Gamma-ray 'glows' are long duration (seconds to tens of minutes) X-ray and gamma-ray emission coming from thunderclouds. Measurements suggest the presence of relativistic runaway electron avalanches (RREA), the same process underlying terrestrial gamma-ray flashes. Here we demonstrate that glows are relatively a common phenomena near the tops of thunderstorms, when compared with events such as terrestrial gamma-ray flashes. Examining the strongest glow measured by the airborne detector for energetic emissions, we show that this glow is measured near the end of a downward RREA, consistent with occurring between the upper positive charge layer and the negative screening layer above it. The glow discharges the upper positive layer by Z9.6 mA, strong enough to be an important charging mechanism of the storm. For this glow, the gamma-ray flux observed is close to the value at which relativistic feedback processes become important, with an avalanche multiplication factor of 4,500.

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.

Energetic Lightning Emissions (ADELE), an array of six gamma-ray detectors, detected a brief burst of gamma rays while flying aboard a Gulfstream V jet near two active thunderstorm cells. The duration and spectral characteristics of the event are consistent with the terrestrial gamma ray flashes (TGFs) seen by instruments in low Earth orbit. A long-duration, complex +IC flash was taking place in the nearer cell at the same time, at a distance of ∼10 km from the plane. The sferics that are probably associated with this flash extended over 54 ms and included several ULF pulses corresponding to charge moment changes of up to 30 C km, this value being in the lower half of the range of sferics associated with TGFs seen from space. Monte Carlo simulations of gamma ray propagation in the Earth's atmosphere show that a TGF of normal intensity would, at this distance, have produced a gamma ray signal in ADELE of approximately the size and spectrum that was actually observed. We conclude that this was the first detection of a TGF from an aircraft. We show that because of the distance, ADELE's directional and spectral capabilities could not strongly constrain the source altitude of the TGF but that such constraints would be possible for TGFs detected at closer range.

We searched for gamma‐ray emission from lightning using the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite by identifying times when RHESSI was near over 2 million lightning discharges localized by the Worldwide Lightning Location Network (WWLLN). We then stacked together the gamma‐ray arrival times relative to the sferic times, correcting for light propagation time to the satellite. The resulting stacked gamma‐ray time profile is sensitive to an average level of gamma‐ray emission per lightning discharge far lower than what can be recognized above background for a single terrestrial gamma‐ray flash (TGF). The summed signal from presumed small, previously unknown TGFs simultaneous with WWLLN discharges is remarkably weak: for the region from 0 to 300 km beneath RHESSI's footprint, (6.2 ± 3.8) × 10−3 detector counts/discharge are measured, as opposed to a typical range of 12–50 detector counts for TGFs identified solely from the gamma‐ray signal. Under the assumption of a broken power law differential distribution of TGF intensities, we find that the index must harden dramatically or cut off just below the sensitivity limit of current satellites and that for most scenarios less than 1% of lightning can produce a TGF that belongs anywhere in the same distribution as those that are observable. For the minority of scenarios where more than a few percent of flashes produce a TGF, most of these “TGFs” are less than 10−4 of the luminosity of the faintest RHESSI TGFs and therefore closer to the luminosity of lightning stepped leaders. The rarity of TGFs holds not only for TGFs simultaneous with the sferic observed by WWLLN but also for any time within 10 ms of the sferic, allowing (for example) for the possibility that different events within the upward propagation of a negative leader in positive intracloud lightning triggered the TGF and WWLLN's detection.