Thunderstorms and Elementary Particle Acceleration;
Yerevan, Armenia, 5-9 October 2015

The problem of how lightning is initiated inside thunderclouds is probably one of the biggest mysteries in the atmospheric sciences. The relationship between thundercloud electrification, lightning activity, wide-band radio emission and particle fluxes has not been yet unambiguously established. One of the most intriguing opportunities opened by the observation of the high-energy processes in the atmosphere (so-called Thunderstorm ground enhancements – TGEs) is their relation to lightning initiation. Lightnings and TGEs are alternative mechanisms for discharging of the atmospheric “electric engine” and synchronized observation of both phenomena helps to understand better the both.
To discuss these high-energy phenomena, the conference on Thunderstorms and Elementary Particle Acceleration was held at the Nor Amberd International Conference Center of the Yerevan Physics Institute (YerPhI) in Armenia. The Cosmic Ray Division of YerPhI and Skobeltsyn Institute of Nuclear Physics of Moscow State University organized the workshop; YerPhI and the Armenian State Committee of Science sponsored it. Thirty scientists and students from the United States, Japan, France, Germany, Israel, Russia, and Armenia attended.

Presentations focused on observations and models of the high-energy emissions in thunderclouds; on termination of particle fluxes by lightnings; multivariate observations of thunderstorm atmospheres from the earth’s
surface and from the space; radio emissions produced by atmospheric discharges and particle fluxes; influence of the Extensive air showers (EASes) on lightning initiations and others.

Discussions covered questions such as the following: Do particle fluxes initiate lightnings? Do EASes helps to unleash -CG lightnings? Is TGE and TGF currents competitive with lightning current? What is mechanism of particle flux termination?

The workshop participants agreed that it would be useful to compare vast amount of experimental data on TGE observed by Armenia, Japanese, Slovakian and USA in various conditions by different particle detectors to check the models of particle origin in thunderclouds. Armenia physicists suggest to locate sensors developed by other groups at Aragats where large TGEs are very often in Spring and Autumn.
With installing of new fast electronics at Aragats it became possible to relate lightning initiation, fast and slow changes of the electric field and particle fluxes on the millisecond scale. Various particle detectors and field meters now are synchronized by GPS receivers providing time stamp with accuracy not worse than few tens of nanosecond. The first large TGE was observed with renewed ASEC facilities at 7 October 2015 during the workshop. The natural “electron accelerator” on Aragats provides several interesting events, which was intensively discussed by participants. During the most interesting 7 October TGE for the first time particle fluxes and lightning were detected on millisecond time scale. On the one-second time scale the termination looks like immediate decline due to deposition of large negative charge into the cloud by the return stroke of lightning. However, on the millisecond time scale it was evident that particle flux was declining successively along with distribution of the deposited charge in the cloud.

The presentation slides and discussion videos are available on the conference website, http://crd.yerphi.am/Conferences/tepa2015/home. More details can be found in the supplemental information in the online version of this meeting report.

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Professor A. Chilingarian gives talk on the results obtained with "Aragats electron accelerator” operated in the thunderclouds above

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One of the TEPA 2015 participants professor Mareev speaks about Thoundercloud electrodynemics