[Paper Review] Electron Acceleration Mechanisms in Thunderstorms

Sébastien Célestin|arXiv (Cornell University)|Dec 31, 2016

Lightning and Electromagnetic Phenomena3 citations

TL;DR

This paper reviews high-energy electron acceleration mechanisms in thunderstorms, focusing on relativistic runaway electron avalanches (RREAs) driven by strong electric fields. It explains that secondary cosmic rays seed these processes, producing terrestrial gamma-ray flashes (TGFs), gamma-ray glows, and electron/positron beams, with upcoming space missions like TARANIS and ASIM set to resolve key uncertainties in TGF origins and particle dynamics.

ABSTRACT

Thunderstorms produce strong electric fields over regions on the order of kilometer. The corresponding electric potential differences are on the order of 100 MV. Secondary cosmic rays reaching these regions may be significantly accelerated and even amplified in relativistic runaway avalanche processes. These phenomena lead to enhancements of the high-energy background radiation observed by detectors on the ground and on board aircraft. Moreover, intense submillisecond gamma-ray bursts named terrestrial gamma-ray flashes (TGFs) produced in thunderstorms are detected from low Earth orbit satellites. When passing through the atmosphere, these gamma-rays are recognized to produce secondary relativistic electrons and positrons rapidly trapped in the geomagnetic field and injected into the near-Earth space environment. In the present work, we attempt to give an overview of the current state of research on high-energy phenomena associated with thunderstorms.

Motivation & Objective

• To provide a comprehensive overview of high-energy atmospheric physics, a rapidly growing interdisciplinary field linking atmospheric science, space physics, and high-energy phenomena.
• To clarify the physical mechanisms behind electron acceleration in thunderstorms, particularly relativistic runaway electron avalanches (RREAs) and thermal runaway processes.
• To highlight the critical role of secondary cosmic rays as seed particles in amplifying high-energy radiation during thunderstorm activity.
• To present future space missions—TARANIS and ASIM—designed specifically to study TGFs and associated high-energy particle bursts with improved temporal and spectral resolution.
• To reconcile conflicting theoretical models of TGF production, including localized acceleration by lightning leaders versus large-scale RREAs in thundercloud fields.

Proposed method

• Synthesizes observational data from satellite missions (BATSE, RHESSI, AGILE, Fermi, and TARANIS) and airborne/ground-based detectors to analyze TGFs and gamma-ray glows.
• Analyzes energy spectra and temporal profiles of TGFs (10 keV to >40 MeV) and correlates them with bremsstrahlung emission from relativistic electrons.
• Models electron acceleration via relativistic runaway electron avalanches (RREAs), using electric fields from lightning leaders and thunderclouds as driving forces.
• Compares two competing theories for TGF production: (1) localized RREAs initiated by lightning leader fields and (2) large-scale RREAs seeded by cosmic-ray air showers or thermal runaway electrons.
• Evaluates detection limitations in current instruments, such as saturation and pile-up effects, and explains how new instruments (e.g., XGRE on TARANIS) will mitigate these issues.
• Integrates data from radio emissions, optical signals, and particle beams to link TGFs with lightning processes, particularly +IC lightning discharges.

Experimental results

Research questions

• RQ1 What physical mechanisms are responsible for the production of terrestrial gamma-ray flashes (TGFs) in thunderstorms?
• RQ2 How do secondary cosmic rays contribute to the initiation and amplification of relativistic electron avalanches in thunderstorm electric fields?
• RQ3 What is the role of lightning leaders—particularly negative leaders with impulsive events—in triggering runaway electron processes and TGFs?
• RQ4 Why do gamma-ray glows persist for seconds before terminating at lightning discharge, and how do they relate to TGFs and RREAs?
• RQ5 How can upcoming missions like TARANIS and ASIM overcome instrumental limitations (e.g., dead time, saturation) to accurately measure TGFs and associated electron/positron beams?

Key findings

• TGFs are short-duration bursts (<1 ms) of high-energy photons (10 keV to >40 MeV), with a global occurrence rate estimated at ~400,000 per year detectable by Fermi-GBM.
• TGFs are strongly correlated with intracloud lightning discharges, particularly +IC lightning, and originate from altitudes of 10–14 km, consistent with bremsstrahlung emission from relativistic electrons.
• Gamma-ray glows, which last seconds to minutes, are attributed to sustained RREAs and are well-explained by secondary cosmic ray seeding in thunderstorm electric fields.
• Thunderstorm Ground Enhancements (TGEs) and gamma-ray glows exhibit similar energy spectra, indicating a common origin in RREAs, with ion currents comparable to lightning and precipitation currents.
• Two leading theories for TGF production coexist: (1) localized RREAs initiated by lightning leader fields and (2) large-scale RREAs seeded by cosmic-ray air showers or thermal runaway electrons, both potentially contributing to TGFs.
• The upcoming TARANIS and ASIM missions will feature advanced detectors (e.g., XGRE on TARANIS) with fast timing, reduced dead time, and electron-photon discrimination to resolve TGF and beam dynamics with unprecedented accuracy.