A Rapid Gamma‐Ray Glow Flux Reduction Observed From 20 km Altitude

Abstract

Two gamma‐ray glows were observed by a high‐altitude NASA ER‐2 aircraft flying at 20 km altitude over a thunderstorm in Colorado, USA. The flux of the first glow rapidly intensified and then abruptly decreased within a few tens of milliseconds. On a timescale of seconds, the flux decrease occurred simultaneously with a hybrid intra‐cloud/cloud‐to‐ground lightning discharge beneath the aircraft. However, a more detailed analysis of the discharge dynamics indicated that the discharge activity was unusually calm during the actual period of the flux decrease. The lightning was observed with on‐board antennas, optical sensor, and ground‐based lightning mapping and location networks. Its closest activity was 12 km away from the aircraft, below and slightly ahead the course. The gamma‐ray flux reduction happened roughly in the middle of the lightning development process. The glow spectral analysis for the periods of a weak and strong flux enhancement has been done. The spectra were found to be background‐like and similar to each other.

Characteristics of Low‐Frequency Pulses Associated With Downward Terrestrial Gamma‐Ray Flashes

“…As reported in Wada et al. (2021), 70 gamma‐ray glows, another high‐energy phenomenon in thunderstorms (Chilingarian et al., 2012; Kochkin et al., 2021; Torii et al., 2002; Tsuchiya et al., 2007; Wada et al., 2018), were detected at only the Kanazawa site, in contrast to three detections of downward TGFs in the same area and the same period. Besides the seven events, another one was detected at the Kanazawa site.…”

Multiple Gamma‐Ray Glows and a Downward TGF Observed From Nearby Thunderclouds

“…(1996) observed similar X‐ray emissions. These observations were followed by further aircraft (e.g., Kelley et al., 2015; Kochkin et al., 2017, 2021; Østgaard et al., 2019), balloon (Eack et al., 2000), and ground‐based observations (e.g., Chilingarian et al., 2010; Torii et al., 2002; Tsuchiya et al., 2007), which detected not only X‐rays but also gamma rays up to tens of MeV. These X‐ and gamma‐ray flux intensifications have a duration ranging from tens of seconds to minutes, are associated with thunderclouds themselves, not with lightning discharges, and are now called “gamma‐ray glows” (e.g., Kuroda et al., 2016; Wada, Enoto, Nakamura, et al., 2019), long bursts (e.g., Torii et al., 2009), and/or thunderstorm ground enhancements (Chilingarian et al., 2011).…”

“…Their average spectra have continuous shape up to >10 MeV and is now considered as the bremsstrahlung emission from electrons accelerated mainly through the relativistic runaway electron avalanche (RREA) mechanism (Gurevich et al, 1992). Spectra of gamma-ray glows are similar to those of the TGFs (e.g., Dwyer, 2012;Kochkin et al, 2021;Tsuchiya et al, 2007), and therefore, the RREA mechanism is also considered to be applicable to it. However, the intensity of some of the brightest glows cannot be explained solely by this mechanism (Kelley et al, 2015;, and a further amplification process such as relativistic feedback (Dwyer, 2003(Dwyer, , 2012 is needed.…”

Atmospheric Electron Spatial Range Extended by Thundercloud Electric Field Below the Relativistic Runaway Electron Avalanche Threshold

“…The latter has a limited length scale at which an energetic electron beam ceases under weaker electric fields than the RREA threshold. Previous works (Bowers et al., 2019; Chilingarian, Mailyan, & Vanyan, 2012; Chilingarian et al., 2014; Kochkin et al., 2021; Østgaard et al., 2019) compare gamma ray glow measurements with Monte Carlo simulated photon spectra to contrast RREA and MOS differentiating the requirements and characteristics of each mechanism.…”