Joseph R. Dwyer

A comparison between Monte Carlo simulations of runaway breakdown and terrestrial gamma-ray flash observations

Author(s): Dwyer, JR; Smith, DM | Abstract: Monte Carlo simulations of the runaway breakdown of air are used to calculate the spectra of terrestrial gamma-ray flashes (TGFs), which are then compared with RHESSI and CGRO/BATSE observations. It is found that the recent RHESSI spectrum is not consistent with a source altitude above 24 km but can be well fit by a source in the range of 15-21 km, depending upon the electric field geometry of the source. Because 15 km is not unusual for the tops of thunderstorms, especially at low latitudes, and is lower than typical minimum sprite altitudes, the RHESSI data imply that thunderstorms and not sprites may be the source of these TGFs. On the other hand, the soft energy spectrum seen in some BATSE TGFs is inconsistent with such large atmospheric depths, indicating that there may exist two distinct sources of TGFs, with altitudes below 21 km and above 30 km. Copyright 2005 by the American Geophysical Union.

Relativistic breakdown in planetary atmospheres

In 2003, a new electrical breakdown mechanism involving the production of runaway avalanches by positive feedback from runaway positrons and energetic photons was introduced. This mechanism, which shall be referred to as “relativistic feedback,” allows runaway discharges in gases to become self-sustaining, dramatically increasing the flux of runaway electrons, the accompanying high-energy radiation, and resulting ionization. Using detailed Monte Carlo calculations, properties of relativistic feedback are investigated. It is found that once relativistic feedback fully commences, electrical breakdown will occur and the ambient electric field, extending over cubic kilometers, will be discharged in as little as 2×10−5s. Furthermore, it is found that the flux of energetic electrons and x rays generated by this mechanism can exceed the flux generated by the standard relativistic runaway electron model by a factor of 1013, making relativistic feedback a good candidate for explaining terrestrial gamma-ray flashes...

A ground level gamma-ray burst observed in association with rocket-triggered lightning

energetic radiation was observed at much earlier times, up to 160 ms before the return strokes. Because for such times, the dart leader tip must have been about 1000 m above the ground, it cannot be ruled out that for these events a gamma-ray (>1 MeV) component also originated from the cloud. [3] In this paper, we report an unusual event that occurred during the last rocket-triggered flash of the 2003 season. For this flash, an intense burst of MeV gamma-rays was observed from a distance of 650 m from the lightning channel, not in association with the dart leader or return stroke, but in association with a large current pulse (11 kA) occurring during the initial-stage (during the initial continuous current), about 20 ms after the vaporization of the triggering wire. In triggered lightning, the initial-stage is characterized by a steady current, preceding the return strokes, with superimposed pulses up to several kA in amplitude [Wang et al., 1999]. Considering the large distance of the detectors and the high energy of the gamma-rays, it is plausible that the burst originated in the cloud processes, perhaps many thousands of meters above the ground. This result may greatly facilitate the study of runaway breakdown of air inside thunderclouds [Gurevich et al., 1992], since it implies that observations of this phenomenon from the ground at sea level may be practical.

The lightning-TGF relationship on microsecond timescales

[1] We analyze the count rates of two terrestrial gamma‐ray flashes (TGFs) detected by the Fermi Gamma‐ray Burst Monitor (GBM) with the broadband magnetic fields (1 to 300 kHz) produced by the simultaneous lightning processes. The microsecond‐scale absolute time accuracy for these data, combined with independent geolocations of the source lightning, enable this analysis with higher accuracy than previously possible. In both events, fast discharge‐like processes occur within several tens of microseconds of the gamma‐ray generation, although not with a consistent relationship. The magnetic field data also show a slower signal component produced by a source current that in both events mirrors the gamma‐ray count rate closely in shape and time. This indicates electromagnetic radiation directly associated with the gamma‐ray generation process and thus provides a new means for probing the internal physics of this enigmatic phenomenon. Citation: Cummer, S. A., G. Lu, M. S. Briggs, V. Connaughton, S. Xiong, G. J. Fishman, and J. R. Dwyer (2011), The lightning‐TGF relationship on microsecond timescales, Geophys. Res. Lett., 38, L14810, doi:10.1029/2011GL048099.

