optical emissions associated with runaway breakdown has a maximum and the region where the intensity of optical emissions caused by conventional breakdown and ambient electron heating has a maximum. Other important characteristics of the simulated sprite such as color, shape, altitude, and intensity are in agreement with the observations as well. We also present for the first time simulations of red sprites with a daytime conductivity profile.

Hard X ray survey of energetic electrons from low-earth orbit

Hard X ray and neutron emissions measured in low-Earth orbit are surveyed to develop a global overview of lightning-related energetic-electron precipitation and acceleration processes. Comparison of geographic intensity maps shows the dominance of enhanced hard X ray intensities measured when the satellite was above the continental United States and above the southern Indian Ocean between Madagascar and Australia. This emission is most enhanced during the northern summer months. Lesser although significant enhancements are seen between the Middle East and the Tibetan Plateau, a stretch of ocean off the east Asian coast between the Philippines and Korea, a stretch of equatorial Africa from the Ivory Coast to Mozambique, a region of the eastern equatorial Pacific just west of Columbia, and a patch of the Indian Ocean stretching between the southern tip of India and Indonesia. Although emissions from many of these regions are generally enhanced during the northern summer and fall seasons, none show any regularity relative to local time of day. Many but not all of these enhancements support natural interpretations in terms of lightning-induced energetic-electron precipitation from the terrestrial trapped radiation belts. Electron scattering induced by radio waves from VLF transmitters most likely contributes to this precipitation.

Relativistic electron beams above thunderclouds

Non-luminous relativistic electron beams above thunderclouds have been detected by the radio signals of low frequency 40-400 kHz which they radiate. The electron beams occur 2-9 ms after positive cloud-to-ground light- ning discharges at heights between 22-72 km above thun- derclouds. Intense positive lightning discharges can also cause sprites which occur either above or prior to the elec- tron beam. One electron beam was detected without any lu- minous sprite which suggests that electron beams may also occur independently of sprites. Numerical simulations show that beams of electrons partially discharge the lightning elec- tric field above thunderclouds and thereby gain a mean en- ergy of 7 MeV to transport a total charge of 10 mC up- wards. The impulsive current 3◊ 10 3 Am 2 associated with relativistic electron beams above thunderclouds is di- rected downwards and needs to be considered as a novel ele- ment of the global atmospheric electric circuit.

Geomagnetic pulsation generation as a result of whistler wave scattering

Nonlinear scattering of whistler mode waves by kinetic Alfven waves (KAWs) is considered. The evolution of whistler mode wave decay instability in time and two spatial dimensions is studied under the approximation of two-fluid magnetohydrodynamics. It is shown that efficient coupling between whistler waves and KAWs is possible due to the finite Larmor radius effect for Alfven waves. The considered process can lead to significant enhancement of KAWs. The nonlinear scattering of whistler waves by KAWs is proposed as a possible mechanism of enhancement and generation of geomagnetic pulsations in the magnetosphere and ionosphere of the Earth.

Association of discrete hard X ray enhancements with eruption of Mount Pinatubo

Hard X ray fluxes in the energy range between 50 keV and 140 keV measured using the Army Background Experiment are found to be associated with the injection by Mount Pinatubo of a massive aerosol into the stratosphere June 14–15, 1991. Discrete X ray enhancement events are observed to increase from an average of 1.8 per month to 20 during a 1-month interval beginning 12 days after the Mount Pinatubo eruption. Systematics of these enhancements suggests a mechanism that requires a strong coupling between magnetospheric and atmospheric processes, perhaps associated with upward lightning.