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Terrestrial gamma ray flashes (TGFs) are a class of enigmatic electrical discharges in the Earth’s atmosphere. In this study, we analyze an unprecedentedly large dataset comprised of 2188 TGFs whose signatures were simultaneously measured using space- and ground-based detectors over a five-year period. The Gamma-ray Burst Monitor (GBM) on board the Fermi spacecraft provided the energetic radiation measurements. Radio frequency (RF) measurements were obtained from the Global Lightning Dataset (GLD360). Here we show the existence of two categories of TGFs − those that were accompanied by quasi-simultaneous electromagnetic pulses (EMPs) detected by the GLD360 and those without such simultaneous EMPs. We examined, for the first time, the dependence of the TGF-associated EMP-peak-amplitude on the horizontal offset distance between the Fermi spacecraft and the TGF source. TGFs detected by the GBM with sources at farther horizontal distances are expected to be intrinsically brighter and were found to be associated with EMPs having larger median peak-amplitudes. This provides independent evidence that the EMPs and TGFs are produced by the same phenomenon, rather than the EMPs being from “regular” lightning in TGF-producing thunderstorms.

This work presents the first simultaneous X-ray measurement and high-speed video observation of the propagation of a lightning leader producing X-rays. As a result, the three-dimensional leader distance from the X-ray measurement and, for the first time, the conditions of the preexisting channel during the leader propagation were observed. Although four leaders in this seven-stroke flash followed the same path to ground, X-rays were only observed during the leader before the return stroke with the highest peak current. The fact that the other three leaders following the same path to ground did not produce detectable X-rays confirms the hypothesis that leader line charge density is an important factor that determines X-ray production. The fact that X-rays was recorded only when the leader tip was at a certain portion of the lightning channel confirms that the orientation of the leader plays an important role in the detection of X-rays.

We examined the occurrence characteristics of the lightning observed at the Gaisberg Tower (GBT) in the years 2000 to 2018 and analyzed current waveforms (measured using a 0.25-mΩ shunt) of upward flashes initiated from the tower. During this period, 865 flashes were recorded at the GBT, of which 823 (95%) were upward and 4 (0.5%) were downward. For 18 flashes, the current waveforms were ambiguous and for 20 flashes they were unsuitable for analysis. Of the 823 upward flashes, 651 (79%) were negative, 35 (4.3%) were positive, and 137 (17%) were bipolar. The median initial stage (IS)-current durations in upward negative, positive, and bipolar flashes were 275, 96, and 282 ms, respectively. The median IS-current peaks in these flashes were 1.4, 3.2, and 1.8 kA, respectively. We expanded the traditional classification of bipolar flashes to include five categories. Of the 137 bipolar flashes, 45% were of Type 1S (single reversal of current polarity during IS), 47% of Type 1M (multiple reversals of current polarity during IS), 5.1% of Type 2 [different polarities of current during IS and return stroke (RS)], 1.5% of Type 3 (RSs of different polarities), and 0.73% (one flash) of Type 4 (different polarities of RS and the following continuing current).

It has been argued in the technical literature, and widely reported in the popular press, that cosmic ray air showers (CRASs) can initiate lightning via a mechanism known as relativistic runaway electron avalanche (RREA), where large numbers of high energy and low energy electrons can, somehow, cause the local atmosphere in a thundercloud to transition to a conducting state. In response to this claim, other researchers have published simulations showing that the electron density produced by RREA is far too small to be able to affect the conductivity in the cloud sufficiently to initiate lightning. In this paper, we compare 74 days of cosmic ray air shower data collected in north central Florida during 2013, 2014, and 2015, the recorded CRASs having primary energies on the order of 1016 eV to 1018 eV and zenith angles less than 38 degrees, with Lightning Mapping Array (LMA) data, and we show that there is no evidence that the detected cosmic ray air showers initiated lightning. Furthermore, we show that the average probability of any of our detected cosmic ray air showers to initiate a lightning flash can be no more than 5 percent. If all lightning flashes were initiated by cosmic ray air showers, then about 1.6 percent of detected CRASs would initiate lightning, therefore we do not have enough data to exclude the possibility that lightning flashes could be initiated by cosmic ray air showers.

Monte Carlo simulations are used to determine source properties of terrestrial gamma-ray flashes (TGFs) as a function of atmospheric column depth and beaming geometry. The total mass per unit area traversed by all the runaway electrons (i.e., the total grammage) during a TGF, Ξ, is introduced, defined to be the total distance traveled by all the runaway electrons along the electric field lines multiplied by the local air mass density along their paths. It is shown that key properties of TGFs may be directly calculated from Ξ and its time derivative, including the gamma-ray emission rate, the current moment, and the optical power of the TGF. For the calculations presented in this paper, a standard TGF gamma-ray fluence, F0 = 0.1 cm-2 above 100 keV for a spacecraft altitude of 500 km, and a standard total grammage, Ξ0 = 1018 g/cm2, are introduced, and results are presented in terms of these values. In particular, the current moments caused by the runaway electrons and their accompanying ionization are found for a standard TGF fluence, as a function of source altitude and beaming geometry, allowing a direct comparison between the gamma rays measured in low-Earth orbit and the VLF-LF radio frequency emissions recorded on the ground. Such comparisons should help test and constrain TGF models and help identify the roles of lightning leaders and streamers in the production of TGFs.

