Physics Letters A 373 (2009) 3550–3553 Contents lists available at ScienceDirect Physics Letters A www.elsevier.com/locate/pla An intracloud discharge caused by extensive atmospheric shower A.V. Gurevich a,* , G.G. Mitko a , V.P. Antonova d , A.P. Chubenko a , A.N. Karashtin b , S.V. Kryukov d , A.S. Naumov a , L.V. Pavljuchenko a , M.O. Ptitsyn a , V.A. Ryabov a , S.Ya. Shalamova a , A.L. Shepetov a , Yu.V. Shlyugaev b , L.I. Vildanova c , K.P. Zybin a a P.N.Lebedev Physical Institute of RAS, Moscow, Russia b Research Radiophysics Institute, Nizhny Novgorod, Russia c Tien-Shan Mountain Cosmic Ray Station, Almaty, Kazakhstan d Ionosphere Institute, Almaty, Kazakhstan article info abstract Article history: Received 13 July 2009 Accepted 20 July 2009 Available online 5 August 2009 Communicated by V.M. Agranovich Keywords: Thunderstorm Gamma emission Runaway breakdown The observational evidence of RB-EAS discharge in a thunderstorm atmosphere is presented. After RB- EAS discharge we name a discharge of a special type developing due to the runaway breakdown (RB) mechanism while an extensive atmosphere shower (EAS) passes through a thundercloud electric field. The observations were fulfilled at the Tien-Shan Mountain Cosmic Ray Station. The widely spread system of oscillation detectors, the special EAS trigger array and the HF radio interferometer were used for measurements.  2009 Elsevier B.V. All rights reserved. 1. Introduction A high-energy cosmic ray particle interacting with molecules in the atmosphere generates an extensive atmospheric shower (EAS) that consists of a large number of different elementary particles and fragments of nucleus. The secondary electrons arising from the EAS transformations are the most important for the possi- ble macroscopic influence on conductivity in thunderstorm atmo- sphere. But simple theoretical estimations show that there is no cosmic ray particle of such energy which could be able to af- fect the thunderstorm electric field via conductivity changing. Such possibility was predicted previously [1] based on the runaway breakdown (RB) phenomenon. It is determined by the avalanche multiplication of runaway electrons if the thundercloud electric field E exceeds the critical one E c . The critical field value for normal atmospheric conditions is E c ≈ 2.16 kV/cm. Besides runaways a very large number of slow (thermal) electrons are generated in this process. The last one could change conductivity essentially and lead to the electrical breakdown. To stimulate RB the presence of fast “seed” electron N s is needed, naturally number of RB electrons is growing proportion- ally to N s . The total number of secondary electrons N s in EAS is proportional to the primary’s cosmic ray energy E p . For example, * Corresponding author. E-mail address: alex@lpi.ru (A.V. Gurevich). for E p = 10 15 eV, N s = 10 6 and for E p = 10 16 eV, N s = 10 7 . When EAS crosses the thundercloud, where E > E c a runaway avalanche exponentially increase the number of energetic electrons. Simulta- neously a tremendous number of thermal electrons are generated. Together, they produce an RB-EAS discharge. Due to the presence of a large number of energetic electrons the discharge should be naturally accompanied by the strong pulse of gamma emission [2]. The theory considered RB-EAS as one of the significant pro- cesses in thunderstorm atmosphere. For example, it was supposed, that RB-EAS lies in the background of the intracloud high-attitude NBE discharge (NBE – narrow bipolar event) [3,4]. The specific fea- tures of NBE – enormously powerful radio emission, low optic emission, absence of step-leader, full time development in a few microseconds – are in reasonable agreement with the predictions based on RB-EAS theory [5]. But in observation RB-EAS discharge was never registered previously. In this Letter the first direct observational evidence of RB-EAS discharge in thunderstorm atmosphere is presented. 2. Experimental set-up The experiments were fulfilled at the Tien-Shan Mountain Cos- mic Ray Station (Kazakhstan). The Station is located at the height range 3340 m above the see level, which is an average passage height of thunderclouds at Northern Tien-Shan. Hence, thunder- storms, which are abundant here since the end of May until the beginning of September, do cover the Station. That’s why; the radi- ation detectors being placed at the Station up to the height 3900 m 0375-9601/$ – see front matter  2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2009.07.085