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.

[1] The magnitudes of scattered fields produced during early/fast very low frequency (VLF) events observed at 13 closely spaced ($65 km) sites are compared with those expected for sprite halo disturbances using a numerical model of wave propagation within the Earth-ionosphere waveguide. Three different early/fast events of varying magnitudes are analyzed using three different nighttime ambient lower ionospheric electron density profiles. The electron density disturbances associated with sprite-halo events are determined using a full-wave electromagnetic (FWEM) model. Observed scattered field amplitudes of typical (VLF amplitude changes of 0.2 dB < ÁA < 0.8 dB) Early/fast events agree with model calculations within a factor of two when the peak return stroke currents (as recorded by the National Lightning Detection Network, referred to in this work as NLDN) are used to determine the sprite-halo characteristics. Scattered field amplitudes associated with larger early/fast events (ÁA > 1 dB) are found to be within a factor of seven for peak currents of causative lightning based on NLDN. However, in previous studies, some sprite-producing lightning flashes have exhibited large slow-tail components, indicating substantial continuing currents and implying charge removal up to 2-3 times larger than that inferred from the peak current reported by NLDN. For the cases discussed in this paper, scattered field calculations using disturbances caused by 2-3 times larger charge removal are found to be within a factor of two of the measured values. VLF scattering from electron density changes associated with sprite halos thus appear to be the underlying cause of at least some of the VLF perturbations observed as early/fast events.

[1] Ionospheric effects of energetic electron precipitation induced by controlled injection of VLF signals from a ground based transmitter are observed via subionospheric VLF remote sensing. The 21.4 kHz NPM transmitter in Lualualei, Hawaii is keyed ON-OFF in 30 minute periodic sequences. The same periodicity is observed in the amplitude and phase of the sub ionospherically propagating signals of the 24.8 kHz NLK (Jim Creek, Washington) and 25.2 kHz NLM (LaMoure, North Dakota) transmitters measured at Midway Island. Periodic perturbations of the NLK signal observed at Palmer, Antarctica suggest that energetic electrons scattered at longitudes of NPM continue to be precipitated into the atmosphere as they drift toward the South Atlantic Anomaly. Utilizing a model of the magnetospheric waveparticle interaction, ionospheric energy deposition, and subionospheric VLF propagation, the precipitated energy flux induced by the NPM transmitter is estimated to peak at L $ 2 and $ 1.6 Â 10 À4 ergs s À1 cm À2 . Citation: Inan,

The giant γ‐ray flare from SGR 1806‐20 created a massive disturbance in the daytime lower ionosphere, as evidenced by unusually large changes in amplitude/phase of subionospherically propagating VLF signals. The perturbations of the 21.4 kHz NPM (Lualualei, Hawaii) signal observed at PA (Palmer Station, Antarctica) correspond to electron densities increasing by a factor of ∼100 to ∼103 cm−3 at ∼60 km and ≳1000 to ∼10 cm−3 at ∼30 km altitude. Enhanced conductivity produced by flare onset endured for >1 hour, the time scale determined by mutual neutralization. A brief (∼100 ms) low frequency (∼3 to 6 kHz) emission is also observed during the flare onset.

Chorus waves, among the most intense electromagnetic emissions in the Earth’s magnetosphere, magnetized planets, and laboratory plasmas, play an important role in the acceleration and loss of energetic electrons in the plasma universe through resonant interactions with electrons. However, the spatial evolution of the electron resonant interactions with electromagnetic waves remains poorly understood owing to imaging difficulties. Here we provide a compelling visualization of chorus element wave–particle interactions in the Earth’s magnetosphere. Through in-situ measurements of chorus waveforms with the Arase satellite and transient auroral flashes from electron precipitation events as detected by 100-Hz video sampling from the ground, Earth’s aurora becomes a display for the resonant interactions. Our observations capture an asymmetric spatial development, correlated strongly with the amplitude variation of discrete chorus elements. This finding is not theoretically predicted but helps in understanding the rapid scattering processes of energetic electrons near the Earth and other magnetized planets.

We present calibrated measurements of ELF waves generated by modulated HF heating of the auroral electrojet by the High frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska, and detected after propagating more than 4400 km in the Earth‐ionosphere waveguide to Midway Atoll. The magnitude of the 2125 Hz wave received at Midway Atoll is consistent with the radiation from a horizontal dipole located at the altitude of the maximum Hall conductivity variation (created by modulated HF heating) and radiating ∼4–32 W. The HF‐ELF conversion efficiency at HAARP is thus estimated to be ∼0.0004–0.0032% for the 2125 Hz wave generated using sinusoidal amplitude modulation.

We present detailed observations of the onset of amplitude saturation in ELF/VLF waves generated via modulated HF heating of naturally‐forming, large‐scale current systems, such as the auroral electrojet. Broadband ELF/VLF measurements at a ground‐based receiver located near the High‐Frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska, exhibit variations in signal amplitude which are qualitatively consistent with a hard‐limiting approximation of the saturation process. A method to approximate the saturation curve as a function of HF power from experimental data is presented, and the results indicate that a ∼5–10% reduction in generated ELF signal amplitude is typical at the maximum radiated HF power level (771 kW) for modulation frequencies between 1225 Hz and 3365 Hz. For HF transmissions using sinusoidal amplitude modulation, the saturation dominantly affects the second harmonic of the generated ELF/VLF signal, with amplitudes on average 16% lower than expected at the maximum HF power level.

Modulated heating of the lower ionosphere with the HAARP HF heater is used to excite 1–2 kHz signals observed on a ship‐borne receiver in the geomagnetic conjugate hemisphere after propagating as ducted whistler‐mode signals. These 1‐hop signals are believed to be amplified, and are accompanied by triggered emissions. Simultaneous observations near (∼30 km) HAARP show 2‐hop signals which travel to the northern hemisphere upon reflection from the ionosphere in the south. Multiple reflected signals, up to 10‐hop, are detected, with the signal dispersing and evolving in shape, indicative of re‐amplification and re‐triggering of emissions during successive traversals of the equatorial interaction regions.