The motivation of this work is to understand the effects of Terrestrial Gamma-ray Flashes (TGFs) on the ozone layer. One of the main ozone-destroying mechanisms is the production of NOx in the stratospheric region. NOx from lightning has been considered as a possible cause of ozone depletion but probably little of this NOx is transported from the tropopause to the stratosphere. Since the energetic particles of TGFs travel from ≈ 12 km to space, the resulting ionization can produce NOx directly in the stratosphere. In order to quantify the production of stratospheric NOx from TGFs, we use the Runaway Electron Avalanche Model (REAM) to simulate a typical setup of the acceleration region inside a thundercloud. The photons are then transported through the Earth's atmosphere, where they deposit some of their energy as ionization in the ozone layer. We then calculate the number of NOx molecules produced by considering the average energy required to produce one electron-ion pair. Finally the effect of TGF NOx production is estimated using the global annual rate of TGFs. It is estimated that the NOx production of TGFs is completely negligible compared to other sources, and therefore TGFs have no effect on the ozone layer.

We examined current waveforms of upward flashes initiated from the Gaisberg Tower in 2000 to 2018. Current was measured at the tower-top using a $\pmb{0.25-\text{m}\Omega}$ shunt. During this period, 865 flashes were recorded at the Gaisberg Tower, of which 823 (95%) were upward and 4 (0.5%) were downward. For 18 flashes the current waveforms were ambiguous and for 20 flashes they were unsuitable for analysis. Of the 823 upward flashes, 651 (79%) were negative, 35 (4.3%) were positive, and 137 (17%) were bipolar. The median initial stage (IS)-current durations in upward negative, positive, and bipolar flashes were 275, 96, and 282 ms, respectively. The median IS-current peaks in these flashes were 1.4, 3.2, and 1.8 $\mathbf{kA}$ , respectively. We expanded the traditional classification of bipolar flashes to include five categories. Of the 137 bipolar flashes, 45% were of Type 1S (single current polarity reversal during IS), 47% of Type 1M (multiple reversals of current polarity during IS), 5.1% of Type 2 (different polarities of current during IS and return stroke), 1.5% of Type 3 (return strokes of different polarities), and 0.73% (one flash) of Type 4 (different polarities of return stroke and the following continuing current).

We analyzed in detail currents associated with upward unconnected leaders (UULs) initiated from the Kennedy Space Center (KSC) Industrial Area Tower (IAT). Current measurements of UULs in natural lightning are relatively rare and provide important insights into the development and propagation of upward leaders. Eight UULs were initiated from the KSC-IAT between August 1, 2018 and November 15, 2019. All UULs were positive as they occurred in response to downward negative leaders that attached to ground near the tower. The KSC Mesoscale Eastern Range Lightning Information System (MERLIN) located these nearby strokes at distances ranging from 185 to 783 m from the tower, with the median being 538 m. The peak current for these strokes ranged from 13 to 69.3 kA, with the median being 26.7 kA. From the perspective of charge transferred, these UULs can be considered to be a bipolar lightning phenomenon; they transferred negative charge to ground between the inception of their current and the current-polarity reversal, following which, they effectively transferred positive charge to ground. We labeled the former time-period as the UUL development phase and the latter as the collapse phase. The median duration and charge transferred for the development phase were 789 μs and − 6.4 mC, respectively, and for the collapse phase were 388 μs and 4.7 mC, respectively. Overall, the net charge transferred to ground by UULs was negative. During the development phase, UUL-currents consisted of faster (total durations of the order of 10 μs) impulses overlaid on slower (millisecond-scale) “background” current. The background-to-peak UUL current-pulse amplitudes ranged from 3.4–289.2 A with the median being 30.1 A. The median pulse total duration and full-width at half-maximum (FWHM) were 14.1 μs and 4.9 μs, respectively. The median background-to-peak and 10-to-90% risetimes were 3.2 and 2.0 μs, respectively. Interpulse intervals ranged from 4.2–132.8 μs, with the median being 20 μs. Generally speaking, pulse amplitudes were larger, background currents were higher, and interpulse intervals were shorter at later times during UULs. It is likely that the UULs with higher background currents at later times were more closely approached by a downward leader branch versus the lower-current UULs.

