The Thunderstorms and Elementary Particle Acceleration (TEPA) annual meeting provides an environment for discussing the latest discoveries in high-energy physics in atmosphere and lightning physics. The 2017 meeting was devoted to high-energy processes in the terrestrial atmosphere and near-Earth space and to the interrelation of enhanced particle fluxes and lightning flashes.
Several talks focused on how, in the absence of thunderstorms, because the intensity of galactic cosmic rays is rather stable, the fluxes of all species of cosmic rays as detected at Earth’s surface are also stable. However, during thunderstorms, particle counters and spectrometers suddenly detect high-energy particle bursts and fluxes of low-energy gamma rays that can last several hours. Electric fields in thunderclouds accelerate and multiply electrons, giving them field energy. Attendees noted that these additional electrons may originate bremsstrahlung gamma rays, and the gamma rays in turn may cause the release of neutrons. When the height of the cloud base is not too large, all particles reach detectors and register as huge peaks lasting several minutes.
Attendees also delved into the dynamics of the three dominant types of high-energy emissions originating in the thunderstorm atmosphere that are observed on low-altitude orbits, in the troposphere, and on Earth’s surface: terrestrial gamma ray flashes (TGFs), gamma glows, and thunderstorm ground enhancements (TGEs). The upper dipole between the main negative and positive layers of a thunderstorm accelerates electrons upward out of the atmosphere, producing the TGFs observed by orbiting gamma ray observatories. The bottom dipole between the main negative and lower positive charge regions accelerates electrons downward toward Earth’s surface, where they are observed by particle detectors as TGEs. Finally, the dipole between the main positive charge layer and the negative screening layer on top of the storm clouds also accelerates electrons downward, where they are observed by gamma ray detectors aboard aircraft or balloons as gamma glows.
At the TEPA conference, participants agreed that multiplication and acceleration of electrons, known as a relativistic runaway electron avalanche, is a core process for all three types of particle emissions from thunderclouds and can provide an explanation for all three types of high-energy emissions.
Conference Field Trip
During an excursion to Aragats Research Station of the Yerevan Physics Institute, conference attendees visited new facilities for synchronous detection of cosmic rays, lightning radiation, and meteorological parameters. The electronics system installed in 2016 on the mountain allows, for the first time, simultaneous measurements of particle fluxes and different kinds of atmospheric discharges with nanosecond time resolution.
Measurements taken on Mount Aragats in 2017 demonstrated that lightning flashes abruptly terminate the high-energy portion of TGEs (TGE event reports are located at http://www.crd.yerphi.am/Slide). For several milliseconds, particle fluxes and lightning leaders coexist in the thunderstorm atmosphere. However, after return strokes (or normal-polarity intracloud flashes), the electric field in the cloud is rearranged, and the high-energy particle flux suddenly declines.
The conference participants discussed the reports on this past year’s measurements of TGEs abruptly terminated by lightning flashes. Each abruptly terminated TGE was accompanied by a lightning flash within 10 kilometers of the detector site. The conference visitors found these observations to suggest that TGEs may play a significant role in unleashing lightning flashes. The network of particle detectors and electric field sensors synchronized on microsecond timescales will allow future investigation of the emitting regions of thunderclouds, determining their sizes and time evolution.
Registration is now open for the seventh TEPA, which is scheduled for 17–20 September 2018.
—Ashot A. Chilingarian (email: [email protected]), Cosmic Ray Division, Yerevan Physics Institute, Yerevan, Armenia