Long lasting low energy thunderstorm ground enhancements and possible Rn-222 daughter isotopes contamination

Thunderstorm ground enhancements (TGEs) comprise large particle fluxes coming from the clouds that usually coincide with thunderstorms. Most of TGEs observed at the Aragats research station in Armenia during the last ten years originated from “beams of the electron accelerator” operating in the thunderclouds above the research station. Observed TGEs contain high-energy electrons and gamma rays (as well as neutrons) and usually last a few minutes. Starting from 2014, we use particle detectors tuned for the registration of lower energies particles coming from thunderclouds (starting from 0.3 MeV). In 2016, we already noticed that TGEs measured by particle detectors with a low energy threshold demonstrated a drastically larger duration. The flux of the high-energy particles (with energies up to 40 MeV) lasts 1–10 min; the lowest ones (less than 3 MeV)—more than two hours. All intense TGEs contain a high-energy peak and a prolonged low-energy extension lasting 2–3 h. In the presented paper, we describe examples of long-lasting TGEs and discuss correlations of enhanced particle fluxes with disturbances of the electric field and with precipitation.

Figure 1

Pattern of the storm in Armenia mapped by lightning flashes showing the approaching storm front; the Aragats station position on the map is flagged.

Figure 2

Meteorogical parameters measured on August 17, 2017. On the top of the picture, one-minute time series of the outside temperature and dew point are shown; in the middle—the atmospheric pressure and the disturbances of electric field; in the bottom—the rain rate.

Figure 3

The lightning activity during a large summer storm on Aragats was coherently detected by the network of the four electric mills EFM-100 of the Boltek company (see inset in the top right corner of the picture).

Figure 4

Thunderstorm ground enhancement (TGE) as measured by the first and second crystals of the NaI network (see inset, energy threshold 0.3 MeV).

Figure 5

The differential energy spectra of four subsequent minutes of TGE, recovered from the energy release histograms measured by the N1 and N2 crystals of the NaI network.

Figure 6

Recovery of gamma rays and electron fluxes with the CUBE detector. Scintillators N7 and N8 are 20 cm thick $0.25{\mathrm{m}}^2$ stacked plastics. In the Table inset, the recovered fluxes measured by both thick scintillators are shown.

Figure 7

One-second count rates of the STAND1 detector located nearby the MAKET experimental hall.

Figure 8

The 50-ms time series of the STAND1 upper scintillator count rate (located outdoors nearby the MAKET experimental hall) and of the near surface electric field measurements. The asterisk indicates the time of lightning flash registered by the World-Wide Lightning Location Network (WWLLN, detection at $18:55:33.630$). The horizontal axes started from $18:55:32$; each tick on the axes corresponds to 100 ms.

Figure 9

One-minute time series of the count rates of the 1 cm thick $1{\mathrm{m}}^2$ area plastic scintillator. In the bottom of the frames, we show rain rate in mm per hour.

Figure 10

TGE events registered by the NaI detector. In the top of figures, we show disturbances of the near surface electric field; in the bottom—the rain rate. In the middle—the one-minute count rate of the NaI detector.