On the initiation of lightning in thunderclouds

Abstract

The relationship of lightning and elementary particle fluxes in the thunderclouds is not fully understood to date. Using the particle beams (the so-called Thunderstorm Ground Enhancements - TGEs) as a probe we investigate the characteristics of the interrelated atmospheric processes. The well-known effect of the TGE dynamics is the abrupt termination of the particle flux by the lightning flash. With new precise electronics, we can see that particle flux decline occurred simultaneously with the rearranging of the charge centers in the cloud. The analysis of the TGE energy spectra before and after the lightning demonstrates that the high-energy part of the TGE energy spectra disappeared just after lightning. The decline of particle flux coincides on millisecond time scale with first atmospheric discharges and we can conclude that Relativistic Runaway Electron Avalanches (RREA) in the thundercloud assist initiation of the negative cloud to ground lightning. Thus, RREA can provide enough ionization to play a significant role in the unleashing of the lightning flash.

Figure 1

One-minute time-series of STAND1 detector: the count rate combinations 100 and 010 correspond to signals only from upper and middle scintillation detectors, respectively. Flux enhancement at 13:55–13:56 UT is ~34% (44σ). Electric field decreased from +25 kV/m at 13:53:25 UT to −24 kV/m at 13:55:25 UT. Distances to 4 nearby lightning flashes measured by the same EFM-100 device are 4.8, 1.9, 11.7, and 3.8 km from left to right, respectively. Other 6 lightning flashes shown in the top of picture occurred at distances more than 10 km.

Figure 2

One second time series of 1 cm thick scintillator of STAND1 detector located nearby MAKET experimental hall. Negative lightning abruptly terminated TGE.

Figure 3

50 ms time series of MAKET upper 1-cm thick scintillator count rate and electrostatic field. The time of trigger is denoted by a point occurred at 13:56:34.087 UT (calculated by 11 GPS satellites). WWLLN registered lightning at 13:56:34.087 UT. The electric field starts to rise ~50 ms after trigger, reaching maximum ~200 ms later; the amplitude of the electric field change was ~48.6 kV/m. Particle flux starts to decline at 11:56:34.2 UT.

Figure 4

Energy spectrum of TGE exteneded up to 30 MeV measured by 2 NaI spectrometers during 4 minutes of TGE.

Figure 5

3-dimensional histogram of the energy release measured by the 60-cm thick plastic scintillator during the time interval from 13:52:22 UT to 13:57:22 UT. The red arrow shows lightning occurred at 13:56:34.

Figure 6

June TGE events abruptly terminated by nearby lightning (a,c,d) and – not terminated (b). The upper 1-cm thick plastic scintillator of STAND1 detector located nearby MAKET experimental hall measures one-second count rate. Electrostatic field and distance to lightning are measured and estimated by the EFM 100 electric mill located nearby GAMMA array.

Figure 7

Cartoon illustrated TGE and TGF initiation above Aragats mountain. This figure is not covered by the CC BY licence. [Credits to Cosmic Ray Division (CRD)]. All rights reserved, used with permission.