Abundances of Heavy and Ultraheavy Ions in 3He-rich Solar Flares

We have surveyed 3He-rich solar energetic particle (SEP) events over the period 1997 September-2003 April in order to characterize abundances of heavy ions near 400 keV nucleon-1. The first part of the study focuses on 20 distinct SEP events that show the previously observed pattern in which, relative to O, heavy ions through Fe are enriched, with the enrichment increasing with mass. We find that these enrichments are well correlated such that 3He-rich SEP events with high Fe/C also show larger enrichments in other heavy ions. Ultraheavy (UH; taken as 78-220 amu) ions are routinely seen in these events with abundance enhancements correlating with Fe/C but with even larger flare-to-flare variations. In one event with unusually little interplanetary scattering, we are able to estimate the time of heavy- and UH-ion injections at the Sun and find them to be simultaneous. The second part of the study sums up many impulsive-event time periods in order to construct a mass histogram of UH nuclei; this histogram shows broad mass peaks similar to those in compilations of solar system abundances. In this summed period, relative to O, the average enhancement of heavy nuclei increases with mass with values of ~7 for Fe, ~40 for mass 78-100 amu, ~120 for mass 125-150 amu, and ~215 for 180-220 amu. The maximum UH enhancements seen in the most-enriched events are at least a factor of 5 larger. The enhancements are approximately proportional to the particle charge-to-mass ratio raised to a power, as seen previously in large, shock-associated SEP events.

Co‐location of lightning leader x‐ray and electric field change sources

[2] Although X-ray emission from lightning was long predicted [Wilson, 1925], only recently was the production of X rays in cloud-to-ground lightning confirmed. Moore et al. [2001] first reported the detection of energetic radiation emissions immediately preceding the return stroke of natural cloud-to-ground negative lightning, followed by a similar discovery by Dwyer et al. [2003] for rockettriggered lightning. Dwyer et al. [2004] reported that these emissions were composed of multiple, brief bursts of X rays in the 30–250 keV range, with each burst typically lasting less than 1 ms. Further, they showed that the sources of the X-ray bursts traveled from the cloud toward the ground, supporting the view that the leader front is the source of the X rays. Dwyer et al. [2005] compared X-ray and electric field records simultaneously obtained during the stepped leaders of natural negative cloud-to-ground lightning. The conclusion from this analysis was that the production of X-rays is associated with the electric field changes accompanying the stepping of the leader that initiates the first return stroke. Although an obvious temporal correspondence was observed, uncertainties in measurement time delays and oscilloscope trigger times prevented any accurate determination of the exact temporal relationship between the X-ray bursts and the stepping of the leader. Observations of the similarity in X-ray emissions from natural and triggered lightning imply a common mechanism for different types of negative leaders [Dwyer et al., 2005]. The aforementioned discoveries have had an impact on views of lightning electrical breakdown in air, in that lightning can no longer necessarily be considered a conventional low-energy (eV) discharge, but often involves an electron distribution function that includes a significant high-energy (keV to MeV) component. These recent advancements highlight many unknowns regarding leader propagation, the stepping process, and their association with X rays. Among the most pressing of these issues are the intensity of the X rays at the source, the electric field at the leader front, the directionality and attenuation of the X-ray emissions, and the spatial and temporal relationship between the sources of X rays and leader steps. This paper addresses the issue of independently locating the sources of X-ray emissions and the corresponding leader step electric field changes via time-of-arrival (TOA) measurements, which may allow advancement on many of these issues. Leadersinbothnaturalandtriggeredlightningareconsidered.