We report on a terrestrial gamma ray flash (TGF) that occurred on 15 August 2014 coincident with an altitude-triggered lightning at the International Center for Lightning Research and Testing (ICLRT) in North Central Florida. The TGF was observed by a ground-level network of gamma ray, close electric field, distant magnetic field, Lightning Mapping Array (LMA), optical, and radar measurements. Simultaneous gamma ray and LMA data indicate that the upward positive leader of the triggered lightning flash induced relativistic runaway electron avalanches when the leader tip was at about 3.5 km altitude, resulting in the observed TGF. Channel luminosity and electric field data show that there was an initial continuous current (ICC) pulse in the lightning channel to ground during the time of the TGF. Modeling of the observed ICC pulse electric fields measured at close range (100–200 m) indicates that the ICC pulse current had both a slow and fast component (full widths at half maximum of 235 μs and 59 μs) and that the fast component was more or less coincident with the TGF, suggesting a physical association between the relativistic runaway electron avalanches and the ICC pulse observed at ground. Our ICC pulse model reproduces moderately well the measured close electric fields at the ICLRT as well as three independent magnetic field measurements made about 250 km away. Radar and LMA data suggest that there was negative charge near the region in which the TGF was initiated.

In this study we analyze the discharge morphologies of five confirmed negative sprite-parent discharges and the associated charge structures of the thunderstorms that produced them. The negative sprite-parent lightning took place in two thunderstorms that were associated with a tropical disturbance in east central and south Florida. The first thunderstorm, which moved onshore in east central Florida, produced four of the five negative sprite-parent discharges within a period of 17 min, as it made landfall from the Atlantic Ocean. These negative sprite-parents were composed of bolt-from-the-blue (BFB), hybrid intracloud-negative cloud-to-ground (IC-NCG), and multicell IC-NCGs discharges. The second thunderstorm, which occurred inland over south Florida, produced a negative sprite-parent that was a probable hybrid IC-NCG discharge and two negative gigantic jets (GJs). Weakened upper positive charge with very large midlevel negative charge was inferred for both convective cells that initiated the negative-sprite-parent discharges. Our study suggests tall, intense convective systems with high wind shear at the middle to upper regions of the cloud accompanied by low cloud-to-ground (CG) flash rates promote these charge structures. The excess amount of midlevel negative charge results in these CG discharges transferring much more charge to ground than typical negative CG discharges. We find that BFB discharges prefer an asymmetrical charge structure that brings the negative leader exiting the upper positive charge region closer to the lateral positive screening charge layer. This may be the main factor in determining whether a negative leader exiting the upper positive region of the thundercloud forms a BFB or GJ.

This paper reports a modeling study of the modifications of the nighttime lower ionospheric conductivity by electrostatic fields produced by underlying thunderstorms. The model used combines Ohm's law with a simplified lower ionospheric ion chemistry model to self-consistently calculate the steady state nighttime conductivity above a thunderstorm. The results indicate that although the electron density is generally increased, the lower ionospheric conductivity can be reduced by up to 1–2 orders of magnitude because electron mobility is significantly reduced due to the electron heating effect. For a typical ionospheric density profile, the resulting changes in the reflection heights of extremely low frequency and very low frequency waves are 5 and 2 km, respectively.

An isolated conducting sphere with rigid interface is used to model a water droplet, in order to investigate initiation process of streamer from a realistic hydrometeor. The method of image charge can then be adopted to calculate the electric field due to the space charge and its induced charge on the sphere. Numerical simulations show that initiation of negative streamers does not occur, but a steady corona discharge takes place around one tip of the sphere. This work provides a feasible method to study streamer initiation from real hydrometeors with rigid interface, and helps advance the initiation theory of lightning.

An acoustic camera comprising a linear microphone array is used to image the thunder signature of triggered lightning. Measurements were taken at the International Center for Lightning Research and Testing in Camp Blanding, FL, during the summer of 2014. The array was positioned in an end-fire orientation thus enabling the peak acoustic reception pattern to be steered vertically with a frequency-dependent spatial resolution. On 14 July 2014, a lightning event with nine return strokes was successfully triggered. We present the first acoustic images of individual return strokes at high frequencies (>1 kHz) and compare the acoustically inferred profile with optical images. We find (i) a strong correlation between the return stroke peak current and the radiated acoustic pressure and (ii) an acoustic signature from an M component current pulse with an unusual fast rise time. These results show that acoustic imaging enables clear identification and quantification of thunder sources as a function of lightning channel altitude.

Electric field values measured inside thunderclouds have consistently been reported to be up to an order of magnitude lower than the value required for the conventional electrical breakdown of air. This result has made it difficult to explain how lightning frequently occurs in thunderclouds. A few different theories have been offered to explain the lightning initiation process, one of them being the theory of lightning initiation from hydrometeors. According to this theory, lightning can be initiated from electrical discharges originating around thundercloud water or ice particles in the measured thundercloud electric field. These particles, called hydrometeors, are believed to cause significant enhancement of the thundercloud electric field in their vicinity and then initiate streamers that are the precursor discharges for the hot lightning leader channel. Previously, Liu et al. (2012a) reported streamer formation from a model hydrometeor in an electric field value of half of the conventional breakdown threshold (Ek) for air. In this paper, we present modeling results for streamer formation in electric fields as low as one third of the breakdown threshold. According to our results, initiation of stable streamers from thundercloud hydrometeors in a 0.3Ek electric field is possible, only if enhanced ambient ionization levels (e.g., the ionization created by corona discharges around the same or other nearby hydrometeors) are present ahead of the streamer. The magnitude and distribution of this ambient density may be a determining factor on whether the streamer branches, recovers after the prebranching stage, or continues propagating stably. We investigate the streamer branching behavior and characteristics and test a theory that has recently been proposed to explain this phenomenon. We find that the geometry of the streamer head plays an important role in the streamer branching phenomena. The fast radial movement of the maximum streamer head curvature, combined with the slow reduction of the maximum curvature value, eventually leads the streamer head to branching. Finally, we compare our modeling results with laboratory experiments and realistic thundercloud conditions and discuss the implications of this study to lightning initiation and other lightning-related phenomena.

