Runaway Electrons and Terrestrial Gamma Rays

Contents

• Introduction
• Runaway Mechanism
• Monte Carlo Model
• Gamma Rays
• Recent Talks
• Related VLF Group Papers
• Other References
• Related Materials

Introduction

The runaway electron breakdown is modelled using Monte Carlo technique [Lehtinen et al., 1999a] to find the avalanche rates and the direction of the electron beam. We also study the nonuniform properties of the avalanche [Lehtinen et al., 1999b] using the same model.

The Monte Carlo model is applied to the Earth's middle and upper atmosphere above thunderstorms, where the electric fields are strong enough to accelerate the electrons upward. The atmosphere can be described as cylindrically symmetric with a vertical axis [Lehtinen et al., 1997] or translationallly symmetric in horizontal direction [Lehtinen et al., 1999a]. Note that the former allows a description only of vertical geomagnetic field, whereas the latter can describe magnetic field at any dip angle.

We investigate the role of avalanching runaway electrons in the production of red sprites and terrestrial gamma ray flashes. The calculated optical emissions in red sprites associated with the runaways are negligible compared to the emission from thermal electrons in the conventional type of breakdown. This result is obtained for all values of the lightning discharge which causes the runaway breakdown in the middle atmosphere. However, the calculated gamma ray flux, due to bremsstrahlung emissions from relativistic electrons, is of the same order as the terrestrial gamma ray flashes observed by the BATSE detector on the Compton Gamma Ray Observatory.

The energetic electrons leaving the atmosphere enter the radiation belts [Lehtinen et al., 1998]. The electron beam interacts with plasma waves in the ionosphere and magnetosphere and precipitates at the geomagnetically conjugate point. Part of the energetic electrons is trapped in the radiation belt, forming an electron "curtain" as the electrons drift in the longitudinal direction.

Runaway Mechanism

Figure 1. Runaway avalanche in the middle atmosphere.

The most energetic electrons accelerate because the dynamic friction, which is due to collisions, at certain electron energies decreases with increasing energy and has a minimum around 1 MeV.

Figure 2. Dynamic friction function.

Monte Carlo Model

Only particles with energies greater than a few keV are considered. Each particle moves under actions of external electric (E) and magnetic (B) fields and collisions with atmospheric material. Both changes in direction and energy of each particle are calculated. The values of E and B and the angle between them are arbitrary input parameters. The program calculates the change of energetic particle number due to ionization and thermalization. Ionization increases the number of particles, while thermalization decreases it. The model provides outputs of particle configurations in the phase space at any given moments of time.

Gamma Rays

The runaway electrons in the middle atmosphere are accelerated upward by the thundercloud electric field. They collide with the material constituing the atmosphere, producing terrestrial gamma ray flashes (TGF) in a process of bremsstrahlung. Since the electrons are very energetic (~1 MeV), the gamma ray radiation pattern is directed along the beam.

Figure 3. A cartoon describing gamma ray production.

Figure 4 shows a typical terrestrial gamma ray flash observed by the Compton Gamma Ray Observatory (CGRO), published by Fishman et al. [1994].

Figure 4. Observed terrestrial gamma ray flash.

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Recent Talks

Lehtinen, N. G, "Monte Carlo Simulation of Runaway MeV Electron Breakdown in the Presence of a Static Magnetic Field with Application to the Phenomenon of Red Sprites and Terrestrial Gamma Ray Flashes", presented at URSI Meeting (January 4-8, 1999, Boulder, CO, U.S.A.).

Lehtinen, N. G., U. S. Inan and T. F. Bell, "Monte Carlo Studies of Nonuniform Runaway in the Presence of a Magnetic Field", presented at URSI Meeting (January 4-8, 1999, Boulder, CO, U.S.A.).

Related VLF Group Papers

Inan, U. S., S. C. Reising, G. J. Fishman and J. M. Horack, "On the association of terrestrial gamma-ray bursts with lightning and implication for sprites", Geophys. Res. Lett., 23, p. 1017, 1996.

Lehtinen, N. G., M. Walt, U. S. Inan, T. F. Bell and V. P. Pasko, "Gamma-ray emission produced by a relativistic beam of runaway electrons accelerated by quasi-electrostatic thundercloud fields", Geophys. Res. Lett., 23, p. 2645, 1996.
article in PDF format

Lehtinen, N. G., T. F. Bell, V. P. Pasko, and U. S. Inan, "A two-dimensional model of runaway electron beams driven by quasi-electrostatic thundercloud fields", Geophys. Res. Lett., 24, p. 2639, 1997.
article in PDF format(available also from GRL site).

Lehtinen, N. G., T. F. Bell, and U. S. Inan, Monte Carlo simulation of runaway MeV electron breakdown with application to red sprites and terrestrial gamma ray flashes, J. Geophys. Res., 104, p. 24699, 1999.
article in PDF format

Lehtinen, N. G., U. S. Inan and T. F. Bell, Trapped electron curtains produced by thunderstorm driven runaway electrons, submitted to Geophysical Research Letters, 1999.

Other References

Related Materials

• BATSE instrument home page
• Terrestrial Gamma Flashes

• Red sprite mechanisms on VLF Group site
• Red sprites and blue jets page at the University of Alaska, Fairbanks
• Another Sprites site at the University of Alaska, Fairbanks
• NASA Sprites and Jets page
• EXL 98 Team Web -- sprite observations from an airplane in the summer of 1998
• High Altitude Discharges Research at Los Alamos
• Spirits of Another Sort: NASA press release