Terrestrial 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.

Terrestrial gamma‐ray flashes (TGFs) are bright, sub‐millisecond bursts of gamma‐rays, originating within the Earth's atmosphere. Most TGFs have been detected by spacecraft in low‐Earth orbit. Only two TGFs have previously been observed from within our atmosphere: one at ground level and one from an aircraft at 14.1 km. We report on a new TGF‐like gamma‐ray flash observed at ground level, detected by the 19‐station Thunderstorm Energetic Radiation Array (TERA) at the University of Florida/Florida Tech International Center for Lightning Research and Testing (ICLRT). The gamma‐ray flash, which had a duration of 52.7 μs, occurred on June 30, 2009 during a natural negative cloud‐to‐ground lightning return stroke, 191 μs after the start of the stroke. This event is the first definitive association of a gamma‐ray flash with natural CG lightning and is among the most direct links to a specific lightning process so far. For this event, 19 gamma‐rays were recorded, with the highest energy exceeding 20 MeV. The high‐energy radiation exhibited very different behavior from the typical x‐ray emission from lightning. Specifically, the gamma‐ray flash had a much harder energy spectrum, consistent with relativistic runaway electron avalanche (RREA) multiplication; it did not arrive in sub‐microsecond bursts, typical of leader emission from lightning, and it occurred well after the start of the return stroke, which has not been previously observed for the x‐ray emission from lightning. Nevertheless, we present evidence that the source region for the gamma ray flash was the same as that for the preceding leader x‐ray bursts.

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Theoretical studies of particle acceleration by shocks have successfully modeled many observed features of the particle energy spectra and temporal variations; however, spectral slopes do not agree with in situ observations, and intensities show huge variations not explained by the models. Recent observations have shown that trace ions in the solar wind, such as 3 He and He + , are present in the accelerated particles with enhanced abundances. The average heavy ion composition is also found to differ significantly from the solar wind. It is observed that these features correlate significantly with ambient suprathermal material abundances, giving evidence that the suprathermals are the seed population that is actually energized. This raises important new questions such as why the suprathermals are favored over the much more abundant solar wind. Since the suprathermal ion population has many more contributors, and much larger variability than the solar wind, this population needs much more detailed study to allow a closure of theory and observations in interplanetary space.