The terrestrial gamma ray flash (TGF) is an emission of highly energetic radiation produced by or at least in close association with lightning. Previous investigations attempted to isolate the production mechanisms and production altitude(s) of TGFs as well as macrophysical characteristics, while thunderstorm microphysical characteristics were largely ignored. This investigation into thunderstorms and their hydrometeor and flash characteristics utilize temporal and spatial coincident satellite passes between the Reuven Ramaty High Energy Solar Spectroscopic Imager and the Tropical Rainfall Measuring Mission to determine the bulk (or footprint) microphysical properties of two types of study events, the thunderstorm complexes which are associated with TGFs (TGF case) and the thunderstorm complexes which did not produce a TGF detected by Reuven Ramaty High Energy Solar Spectroscopic Imager during the pass (non-TGF case). Results are presented for two different comparison methods. The first case utilizes geographic region weighted by TGF distribution, and the second is based on TGF percentage of occurrence when compared to total flash count of data set. Results show that the associated storms around the TGF location possess differences in the hydrometeor concentrations: cloud liquid water, cloud ice, precipitation water, and precipitation ice. These results take place at different levels of the atmosphere, including the mixed phase region. Additionally, results will show that TGFs are a consistent percentage of observed flashes as the rate of TGFs as a function of Lightning Imaging Sensor flash count is relatively constant.

Small electrostatic fields of values not strong enough to activate electron impact ionization, attachment, and detachment processes can be established in the upper atmosphere by underlying thunderstorms. This paper investigates their effects on the ionospheric densities by using a simplified ion chemistry model. The modeling results indicate that under the steady state condition, the nighttime electron density profile can be reduced by up to ∼40% or enhanced by a factor of up to ∼6 because of the variation of the three-body electron attachment rate constant with the electric field.

Runaway electron avalanches developing in thunderclouds in high electric field become self-sustaining due to relativistic feedback via the production of backward propagating positrons and backscattered X-rays. To date, only positrons created from pair production by gamma rays interacting with the air have been considered. In contrast, direct electron-positron pair production, also known as “trident process,” occurs from the interaction of energetic runaway electrons with atomic nuclei, and so it does not require the generation of a gamma ray mediator. The positrons produced in this process contribute to relativistic feedback and become especially important when the feedback factor value approaches unity. Then the steady state flux of runaway electrons increases significantly. In certain cases, when the strong electrostatic field forms in a narrow area, the direct positrons become one of processes dominating relativistic feedback. Calculations of the direct positron production contribution to relativistic feedback are presented for different electric field configurations.

The evolution of He+-mode electromagnetic ion cyclotron (EMIC) waves is studied inside the geostationary orbit using our global model of ring current (RC) ions, electric field, plasmasphere, and EMIC waves. In contrast to the approach previously used by Gamayunov et al. (2009), however, we do not use the bounce-averaged wave kinetic equation but instead use a complete, nonbounce-averaged, equation to model the evolution of EMIC wave power spectral density, including off-equatorial wave dynamics. The major results of our study can be summarized as follows. (1) The thermal background level for EMIC waves is too low to allow waves to grow up to the observable level during one pass between the “bi-ion latitudes” (the latitudes where the given wave frequency is equal to the O+–He+ bi-ion frequency) in conjugate hemispheres. As a consequence, quasi-field-aligned EMIC waves are not typically produced in the model if the thermal background level is used, but routinely observed in the Earth's magnetosphere. To overcome this model-observation discrepancy we suggest a nonlinear energy cascade from the lower frequency range of ultralow frequency waves into the frequency range of EMIC wave generation as a possible mechanism supplying the needed level of seed fluctuations that guarantees growth of EMIC waves during one pass through the near equatorial region. The EMIC wave development from a suprathermal background level shows that EMIC waves are quasi field aligned near the equator, while they are oblique at high latitudes, and the Poynting flux is predominantly directed away from the near equatorial source region in agreement with observations. (2) An abundance of O+ strongly controls the energy of oblique He+-mode EMIC waves that propagate to the equator after their reflection at bi-ion latitudes, and so it controls a fraction of wave energy in the oblique normals. (3) The RC O+ not only causes damping of the He+-mode EMIC waves but also causes wave generation in the region of highly oblique wave normal angles, typically for θ > 82°, where a growth rate γ > 10−2rad/s is frequently observed. The instability is driven by the loss cone feature in the RC O+ distribution function, where ∂F/∂v⟂>0 for the resonating O+. (4) The oblique and intense He+-mode EMIC waves generated by RC O+ in the region L≈2–3 may have an implication to the energetic particle loss in the inner radiation belt.

