Terrestrial gamma-ray flashes initiated by positive leaders

Terrestrial gamma-ray flashes (TGFs) are powerful submillisecond bursts of gamma rays produced by thunderstorms. To date, most TGFs have been observed by spacecraft in low-Earth orbit and have been found to be associated with negative intracloud lightning leaders. In recent years, TGFs have also been measured on the ground as downward beams originating from the overhead storms. While the majority of these ground-level TGFs appear to be associated with negative lightning leaders, similar to the TGFs seen from space, others are associated with upward-propagating positive leaders. In this paper, Runaway Electron Avalanche Model Monte Carlo simulations, modified to include low-energy electron and ion currents and self-consistent electric fields, are used to model TGF production by the relativistic feedback mechanism initiated by positive leaders. It is found that intense bursts of gamma rays are produced by positive leaders, similar to the observed ground-level TGFs. It is also found that these events produce dangerous radiation doses in excess of 1 Sievert and so may be of concern for aviation safety.

Figure 1

Histogram of greater than 1 MeV gamma rays during the 2003 ground-level TGF as shown in Ref. [7].

Figure 2

Gamma-ray pulses recorded by a NaI(Tl)/PMT detector during the 2014 ground-level TGF [8].

Figure 3

Sea-level equivalent electric field along and above the leader channel versus altitude immediately before the TGFs for the three cases. The horizontal dashed line shows the RREA threshold field.

Figure 4

Top: close-up of the sea-level equivalent electric field along and above the leader channel versus altitude immediately before the TGF for case B. The horizontal dashed line shows the RREA threshold field. Bottom: rate of runaway electrons passing downward through a horizontal plane versus altitude.

Figure 5

Left: electric field strength produced by an upward positive leader that initiated a ground-level TGF. The leader channel is seen as the vertical purple line in the center. The black lines with arrows show the average trajectories of the runaway electrons. Right: electric field strength versus height immediately after the TGF (black) and prior to the lightning leader (blue).

Figure 6

Top: gamma-ray rate at 3.5 km vs time for the three simulated TGFs discussed in this paper. Bottom: electric currents injected into the top of the leader channel by the relativistic feedback discharge for the same three simulated TGFs.

Figure 7

Whole-body radiation dose received by an individual inside an aircraft vs attitude for the three simulated TGFs.

Figure 8

Electric potential difference required to make a TGF vs the average ISA sea-level equivalent electric field inside the avalanche region. The solid curve is from REAM Monte Carlo simulations for a uniform field with limited lateral extent ($R=L/2$). The black symbols are potential differences in the avalanche region, and the green symbols are the total potential differences between the ground and the top of the simulation volume for the three simulated TGFs in this paper. The vertical dashed line is the RREA threshold field, $E_{\mathrm{th}}$.