The source altitude, electric current, and intrinsic brightness of terrestrial gamma ray flashes

Many details of how thunderstorms generate terrestrial gamma ray flashes (TGFs) and other forms of high-energy radiation remain uncertain, including the basic question of where they are produced. We exploit the association of distinct low-frequency radio emissions with generation of terrestrial gamma ray flashes (TGFs) to directly measure for the first time the TGF source altitude. Analysis of two events reveals source altitudes of 11.8 ± 0.4 km and 11.9 ± 0.9 km. This places the source region in the interior of the thunderstorm between the two main charge layers and implies an intrinsic TGF brightness of approximately 10 18 runaway electrons. The electric current in this nontraditional lightning process is found to be strong enough to drive nonlinear effects in the ionosphere, and in one case is comparable to the highest peak current lightning processes on the planet.

Abundances and Energy Spectra of Corotating Interaction Region Heavy Ions Observed during Solar Cycle 23

Using instruments on the ACE spacecraft, we surveyed the heavy-ion spectra and composition over the range He-Fe for 41 corotating interaction regions (CIRs) during 1998-2007. Below ~1 MeV nucleon^(−1) the spectra are power laws in kinetic energy nucleon^(−1) with an average spectral index of 2.51 ± 0.10, rolling over above ~1 MeV nucleon^(−1) to power-law spectra with an average index of 4.47 ± 0.17. The spectral shapes for different species are similar, leading to relative abundances that are constant over our energy range, even though the intensities cover up to 8 orders of magnitude. Relative to oxygen, the measured abundances at 385 keV nucleon^(−1) for ^4He, C, N, Ne, Mg, Si, S, Ca, and Fe are 273 ± 72, 0.760 ± 0.023, 0.143 ± 0.005, 0.206 ± 0.009, 0.148 ± 0.006, 0.095 ± 0.005, 0.028 ± 0.002, 0.007 ± 0.001, and 0.088 ± 0.007, respectively. Except for an overabundance of ^4He and Ne, the abundances are quite close to that of the fast solar wind. We have found ^3He/^4He ratios to be enhanced over solar wind values in ~40% of the CIRs. The Fe/O ratio in individual CIRs is observed to vary over a factor of ~10 and is strongly correlated with the solar wind Fe/O ratio measured 2-4 days preceding each CIR. Taken together with previous studies showing the presence of pickup He^+ in CIRs, the observational data provide evidence that CIR energetic particles are accelerated out of a suprathermal ion pool that includes heated solar wind ions, pickup ions, and remnant suprathermals from impulsive solar energetic particle events.

The rarity of terrestrial gamma‐ray flashes

We report on the first search for Terrestrial Gamma-ray Flashes (TGFs) from altitudes where they are thought to be produced. The Airborne Detector for Energetic Lightning Emissions (ADELE), an array of gamma-ray detectors, was flown near the tops of Florida thunderstorms in August/September 2009. The plane passed within 10 km horizontal distance of 1213 lightning discharges and only once detected a TGF. If these discharges had produced TGFs of the same intensity as those seen from space, every one should have been seen by ADELE. Separate and significant nondetections are established for intracloud lightning, negative cloud-to-ground lightning, and narrow bipolar events. We conclude that TGFs are not a primary triggering mechanism for lightning. We estimate the TGF-to-flash ratio to be on the order of 10^(−2) to 10^(−3) and show that TGF intensities cannot follow the well-known power-law distribution seen in earthquakes and solar flares, due to our limits on the presence of faint events.

Lightning leader altitude progression in terrestrial gamma‐ray flashes

Radio emissions continue to provide insight into the production of terrestrial gamma ray flashes (TGFs) by thunderstorms, including the critical question of the conditions under which they are generated. We have identified several TGF-associated lightning radio emissions in which the altitudes of in-cloud lightning leader pulses that precede and follow the TGF can be measured. We combine these with high absolute timing accuracy TGF observations from the Fermi satellite to determine the development of the lightning channel before, during, and after the TGF production. All of these TGFs were produced several milliseconds after the leader had initiated and when the leaders reached 1–2 km in length. After the TGFs, the leaders all continued to ascend for several more kilometers with no dramatic change in their characteristics, although they all exhibited high average velocities of 0.8–1.0 × 106 m/s. Implications in the context of TGF models are discussed. These results paint the first clear picture of the lightning processes that occur before, during, and after TGF production.