Simplified equations describing the transport and energy spectrum of runaway electrons are derived from the basic kinematics of the continuity equations. These equations are useful in modeling the energy distribution of energetic electrons in strong electric fields, such as those found inside thunderstorms. Dwyer and Babich (2011) investigated the generation of low-energy electrons in relativistic runaway electron avalanches. The paper also developed simple analytical expressions to describe the detailed physics of Monte Carlo simulations of relativistic runaway electrons in air. In the current work, the energy spectra of the runaway electron population are studied in detail. Dependence of electron avalanche development on properties such as the avalanche length, radiation length, and the effective Moller scattering efficiency factor are discussed in detail. To describe the shapes of the electron energy spectra for a wide range of electric field strengths, the diffusion term responsible for random deviation of electron energy ionization loss from the mean value is added to the kinetic equation. We find that the diffusion in energy space helps maintain an exponential energy spectrum for electric fields that approach the runaway electron threshold field.

We investigate the structure of X-ray emissions from downward triggered lightning leaders using a pinhole-type X-ray camera (XCAM) located at the International Center for Lightning Research and Testing. This study builds on the work of Dwyer et al. (2011), which reported results from XCAM data from the 2010 summer lightning season. Additional details regarding the 2010 data are reported here. During the 2011 summer lightning season, the XCAM recorded 12 out of 17 leaders, 5 of which show downward leader propagation. Of those five leaders, one dart-stepped leader and two chaotic dart leaders are the focus of this paper. These three leaders displayed unique X-ray emission patterns: a chaotic dart leader displayed a diffuse structure (i.e., a wide lateral “spraying” distribution of X-rays), and a dart-stepped leader and a chaotic dart leader exhibited compact emission (i.e., a narrow lateral distribution of strong X-ray emission). These two distinct X-ray emission patterns (compact and diffuse) illustrate the variability of lightning leaders. Using Monte Carlo simulations, we show that the diffuse X-ray source must originate from a diffuse source of energetic electrons or possibly emission from several sources. The compact X-ray sources originate from compact electron sources, and the X-ray source region radius and electric charge contained within the X-ray source region were between 2 and 3 m and on the order of 10–4 C, respectively. For the leaders under investigation, the X-ray source region average currents were determined to be on the order of 102 A.

[1] Compact intracloud discharges (CIDs) are sources of the powerful, often isolated radio pulses emitted by thunderstorms. The VLF-LF radio pulses are called narrow bipolar pulses (NBPs). It is still not clear how CIDs are produced, but two categories of theoretical models that have previously been considered are the Transmission Line (TL) model and the Relativistic Runaway Electron Avalanche-Extensive Air Showers (RREA-EAS) model. In this paper, we perform numerical calculations of RREA-EASs for various electric field configurations inside thunderstorms. The results of these calculations are compared to results from the other models and to the experimental data. Our analysis shows that different theoretical models predict different fundamental characteristics for CIDs. Therefore, many previously published properties of CIDs are highly model dependent. This is because of the fact that measurements of the radiation field usually provide information about the current moment of the source, and different physical models with different discharge currents could have the same current moment. We have also found that although the RREA-EAS model could explain the current moments of CIDs, the required electric fields in the thundercloud are rather large and may not be realistic. Furthermore, the production of NBPs from RREA-EAS requires very energetic primary cosmic ray particles, not observed in nature. If such ultrahigh-energy particles were responsible for NBPs, then they should be far less frequent than is actually observed.

This paper describes a method of simulating solar energetic particle propagation through the magnetic fields of the solar corona and interplanetary medium. The simulation code is based on the focus transport equation of energetic particles in 3-d magnetic fields, which contains all the particle transport mechanisms, including streaming, convection, gradient/curvature drift, adiabatic focusing, pitch angle scattering by Alfvénic magnetic field fluctuations and perpendicular diffusion due to the random walk of field lines. In the simulation, particles are injected at their source in the corona, and their guiding center trajectories are calculated using stochastic differential equations. Because of the vastly different time scales of particle transport mechanisms included in the equation, we use the 4-th order Runge-Kutta method to integrate the particle streaming and adiabatic focusing terms, while the stochastic terms of pitch angle scattering and perpendicular diffusion are integrated with the Euler scheme. The model is applied to the 2017 September 10 solar energetic particle event. With perpendicular diffusion, we are able to explain SEP observations from Earth and STEREO-A. A pattern of SEP precipitation on the solar surface is also predicted.

