N. G. Lehtinen

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.

γ-Ray emission produced by a relativistic beam of runaway electrons accelerated by quasi-electrostatic thundercloud fields

In an experiment described by Fishman et al. [1994], high energy photons of atmospheric origin were detected by the Burst and Transient Source Experiment (BATSE) detectors, located on the Compton Gamma Ray Observatory (CGRO). In this paper we assess the possibility that the bursts may be bremsstrahlung produced by relativistic (>1 MeV) runaway electron beams accelerated in an avalanche process by quasi-electrostatic thundercloud fields. We consider the height-dependent density profile of the relativistic electrons specified as a function of time in the context of a previously reported runaway model [Bell et al., 1995]. The electron beam is modeled as a vertical cylinder with radius 10 km, and numerical estimates are provided of γ-ray fluxes which would be observed at the satellite. The predicted fluxes at the satellite altitude and at horizontal distances of up to 500 km from the source are found to be comparable to the experimental data.

Production of terrestrial gamma‐ray flashes by an electromagnetic pulse from a lightning return stroke

[1] Recent observations of terrestrial gamma-ray flashes (TGFs) (Smith et al., 2005) suggest the need for a new mechanism of TGF production. Consideration of relativistic runaway electron (RRE) avalanche driven by electromagnetic impulses (EMP) radiated by rapidly moving lightning return strokes indicates that TGFs can be produced by discharges with peak return stroke currents I p > 450-700 kA with velocities v rs /c = 0.99-0.995.

A two-dimensional model of runaway electron beams driven by quasi-electrostatic thundercloud fields

Intense, transient quasi-electrostatic (QE) fields, which exist above thunderclouds following a positive cloud-to-ground lightning discharge, can produce an upward travelling runaway electron (REL) beam. A new two-dimensional (2D) REL-QE model is developed, expanding the previously reported 1D model [Bell et al., 1995] and incorporating the QE [Pasko et al., 1997a] and the recently developed electrostatic heating (ESH) [Pasko et al., 1997b] models. The new model gives the lateral electron distribution in the beam and allows us to determine the ionospheric effects and the optical luminosities resulting from the simultaneous action of the QE fields on the ambient electrons and the runaway electrons. The model is self-consistent and includes the changes in space charge and conductivity due to the REL. Optical emissions and γ-ray emissions [Lehtinen et al., 1996a] are calculated and compared to experimental observations of Sprites and terrestrial γ-ray flashes (TGF). It is shown that the structure of the electric field and the optical emissions can be significantly affected by the REL.

Effects of thunderstorm‐driven runaway electrons in the conjugate hemisphere: Purple sprites, ionization enhancements, and gamma rays

The presence of energetic runaway electron beams above thunderstorms is suggested by observations of terrestrial gamma ray flashes [Fishman et al., 1994], as well as by theoretical work [Roussel-Dupre and Gurevich, 1996; Lehtinen et al., 1999], although such beams have not been directly measured. In this paper we consider possible measurable effects of such beams in the conjugate hemisphere as a means to confirm their existence and quantify their properties. High-density relativistic runaway electron beams, driven upward by intense lightning-generated mesospheric quasi-static electric fields, have been predicted [Lehtinen et al., 2000] to be isotropized and thermalized during their interhemispherical traverse along the Earths magnetic field lines so that only ∼10% of the electrons which are below the loss cone should arrive at the geomagnetically conjugate ionosphere. As they encounter the Earths atmosphere, the energetic electrons would be scattered and produce light and ionization, much like a beam of precipitating auroral electrons. A Monte Carlo approach is used to model the interaction of the downgoing electrons with the conjugate atmosphere, including the backscattering of electrons, as well as production of optical and gamma ray emissions and enhanced secondary ionization. Results indicate that these conjugate ionospheric effects of the runaway electron beam are detectable and thus may be used to quantify the runaway electron mechanism.

