Radar Diagnosis of the Thundercloud Electron Accelerator

Abstract

Thunderstorm Ground Enhancements (TGEs) refer to correlated enhancements in surface electric field and gamma ray flux that are manifestations of electron runaway in the storm overhead. The electric field enhancements can be of positive or negative polarity. In this study, altitude‐resolved S‐band radar observations of graupel are used to demonstrate distinct differences in storm structure linked with these “positive” and “negative” TGEs. The physical interpretation rests on the well‐established temperature‐dependent tripole structure of thunderstorms, with the main negative charge of the tripole acting as an electron repeller. This interpretation is supported by case studies showing altitude‐stable convection, with shallow (deep) development linked with “positive” (“negative”) TGEs, and by case studies of collapsing storms that show upper dipole dominance early and lower inverted dipole dominance later when graupel particles descend from a colder to warmer temperature domain. In the case of many TGEs on Mt Aragats (3.2 km MSL), the temperature‐dependent altitude of downward electron acceleration and avalanching may be sufficiently distant (>500 m) from surface detectors that the energetic electrons (1–10 MeV) are not likely avalanche/runaway electrons. Instead, they are Compton‐scattered and pair‐produced electrons from bremsstrahlung gamma radiation emanating from the high‐field avalanche region aloft. These inferences are consistent with GEANT4 calculations that identify the physical origins of energetic electrons at the surface.

Termination of Downward‐Oriented Gamma‐Ray Glow by Normal‐Polarity In‐Cloud Discharge Activity

“…R. Williams, 1989), the electric field between the lower positive and the middle negative layers can accelerate electrons downward. E. Williams et al (2022) discussed this possibility with surface electric-field and radar observations for TGEs at Mt. Aragats in summer thunderstorms.…”

“…E. Williams et al. (2022) discussed this possibility with surface electric‐field and radar observations for TGEs at Mt. Aragats in summer thunderstorms.…”

Meteorological Aspects of Gamma‐Ray Glows in Winter Thunderstorms

“…While electric‐field observations on the ground can estimate electric fields in a surface layer of a thundercloud (e.g., Chilingarian & Mkrtchyan, 2012; Østgaard et al., 2019), radar observations can survey three‐dimensionally a thundercloud system. However, systematic radar studies of thunderclouds associated with gamma‐ray glows have not been performed, except an earlier attempt (Williams et al., 2019). Here we report gamma‐ray glows at a single observation site during winter thunderstorms in Japan.…”

Negative Excursion of Surface Electric Fields During Gamma‐Ray Glows in Winter Thunderstorms

“…The output signals from the photomultipliers are digital-sampled, processed, and stored by our signal processing unit composed of a field-programmable gate array and a Raspberry Pi (Yuasa et al, 2020). While distinguishing energetic electrons from high-energy photons is becoming important (Chilingarian et al, 2022;E. Williams et al, 2022), our detectors did not have the ability to discriminate electrons from photons at this time.…”

“…These cases are similar to negative TGEs (TGEs with negative E-field variation) detected at the Aragats Space Environment Center. E. Williams et al (2022) analyzed that negative TGEs occur when vertical convection is active. It means that the upper positive and middle negative layers are dominant, and the amount of positive charge in the LPCR is relatively small.…”