[1] Modeling results of sprite streamer formation from large ionospheric inhomogeneities or patches (several tens to hundreds of meter wide) indicate that positive streamers can be initiated at subbreakdown conditions from the inhomogeneities with a density comparable to sprite halo densities. For spherical patches with a given radius, the minimum density required for streamer initiation decreases with increasing ambient field. For a given density, the minimum size of the inhomogeneity decreases with increasing ambient field. The modeling results on the associated optical emissions show that a luminous spherical-like cap appears around the lower tip of the ionization patch before streamer initiation, and the streamer is initiated from the bottom of this cap, which appears to be similar to streamer initiation from luminous structures in the lower ionosphere recorded by recent high-speed videos. Our study suggests that if the sprite halo front is unstable, inhomogeneities developing from it can initiate sprite streamers at subbreakdown conditions.

[1] The source of terrestrial gamma-ray flashes (TGFs) has been an active area of research since their discovery by Compton Gamma-Ray Observatory/Burst and Transient Source Experiment in 1994. These intense bursts of gamma rays originate within thunderclouds via the rapid production of relativistic runaway electrons, accelerated by thundercloud/lightning electric fields to multi-MeV energies. Recent studies have shown that electrical discharges within thunderclouds caused by these runaway electrons can generate currents that rival those of lightning and so could serve as an alternative discharge path for thunderclouds. In particular, the discharges responsible for TGFs produce some of the largest amplitude VLF-LF radio pulses from thunderstorms and, indeed, were previously misidentified as lightning. In this article, we shall provide a brief review of TGFs and present calculations of their electrical properties inside thunderclouds and their gamma-ray and optical emissions. We shall show that the lightning-like events responsible for TGFs emit relatively little visible light and, thus, are inherently dark.

This paper explores the impact of the geomagnetic field on the relativistic runaway electron avalanche length, λe−. Coleman and Dwyer [2006] developed an analytical fit to Monte Carlo simulations using the Runaway Electron Avalanche Model (REAM). In this work, we repeat this process but with the addition of the geomagnetic field in the range of [100,900]/nμT, where n is the ratio of the density of air at altitude to the sea level density. As the ambient electric field approaches the runaway threshold field (Eth≈ 284 kV/m sea level equivalent), it is shown that the magnetic field has an impact on the orientation of the resulting electron beam. The runaway electrons initially follow the vertically oriented electric field but then are deflected in the v×B direction, and as such the electrons experience more dynamic friction due to the increase in path length. This will be shown to result in a difference in the avalanche length from the case where B = 0. It will also be shown that the average energy of the runaway electrons will decrease while the required electric field to produce runaway electrons increases. This study is also important in understanding the physics of Terrestrial Gamma-ray Flashes (TGFs). Not only will this work impact relativistic feedback rates determined from simulations, it may also be useful in studying spectroscopy of TGFs observed from balloon and aircraft measurements. These models may also be used in determining beaming properties of TGFs originating in the tropical regions seen from orbiting spacecraft.

This paper reports a study on the inception condition of positive corona discharges around thundercloud hydrometeors that are simulated as a spherical point electrode. The corona inception from the hydrometeor is investigated using a model developed by Naidis (2005), which suggests that the discharge becomes self-sustaining when the number of ionizing photons produced by all secondary electron avalanches is equal to that by a primary avalanche. For an isolated, charged hydrometeor in dry air of varying pressure, the results show that the onset of positive corona discharges from a large hydrometeor at high pressure requires a stronger avalanche multiplication than those from a small hydrometeor at low pressure. The onset voltage and surface electric field are obtained for thundercloud hydrometeors of different radii. For a hydrometeor of a radius of 1 mm at atmospheric pressure, the onset surface field reaches about 2.75 times the conventional breakdown threshold field. In humid air (a gas mixture of dry air and water vapor), it is found that a stronger avalanche is required for the corona onset, and the effect of humidity is more pronounced for large hydrometeors. The absorption of the ionizing photons by water vapor is the main reason why a stronger avalanche is required for the corona inception in humid air. As corona discharges from an isolated hydrometeor remove the charge on it, this discharging mechanism sets an upper limit on hydrometeor charge that agrees reasonably with the reported observations of maximum precipitation charge. Finally, we discuss the corona inception condition from a charged hydrometeor in non-zero ambient field of thundercloud condition.

