Particle bursts from thunderclouds: Natural particle accelerators above our heads

Strong electrical fields inside thunderclouds give rise to fluxes of high-energy electrons and, consequently, gamma rays and neutrons. Gamma rays and electrons are currently detected by the facilities of low orbiting satellites and by networks of surface particle detectors. During intensive particle fluxes, coinciding with thunderstorms, series of particle bursts were detected by the particle detectors of Aragats Space Environmental Center at an altitude of 3250 m. We classify the thunderstorm ground enhancements in 2 categories, one lasting microseconds, and the other lasting tens of minutes. Both types of events can occur at the same time, coinciding with a large negative electric field between the cloud and the ground and negative intracloud lightning. Statistical analysis of the short thunderstorm ground enhancement bursts sample suggests the duration is less than $50\mu \mathrm{s}$ and spatial extension is larger than $1000{\mathrm{m}}^2$. We discuss the origin of thunderstorm ground enhancements and its connection to the terrestrial gamma flashes detected by orbiting gamma-ray observatories.

Figure 1

Particle detectors and field meters of the Aragats Space Environmental Center operation during the 2010 measurement campaign.

Figure 2

One-minute time series detected by the Maket array. (a) Count rate of the standalone outer detector (energy threshold $\sim 7\mathrm{MeV}$); (b) count rates of the “EAS hardware trigger”—9 scintillators give a signal within $1\mu \mathrm{s}$); (c) count rate of the EAS software triggers—off-line selection of events where all 16 scintillators give a signal within $1\mu \mathrm{s}$.

Figure 3

Scatter plot of the registered at quiet weather Maket triggers; pure EAS events.

Figure 4

Two-way classification of the showers detected on 19 September, 2009, and 4 October, 2010, during thunderstorms.

Figure 5

Integral density distribution of the events from the joint $\mathrm{TGE}+\mathrm{EAS}$, statistically reconstructed TGE, and pure EAS classes.

Figure 6

Numbers of events detected with 9–16 scintillators of the Maket array for the two classes of events, both containing 465 events at hardware trigger conditions (9 scintillators fired).

Figure 7

Distribution of the Maket hardware triggers by the number of triggers per second at 3 minutes of 19 September, 2009.

Figure 8

The temporal structure of the electrical field disturbances and the time series of the Maket triggers detected on 4 October, 2010.

Figure 9

Disturbed electric field and count rate of the gamma rays (energy $>10\mathrm{MeV}$) measured by the 01 combination of the Aragats Solar Neutron Telescope: 4 October, 2010, 18:22–18:24.

Figure 10

Symmetry of the TGFs and TGEs.

Figure 11

Time series of the AMMM outdoor 5 cm scintillator and the Maket outdoor 5 cm scintillators located at a distance of 400 m from AMMM; TGE on 2 November, 2009.

Figure 12

Time series of the ASNT corresponding to different energy releases during TGE on 19 September, 2009.

Figure 13

Time series of ASNT corresponding to different directions of the incoming particle flux; TGE on 21 May, 2009.

Figure 14

SEVAN detection of the October 4 TGE. Gray curve—the time series of low-energy neutral particles (the coincidence 010); black curve—the high-energy charged particles (the coincidence in all 3 layers of detector, particle traverse 10 cm of lead).