Ionization of the lower ionosphere by γ-rays from a Magnetar: Detection of a low energy (3–10 keV) component

A gigantic periodic flare from the soft γ repeater SGR 1900+14 produced enhanced ionization at ionospheric altitudes of 30 to 90 km, which was observed as unusually large amplitude and phase changes of very low frequency (VLF) signals propagating in the Earth-ionosphere waveguide. The VLF signals remained perturbed for ∼5 min and exhibited the 5.16 s periodicity of the giant flare detected on the Ulysses spacecraft [Hurley et al., 1999]. Quantitative analysis indicates the presence of an intense initial low energy (3–10 keV) photon component that was not detectable by the Ulysses instrument.

Confining the angular distribution of terrestrial gamma ray flash emission

[1] Terrestrial gamma ray flashes (TGFs) are bremsstrahlung emissions from relativistic electrons accelerated in electric fields associated with thunder storms, with photon energies up to at least 40 MeV, which sets the lowest estimate of the total potential of 40 MV. The electric field that produces TGFs will be reflected by the initial angular distribution of the TGF emission. Here we present the first constraints on the TGF emission cone based on accurately geolocated TGFs. The source lightning discharges associated with TGFs detected by RHESSI are determined from the Atmospheric Weather Electromagnetic System for Observation, Modeling, and Education (AWESOME) network and the World Wide Lightning Location Network (WWLLN). The distribution of the observation angles for 106 TGFs are compared to Monte Carlo simulations. We find that TGF emissions within a half angle >30° are consistent with the distributions of observation angle derived from the networks. In addition, 36 events occurring before 2006 are used for spectral analysis. The energy spectra are binned according to observation angle. The result is a significant softening of the TGF energy spectrum for large (>40°) observation angles, which is consistent with a TGF emission half angle (<40°). The softening is due to Compton scattering which reduces the photon energies.

EXPERIMENTAL STUDY OF ACOUSTIC ULTRA-HIGH-ENERGY NEUTRINO DETECTION

An existing array of underwater, large-bandwidth acoustic sensors has been used to study the detection of ultra-high-energy (UHE) neutrinos in cosmic rays. Acoustic data from a subset of seven hydrophones located at a depth of ~1600 m have been acquired for a total live time of 195 days. For the first time, a large sample of acoustic background events has been studied for the purpose of extracting signals from super-EeV showers. As a test of the technique, an upper limit for the flux of UHE neutrinos is presented, along with considerations relevant to the design of an acoustic array optimized for neutrino detection.

Trapped energetic electron curtains produced by thunderstorm driven relativistic runaway electrons

Relativistic runaway electron beams driven upward by intense lightning-generated quasielectrostatic (QE) fields undergo intense interactions with the background magnetospheric plasma, leading to rapid nonlinear growth of Langmuir waves. The beam electrons are strongly scattered by the waves in both pitch angle and energy, within one interhemispheric traverse along the Earths magnetic field lines. While those electrons within the loss cone precipitate out, most of the electrons execute bounce and drift motions, forming detectable trapped curtains of energetic electrons.

Sensitivity of an underwater acoustic array to ultra-high energy neutrinos

Abstract We investigate the possibility of searching for ultra high energy neutrinos in cosmic rays using acoustic techniques in ocean water. The type of information provided by the acoustic detection is complementary to that of other techniques, and the filtering effect of the atmosphere, imposed by the fact that detection only happens if a shower fully develops in water, would provide a clear neutrino identification. We find that it may be possible to implement this technique with very limited resources using existing high frequency underwater hydrophone arrays. We review the expected acoustic signals produced by neutrino-induced showers in water and develop an optimal filtering algorithm able to suppress statistical noise. The algorithm found is computationally appropriate to be used as a trigger for the signal processors available on existing arrays. We estimate the noise rates for a trigger system on a very large size hydrophone array of the US Navy and find that, while a higher density of hydrophones would be desirable, the existing system may already provide useful data.