The dominant and unexpected feature in the first Interstellar Boundary EXplorer (IBEX) maps is a ribbon of the enhanced energetic neutral atom (ENA) emissions. Presenting the first results from IBEX, McComas et al. identified six possible mechanisms of ribbon formation. One of the mechanisms, the so-called secondary ENA mechanism, was already quantitatively elaborated by Heerikhuisen et al., and they successfully reproduced the main features of the ribbon. We further study the "secondary ENA" mechanism by quantifying a previously omitted stage of the proton evolution between two consecutive acts of the charge-exchange in the outer heliosheath (OHS). The main findings can be summarized as follows. (1) The neutrals supplied by the supersonic near-equatorial solar wind dominate the near-equatorial source of the keV ENAs in the OHS compared to the inner heliosheath contribution. (2) The ribbon of the observed width can be produced even if only the large-scale (~102-104 AU) interstellar turbulence operates but the resulting pitch angle distribution functions (PADFs) are unstable with respect to the ion cyclotron wave generation around the locus where the line of sight from IBEX to the ribbon is perpendicular to the interstellar magnetic field beyond the heliopause. (3) A combination of the large-scale interstellar turbulence and a small-scale (~10–5to10–4 AU) turbulence generated by an unstable PADF of the energetic protons is able to make PADF marginally stable. In this case, the ribbon is still narrow because only a small part of the proton phase space distribution function can resonate with a locally generated ion cyclotron turbulence. (4) A concurrent operation of the large-scale interstellar turbulence and the locally generated small-scale turbulence might be responsible for the localized emission structures observed in the IBEX ribbon.

In this study, the first electric field pulse of cloud and cloud-to-ground discharges were analyzed and compared with other pulses of cloud discharges. Thirty eight cloud discharges and 101 cloud-t ...

In this paper, based on theoretical estimation of the achievable electric fields during the physical development process of a long spark under different conditions, we show that the encounter of negative and positive streamer fronts just before the final breakdown is one scenario, under which the observed X-ray bursts in long sparks is highly possible. Our calculations show that for example in an 80 cm long rod–sphere air gap at atmospheric pressure with negative lightning impulse breakdown voltage of about 925 kV, electrons are accelerated to values in the range of 100–300 keV during the encounter. Subsequently, these electrons gain more energy moving through the potential gradient of the positive streamer region. The total gain of energy by electrons may reach 300–500 keV. The results also show that negative discharges can produce more energetic electrons than positive. If the suggested mechanism of X-ray production in long sparks is correct, then the X-ray burst may consist of several pulses closely spaced in time. Time resolved photography in simultaneous measurement of X-rays would be able to confirm this prediction.

We examine the distribution and propagation of energy in the plasma sheet and lobes using observations and simulations for three substorms. The substorms occurred on 9 March 1995, 10 December 1996, and 27 August 2001 and have been simulated using the Lyon-Fedder-Mobarry magneto-hydrodynamic code. All three events occur over North America and show a clear substorm current wedge over the ground magnetometer chains of Alaska, Canada, and Greenland. The three simulations show the thinning of the plasma sheet during the growth phase of the event and an increase in the relative amount of thermal energy due to the compression of the plasma sheet. Generally, the total lobe energy, polar cap flux, and lobe magnetic field strength simultaneously increase during the growth phase, and polar cap flux and total lobe energy only start dropping at substorm onset, as measured by the CANOPUS magnetometer chain. Starting at time of onset and continuing throughout the expansion phase a transfer of magnetic energy from the lobes into the plasma sheet occurs, with the increase in the plasma sheet energy ranging from 30–40% of the energy that is released from the lobes.

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.

This paper presents a model calculation of solar energetic particle propagation in a three-dimensional interplanetary magnetic field. The model includes essentially all the particle transport mechanisms: streaming along magnetic field lines, convection with the solar wind, pitch-angle diffusion, focusing by the inhomogeneous interplanetary magnetic field, perpendicular diffusion, and pitch-angle dependent adiabatic cooling by the expanding solar wind. We solve the Fokker–Planck transport equation with simulation of backward stochastic processes in a fixed reference frame in which any spacecraft is roughly stationary. As an example we model the propagation of those high-energy (E 10 MeV) solar energetic particles in gradual events that are accelerated by large coronal mass ejection shocks in the corona and released near the Sun into interplanetary space of a Parker spiral magnetic field. Modeled with different scenarios, the source of solar energetic particles can have a full or various limited coverages of latitude and longitude on the solar surface. We compute the long-term time profiles of particle flux and anisotropy at various locations in the heliosphere up to 3 AU, from the ecliptic to high latitudes. Features from particle perpendicular diffusion are revealed. Our simulation reproduces the observed reservoir phenomenon of solar energetic particles with constraints on either solar particle source or the magnitude of perpendicular diffusion.

Galactic cosmic-ray composition reflects the effects of nuclear interactions during propagation through the interstellar medium. In order to use measurements to determine the source of cosmic rays, one needs a model to deconvolute the propagation effect. This paper presents a new numerical method to solve cosmic-ray diffusive transport equations with a complete network of nuclear interactions. The new method uses the time backward Markov stochastic processes to solve the transport equation by tracing the particles' trajectories starting from the solar system back to their sources in the Galaxy. To make the calculation efficient for large arrays of equations for many cosmic-ray species, a matrix representing the composition of all cosmic-ray heavy nuclei and location is used. The results for abundance ratios of key elements such as B/C and sub-Fe/Fe compared with observations from HEAO-3, ACE, and Ulysses and for isotopic ratios such as 36Cl/37Cl and 54Mn/55Mn are shown to be consistent with other numerical approaches. The ability to track particles through the Galaxy, which is inherent in this technique, offers new opportunities for investigations. For example, one can look at the spatial and propagation time distributions and element composition of source particles that arrive in the solar system.

