J. R. Dwyer

First results on terrestrial gamma ray flashes from the Fermi Gamma‐ray Burst Monitor

[1]xa0The Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope detected 12 intense terrestrial gamma ray flashes (TGFs) during its first year of observation. Typical maximum energies for most of the TGFs are ∼30 MeV, with one TGF having a 38 MeV photon; two of the TGFs are softer and longer than the others. After correcting for instrumental effects, a representative bright TGF is found to have a fluence of ∼0.7 photons cm−2. Pulses are either symmetrical or have faster risetimes than fall times; they are well fit with Gaussian or lognormal functions. The fastest risetime observed was 7 μs, constraining the source radius to be less than about 2 km from the velocity of light. TGFs with multiple pulses separated in time have been known since their discovery; the GBM sample also includes clear cases of partially overlapping pulses. Four TGFs are associated with lightning locations from the World Wide Lightning Location Network. With the several μs absolute time accuracy of GBM, the time order can be confidently identified: one TGF occurred before the lightning, two were simultaneous, and one TGF occurred after the lightning.

Electron-positron beams from terrestrial lightning observed with Fermi GBM

[1]xa0Terrestrial Gamma-ray Flashes (TGFs) are brief pulses of energetic radiation observed in low-earth orbit. They are associated with thunderstorms and lightning and have been observed both as gamma-ray and electron flashes depending on the position of the spacecraft with respect to the source. While gamma-ray TGFs are detected as short pulses lasting less than 1 ms, most TGFs seen by the Fermi Gamma-ray Burst Monitor (GBM) with durations greater than 1 ms are, instead, the result of electrons traveling from the sources along geomagnetic field lines. We perform spectral analysis of the three brightest electron TGFs detected by GBM and discover strong 511 keV positron annihilation lines, demonstrating that these electron TGFs also contain substantial positron components. This shows that pair production occurs in conjunction with some terrestrial lightning and that most likely all TGFs are injecting electron-positron beams into the near Earth environment.

Heavy-Ion Elemental Abundances in Large Solar Energetic Particle Events and Their Implications for the Seed Population

We have surveyed the ~0.1–10 MeV nucleon to the -1 abundances of heavy ions from 3He through Fe in 64 large solar energetic particle (LSEP) events observed on board the Advanced Composition Explorer from 1997 November through 2005 January. Our main results are (1) the 0.5–2.0 MeV nucleon to the -1 3He/ 4He ratio is enhanced between factors of ~2–150 over the solar wind value in 29 (~46%) events. (2) The Fe/O ratio in most LSEP events decreases with increasing energy up to ~60 MeV nucleon to the -1. (3) The Fe/O ratio is independent of CME speed, flare longitude, event size, the 3He/4He ratio, the pre-event Fe/O ratio, and solar activity. (4) The LSEP abundances exhibit unsystematic behavior as a function of M/Q ratio when compared with average solar wind values. (5) The survey-averaged abundances are enhanced with increasing M/Q ratio when compared with quiet coronal values and with average gradual SEP abundances obtained at 5–12 MeV nucleon to the -1. (6) The event-to-event variations in LSEP events are remarkably similar to those seen in CME-driven IP shocks and in 3He-rich SEP events. The above results cannot be explained by simply invoking the current paradigm for large gradual SEP events, i.e., that CME-driven shocks accelerate a seed population dominated by ambient coronal or solar wind ions. Instead, we suggest that the systematic M/Q-dependent enhancements in LSEP events are an inherent property of a highly variable suprathermal seed population, most of which is accelerated by mechanisms that produce heavy-ion abundances similar to those observed in impulsive SEP events. This heavy-ion-enriched material is subsequently accelerated at CME-driven shocks near the Sun by processes in which ions with higher M/Q ratios are accelerated less efficiently, thus causing the Fe/O ratios to decrease with increasing energy.

