Gamma-ray Showers Observed at Ground Level in Coincidence With Downward Lightning Leaders

Abstract: Bursts of gamma ray showers have been observed in coincidence with downward propagating negative leaders in lightning flashes by the Telescope Array Surface Detector (TASD). The TASD is a 700~square kilometer cosmic ray observatory located in southwestern Utah, U.S.A. In data collected between 2014 and 2016, correlated observations showing the structure and temporal development of three shower-producing flashes were obtained with a 3D lightning mapping array, and electric field change measurements were obtained for an additional seven flashes, in both cases co-located with the TASD. National Lightning Detection Network (NLDN) information was also used throughout. The showers arrived in a sequence of 2--5 short-duration ($\le$10~$\mu$s) bursts over time intervals of several hundred microseconds, and originated at an altitude of $\simeq$3--5 kilometers above ground level during the first 1--2 ms of downward negative leader breakdown at the beginning of cloud-to-ground lightning flashes. The shower footprints, associated waveforms and the effect of atmospheric propagation indicate that the showers consist primarily of downward-beamed gamma radiation. This has been supported by GEANT simulation studies, which indicate primary source fluxes of $\simeq$$10^{12}$--$10^{14}$ photons for $16^{\circ}$ half-angle beams. We conclude that the showers are terrestrial gamma-ray flashes (TGFs), similar to those observed by satellites, but that the ground-based observations are more representative of the temporal source activity and are also more sensitive than satellite observations, which detect only the most powerful TGFs.

Overview of Gamma-ray Showers Associated with Downward Lightning Leaders

The paper entitled "Gamma-ray Showers Observed at Ground Level in Coincidence With Downward Lightning Leaders" by Abbasi et al. examines the intersection of atmospheric physics and high-energy astrophysics by investigating gamma-ray emissions coincident with lightning phenomena. Using data from the Telescope Array Surface Detector (TASD), the researchers provide a detailed account of terrestrial gamma-ray flashes (TGFs) from downward lightning leaders and their detection from a ground-based perspective.

Key Findings

The study reports the occurrence of gamma-ray showers observed during downward negative leader progression at the onset of cloud-to-ground lightning. These showers are notably detected at significantly lower altitudes (3-5 kilometers above ground) compared to previous satellite-based observations, which typically focus on gamma-ray flashes within the atmosphere at much higher altitudes. The TASD, covering a substantial area of 700 square kilometers, allowed the identification of gamma-ray bursts with unprecedented temporal resolution and sensitivity, thus elucidating the initiation process of TGFs from the ground perspective.

An important numerical result highlighted is the estimated primary photon flux ranging from $10^{12}$ to $10^{14}$ photons directed downward. These figures underscore how terrestrial phenomena can produce gamma-ray bursts that are comparable in nature yet distinct in resolution and composition from those detected by satellites.

Methodological Advances

The employment of the TASD supplemented by a Lightning Mapping Array (LMA) provided insights into the spatial and temporal characteristics of these gamma-ray showers. Unlike satellite observations, which might capture only the most intense gamma-ray flashes, the field measurements presented here enable a more comprehensive understanding of the TGFs' formation phase and duration. Moreover, the integration with electric field measurement tools adds a layer of detail in discerning the electric breakdown characteristics during these events.

Implications and Future Directions

This research opens intriguing possibilities for investigating TGFs and their association with lightning, offering potential pathways to explore atmospheric electricity and energetic particle physics further. Practically, these findings could lead to improved predictive models for thunderstorm activity and possibly inform safety protocols for lightning-prone areas.

Theoretically, these observations contribute to the broader understanding of energetic particle generation in thunderstorms, with implications for the relativistic runaway electron avalanche (RREA) model of TGFs. The distinct resolution offered by ground-based detection suggests new approaches to studying electron avalanche phenomena under increasing environmental pressures closer to the Earth's surface.

Moving forward, expanding the TASD array and integrating additional sensors might refine the observational capabilities, leading to more detailed analyses of gamma-ray production and propagation dynamics in different atmospheric conditions. Enhanced temporal and spatial data could further elucidate the characteristics of TGFs, providing deeper insights into the mechanisms driving these high-energy events.

Conclusion

The study by Abbasi et al. serves as a foundational piece in redefining how TGFs are understood, highlighting the significant role of downward lightning leaders in their generation. The implications span both practical applications in atmospheric science and theoretical advancements in the physics of high-energy particle generation, suggesting a fruitful avenue for continued research in these domains.