Intensities of ∼1–10 MeV relativistic electrons in several energy channels of the high-energy telescope (HET) on Ulysses increase dramatically during its flybys of Jupiter in 1992 and 2004. Perpendicular diffusion coefficients of these particles are derived by fitting the spatial profile of Jovian electron intensity to a diffusion-convection model of particle transport. It is found that the latitudinal diffusion coefficient during the 2004 Jupiter flyby has to be enhanced from its value during the 1992 Jupiter flyby and it is also enhanced relative to the radial perpendicular diffusion. Such an enhancement of latitudinal particle transport was implied previously through the observations of Jovian electrons, cosmic rays and solar energetic particles at high heliographic latitudes, and now this requirement extends further to low latitude region of the heliosphere. Energy dependence of the perpendicular diffusion coefficient is obtained quite precisely through the variation in the slope of energy spectrum of Jovian electrons. The perpendicular diffusion coefficient increases with energy, which can put a tight constraint on models of the particle transport coefficient. The newest nonlinear guiding center (NLGC) theory of perpendicular diffusion is consistent with this observation, but only when it is combined with a parallel diffusion coefficient from the quasilinear theory in a slab magnetic turbulence without dynamic damping.

Although the production of X-rays from natural and rocket-triggered lightning leaders have been studied in detail over the last 10 years, the energy spectrum of the X-rays has never been well measured because the X-rays are emitted in very short but intense bursts that result in pulse pileup in the detectors. The energy spectrum is important because it provides information about the source mechanism for producing the energetic runaway electrons and about the electric fields that they traverse. We have recently developed and operated the first spectrometer for the energetic radiation from lightning. The instrument is part of the Atmospheric Radiation Imagery and Spectroscopy (ARIS) project and will be referred to as ARIS-S (ARIS Spectrometer). It consists of seven 3′′ NaI(Tl)/photomultiplier tube scintillation detectors with different thicknesses of attenuators, ranging from no attenuator to more than 1′′ of lead placed over the detector (all the detectors are in a 1/8′′ thick aluminum box). Using X-ray pulses preceding 48 return strokes in 8 rocket-triggered lightnings, we found that the spectrum of X-rays from leaders is too soft to be consistent with Relativistic Runaway Electron Avalanche. It has a power law dependence on the energies of the photons, and the power index, λ, is between 2.5 and 3.5. We present the details of the design of the instrument and the results of the analysis of the lightning data acquired during the summer of 2012.

Observation of upward electrical discharges from thunderstorms has been sporadically reported in the scientific literature. According to their terminal altitudes, they are classified as starters (20–30 km), jets (40–50 km) and gigantic jets (70–90 km). They not only have a significant impact on the occupied atmospheric volumes but also electrically couple different atmospheric regions. However, as they are rare and unpredictable, our knowledge of them has been built on observations that typically record only one type of such discharges. Here we report a close-distance observation of seven upward discharges including one starter, two jets and four gigantic jets above Tropical Depression Dorian. Our optical and electromagnetic data indicate that all events are of negative polarity, suggesting they are initiated in the same thundercloud charge region. The data also indicate that the lightning-like discharge channel can extend above thunderclouds by about 30 km, but the discharge does not emit low-frequency electromagnetic radiation as normal lightning., Upward electrical discharges from thunderstorms were discovered recently, and only a very limited set of observations exist because they are rare and unpredictable. Here, the authors present recordings of different types of the discharge above a storm, which contradict current theories of their origins.

An example of Galactic cosmic-ray modulation in a fully three-dimensional heliosphere is presented here. We use a stochastic particle method to solve for modulation without requiring symmetric boundaries or fields. We include all typical modulation terms, including full three-dimensional drift. We have applied this to an MHD heliosphere appropriate for solar minimum conditions. This field includes nonradial solar wind velocity components, as well as a built-in nonspherical termination shock. Parameters that are of interest in modulation can be analyzed in detail, particularly the momentum change of cosmic rays during their transport through the heliosphere. We show radial profiles of modulation at different energies, latitude, and longitude, as well as the more traditional modulated spectra at various locations. Finally, we are able to show in detail where particles at a particular energy and location gain and lose energy. We are able to conclude that the radial profiles of modulation are strongly dependent on the latitudinal angle with respect to the stagnation point. We see that modulation of GCRs near the stagnation point takes place mostly upstream of the shock but that modulation continues far outside the shock as the stochastic particles will random walk across the shock several times before entering our observation region. The modulation profiles remain similar with differing longitude, although the exact shape of the radial profile and spectra are strongly dependent on the termination shock location. Cosmic-ray intensity at high energies can be higher than the ISM intensity, while at low energies cosmic rays still experience solar modulation. The termination shock and heliosheath regions were important to modulation in the outer heliosphere. Finally, the sign of the latitudinal gradient is not a simple function of the qA direction, in general.