Associations between Fermi Gamma-ray Burst Monitor terrestrial gamma ray flashes and sferics from the World Wide Lightning Location Network

[1]xa0We report on a search for correlations between terrestrial gamma ray flashes (TGFs) detected by the Fermi Gamma-ray Burst Monitor (GBM) and lightning strokes measured using the World Wide Lightning Location Network (WWLLN). We associate 15 of a total 50 GBM-detected TGFs with individual discharges. We establish the relative timing between the TGF and the lightning stroke to an accuracy of <50 μs, and find that in 13 of these 15 lightning-TGF associations, the lightning stroke and the peak of the TGF are simultaneous to ∼40 μs. This suggests that a large fraction of TGFs are coincident with lightning discharges. The two nonsimultaneous associations do not show a consistent TGF-lightning stroke temporal sequence. All 15 associations are with sferics within 300 km of the subspacecraft position. For those TGFs not correlated with a particular lightning stroke, we find storm activity within 300 km of the subspacecraft position in all but four of the TGFs. For three of these four TGFs, we find storm activity very close to one of the magnetic footprints of the spacecraft position. We associate the subspacecraft TGFs with gamma ray events and the footprint events with electrons traveling along magnetic field lines before hitting the Fermi spacecraft.

Observations of stepping mechanisms in a rocket-and-wire triggered lightning flash

[1]xa0We present 10 high-speed video images that depict the bottom 150 m of a downward-negative, dart-stepped leader in a rocket-and-wire triggered flash, recorded at 240 kiloframes per second (4.17 μs frame integration time), along with correlated measurements of the X-ray emission at 50 m, electric field derivative (dE/dt) at 80 m, and the rocket-launch-tower current beneath the leader. We observed discrete segments of secondary channel that exhibited luminosity above that of the surrounding corona streamers and were distinctly separate and beneath the downward-extending leader channel. These segments appear similar to the space stems or space leaders that have been imaged in long negative laboratory sparks. Multiple simultaneous pulses in X-ray emission, dE/dt, and launch tower current were recorded during the time that the leader steps were imaged. The leader extended at an average downward speed between 2.7 × 106 and 3.4 × 106 m s−1.

Characteristics of energetic (≳30 keV/nucleon) ions observed by the Wind/STEP instrument upstream of the Earth's bow shock

We investigate here the characteristics of energetic ions (0.03-2.0 MeV/nucleon) during 1225 upstream events observed by the Energetic Particles: Anisotropy, Composition, and Transport/Suprathermal Energetic Particle (EPACT/STEP) instrument on board the Wind spacecraft from 1994 day 325 to 1999 day 92. We find that (1) the event occurrence rate showed significant variations with changes in the solar cycle, (2) the occurrence rate increased when both the solar wind speed and the geomagnetic activity index were enhanced, (3) most events were observed within ±80 R E in Y GSE and inside ∼100 R E in X GSE , although the events occurred at all locations of the Wind orbit, (4) ∼73% of the events were observed when the interplanetary magnetic field was radial and in the ecliptic plane, and when the spacecraft was most likely magnetically connected to the bow shock, (5) the events lasted typically between 10 min to 3 hours and exhibited strong sunward field-aligned flow, (6) ∼25% of the ion events were accompanied by 20-48 keV electrons as measured by the Wind/3DP instrument, (7) the energy spectra 100-300 keV protons and 30-300 keV/nucleon He-Fe during ∼70% of the events obeyed power laws with γ between 3 and 5, while the energy spectra of He and CNO for ∼30% of the events softened above ∼80 keV/nucleon obeyed power laws with γ ∼3-5, (8) the total energy ion spectrum above ∼0.5 MeV energy was dominated by heavier ions during the events, (9) a substantial fraction (≥40%) of the spectra for all species extended above ∼150 keV/e, and (10) the heavy ion composition of the events was similar to typical solar wind values. We compare the above findings with the main predictions of the magnetospheric leakage and the Fermi acceleration models and find that neither model can satisfactorily account for our results. We highlight the new challenges and requirements for both models.

