Key points
The most powerful electron accelerators operated in thunderclouds send billions and billions of MeV particles in direction of the earth’s surface per second;
These electrons are precursors of the lightning flashes by opening ionized channels to the lightning leaders;
The strong accelerating electric field extends very low to the earth’s surface
Introduction
It is widely accepted, that the cloud charge structure for a typical thunderstorm contains an upper positive charge region consisting of ice crystals, a main negative charge region consisting of both graupel and ice crystals, and a lower positive charge region consisting of graupel. The electric charge of graupel is positive at temperatures warmer than -10° C, and negative at temperatures cooler than -10° C (Takahashi, 1978, Wada et al., 2021). In review (Williams, 1989) was stated that the tripolar structure of thunderstorms is supported by a wide variety of observations and that temperature appears to be the most important single parameter in controlling the polarity of charge acquired by the precipitation particles. When graupel falls into the region warmer than ≈ -10° C, a charge reversal will occur in the central part of the storm, and the graupel population will change the charge from negative to positive. Large and dense graupel population either suspended in the middle of the thunderstorm cloud or falling toward the earth’s surface constitutes a “moving” lower positive charge region (LPCR). The dipole formed by the LPCR and relatively stable main negative (MN) charge region significantly intensify the electric field of the dipole formed by the MN and its mirror image in the ground (MN-MIRR, first scenario of RREA initiation, see Fig.1 in Chilingarian et al., 2021a). A free electron entering the strong and extended electric field accelerates and unleashes the relativistic runaway electron avalanches (RREA, Gurevich et al., 1992). The RREA is a threshold process, which occurred only if the electric field exceeds the critical value in a region of the vertical extent of about 1–2 km. When the second scenario of the RREA origination (MN-MIRR and MN-LPCR) is realized the electric field in the cloud frequently surpasses the critical value and an intense RREA ends up in an extreme thunderstorm ground enhancement (TGE, Chilingarian et al., 2010, 2011) exceeding the background level of gamma rays and electrons up to hundred times (Chum et al., 2021). After the graupel fall, or after the lightning flash consuming positive charge of the LPCR, the surface electric field again is controlled by the main negative charge region only (see the detail of the RREA initiation model in (Chilingarian et al, 2020 and 2021a).
At Aragats research station we develop a new approach for understanding thunderclouds charge structure based on the new kind of messengers, namely, the secondary cosmic rays modulated by the intracloud electric field (Chilingarian et al., 2021b). The 24/7 monitoring of almost all species of secondary cosmic rays is going on near 12 years. We simultaneously observe graupel fall, the outside temperature (a proxy of the intracloud temperature), the near-surface electric field (a proxy of the intracloud electric field), and fast-changing during thunderstorms neutral and charged particles count rates. Such a multisensory approach allows us to get insight into the charge structure of the thundercloud using particle physics methods.
Comparative analysis of 23 - 25 May 2021 thunderstorms
According to the adopted approach of the multivariate correlation analysis, we use as many as possible measurements for characterizing high-energy processes in the atmosphere. The count rate of electron and gamma ray fluxes, as well, as the energy release histograms, are registered by the large spectrometer allowing to recover differential energy spectra of both charged and neutral fluxes (Aragats solar neutron telescope, ASNT, Chilinarian et al., 2017a). The lightning identification and distance to lightning flash estimation are done by monitoring of disturbances of the near-surface (NS) electric field with the network of EFF-100 electric mils of BOLTEC company. Meteorological measurements are made with DAVIS weather station. The moon-glow panoramic cameras are used for the monitoring of skies above Aragats.
On May 23, 2021 a large storm was unleashed in direct vicinity of the Aragats research station. Storm duration was 6 hours with more than hundred nearby lightning flashes. NS electric field shows many episodes of deep negative and deep positive (-20 - +20 kV/m) electric field excursions. The electric field direction reversals after lightning strikes were very fast (a few seconds). In the first part of storm numerous attempts to start TGE were registered by the ASNT spectrometer. In Fig 1, we present a 20-minute period of the thunderstorm with 2 TGEs terminated by the lightning flash on the initial stage of development. The duration of TGEs was ≈20 sec, the NS electric field was in the negative domain, the amplitude of the NS field surge caused by the terminating lightning flash was ≈ 50 kV/m (see Table 1). In the frame a) we show the descending distance to the cloud base, estimated by one of atmospheric thermodynamics’ calculators available on the internet which uses measurements of the outside temperature and dew point (Omni, 2019). In frames b) we show the shots of the panoramic camera monitoring skies above Aragats. The characteristic patterns on the camera glass are due to graupel fall (see Chilingarian et al., 2021c). Not very intense graupel fall continues before and during observed two TGEs. It is interesting to note that a new TGE started just after lightning kills the previous one during the electric field recovering stage. This is evidence of the largely electrified atmosphere, when a new started TGE opens path to the lightning leader (Chilingarian et al., 2017b).