COMPOSITION AND SPECTRAL PROPERTIES OF THE 1 AU QUIET-TIME SUPRATHERMAL ION POPULATION DURING SOLAR CYCLE 23

We have surveyed the spectral and compositional properties of suprathermal heavy ions during quiet times from 1995 January 1 to 2007 December 31 using Wind/Energetic Particles: Anisotropy, Composition, and Transport/SupraThermal-through-Energetic Particle Telescope and Advanced Composition Explorer/Ultra-low Energy Isotope Spectrometer at energies between 0.04 and 2.56 MeV nucleon–1. We find the following. (1) Quiet-time Fe/O and C/O abundances are correlated with solar cycle activity, reflecting corresponding values measured in solar energetic particle and interplanetary (IP) shock events during solar maximum, and those measured in the solar wind and corotating interaction regions (CIRs) during solar minimum conditions. (2) The 3He/4He ratio lies in the 3%-8% range during the quiet times of 1998-2004 with finite 3He detected on ~27.4% of the days. This ratio drops to 0.3%-1.2% during 2005-2007 and finite 3He is detected on ~5% of the days. (3) All heavy-ion species exhibit suprathermal tails between 0.04 and 0.32 MeV nucleon–1 with spectral indices ranging from ~1.27 to 2.29. These tails sometimes extend above ~2 MeV nucleon–1 with Fe spectra rolling over at lower energies than those of CNO. (4) The suprathermal tail spectral indices of heavier species (i.e., Fe) are harder than those of the lighter ones (i.e., CNO). These indices do not exhibit a clear solar cycle dependence and for ~50% of the time, they deviate significantly from the 1.5 value. These compositional observations provide evidence that even during the quietest times in IP space, the suprathermal population (3He and C-through-Fe) consists of ions from different sources whose relative contributions vary with solar activity. The heavy-ion energy spectra exhibit suprathermal tails with variable spectral indices that do not exhibit the spectral index of 1.5 predicted by some recent models.

Thunderstorm characteristics associated with RHESSI identified terrestrial gamma ray flashes

[1]xa0The characteristics of thunderstorms that produce terrestrial gamma-ray flashes (TGFs) observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are determined using climatological and meteorological data. RHESSI observed TGFs follow diurnal, seasonal, and geographic patterns that are very similar to those of thunderstorms confirming, in part, that these events are directly connected to thunderstorm activity. The TGF producing thunderstorms are shown to be closely associated with tall (ranging from 13.6 km to 17.3 km) tropical thunderstorm systems, a finding that is consistent with theoretical expectations from models of relativistic breakdown that relate the source region to the spectral signatures observed by RHESSI. Unlike sprites, there appears to be no predilection for TGFs to occur with large thunderstorm complexes. Rather, TGF producing thunderstorms are shown to range in areal extent by several orders of magnitude. Analysis of a single TGF event within the Mozambique Channel indicates an elevated mixed phase (both liquid water and ice present) level of approximately 6 km which is consistent with the climatological findings.

Solar Cycle Variations in the Composition of the Suprathermal Heavy-Ion Population near 1 AU

We have measured the annual variation in the quiet-time composition of interplanetary suprathermal ions between 0.04 and 1 MeV nucleon-1 from 1994 November 20 through 2006 January 1. Our results show the following: (1) The C/O and Fe/O ratios are similar to the corresponding values measured in the solar wind and corotating interaction regions (CIRs) during solar minimum conditions of 1994-1997 and 2005. (2) During periods of increased solar activity between 1998 and 2002, the C/O ratio is similar to that measured in solar energetic particle (SEP) events, while the Fe/O ratio lies between the values measured in coronal mass ejection-driven interplanetary shock and SEP events. (3) The 3He/4He ratio lies in the 1.5%-5% range during 1997-2005 August, but it dropped by an order of magnitude during 2005. We conclude that the composition of the suprathermal ion population between ~6 and 30 times the solar wind speed near 1 AU is dynamic and is essentially dominated by ions accelerated in SEP events during solar maximum conditions and by suprathermal solar wind ions and/or those accelerated in CIRs during solar minimum conditions.

Spatial and energy distributions of X‐ray emissions from leaders in natural and rocket triggered lightning

[1]xa0Energetic radiation is known to be produced by lightning. To investigate these emissions, ground-based observations are being conducted at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL where measurements of energetic radiation from both natural and rocket-triggered lightning discharges are recorded. In the present study, data from two natural negative cloud-to-ground stepped leaders and one rocket-triggered “chaotic” dart leader are analyzed in detail to investigate X-ray energy spectra and spatial X-ray distributions around the source. These measurements are compared with Monte Carlo simulations of runaway electron propagation with the goal of understanding the underlying mechanism of runaway electron production and their role in lightning initiation and propagation. We show that the energetic electrons that produce X-rays exhibit a characteristic energy less than 3xa0MeV for two natural and one rocket-triggered leaders investigated. In addition to studying these three leaders, energetic electron luminosity, total energy, and energetic electron per meter are compared to the following return stroke currents for 28 leaders from 12 different triggered flashes. Electron luminosity is found to increase exponentially with return stroke current up to about 10 kA and to be roughly constant for larger currents. The maximum electron luminosity, which is determined indirectly through X-ray luminosities, is on the order of 1017 electrons/s, which is less than the value found from theoretical calculations.