Observation of variations in cosmic ray single count rates during thunderstorms and implications for large-scale electric field changes

We present the first observation by the Telescope Array Surface Detector (TASD) of the effect of thunderstorms on the development of cosmic ray single count rate intensity over a $700{\mathrm{km}}^2$ area. Observations of variations in the secondary low-energy cosmic ray counting rate, using the TASD, allow us to study the electric field inside thunderstorms, on a large scale, as it progresses on top of the $700{\mathrm{km}}^2$ detector, without dealing with the limitation of narrow exposure in time and space using balloons and aircraft detectors. In this work, variations in the cosmic ray intensity (single count rate) using the TASD, were studied and found to be on average at the $\sim (0.5–1)\%$ and up to 2% level. These observations were found to be both in excess and in deficit. They were also found to be correlated with lightning in addition to thunderstorms. These variations lasted for tens of minutes; their footprint on the ground ranged from 6 km to 24 km in diameter and moved in the same direction as the thunderstorm. With the use of simple electric field models inside the cloud and between cloud to ground, the observed variations in the cosmic ray single count rate were recreated using CORSIKA simulations. Depending on the electric field model used and the direction of the electric field in that model, the electric field magnitude that reproduces the observed low-energy cosmic ray single count rate variations was found to be approximately between 0.2 GV–0.4 GV. This in turn allows us to get a reasonable insight on the electric field and its effect on cosmic ray air showers inside thunderstorms.

Figure 1

Top: The Telescope Array, consisting of 507 scintillator SDs on a 1.2 km grid over a $700{\mathrm{km}}^2$ area. The SD scintillators are enclosed by three fluorescence detectors shown in filled triangles together with their field of view in solid lines. The northernmost fluorescence detector is called Middle Drum while the southern fluorescence detectors are referred to as Black Rock Mesa and Long Ridge. The filled circle in the middle equally spaced from the three fluorescence detectors is the central laser facility used for atmospheric monitoring and detector calibration. Bottom: Schematic sketch of the upper and lower 1.2 cm thick plastic scintillator layers inside the scintillator box, the 1 mm stainless steel plate, the 104 wavelength-shifting (WLS) fibers and the photomultiplier tubes. These items are enclosed in a stainless steel box, 1.5 mm thick on top and 1.2 mm thick on the bottom [13].

Figure 2

Time evolution of the intensity variation of the single count rate change $(\frac{N_c-N_p}{N_p})\%$ or $(\mathrm{\Delta }N/N)\%$ on the 09/27/2014 thunderstorm. Each time frame is ten minutes in duration. The starting time in UTC is denoted on each frame. The black and gray crosses marks are the intracloud and cloud-to-ground lightning sources detected by the NLDN for each frame. The two yellow and pink stars point at the two detectors [1516 (denoted in pink) and 1015 (denoted in yellow)] plotted in Fig. 3.

Figure 3

Rate variation vs time and temperature variation vs time for two detectors numbered (1516 and 1015). Here 1516 shows a deficit in the rate variation ($-0.8\%$) and 1015 shows an excess in the rate variation ($+1.3\%$).

Figure 4

The energy distributions of the muons and electromagnetic components of the EAS at 1400 m. The distribution of particles ($e^{\pm },{\mu }^{\pm },\gamma$) included in this plot are without electric field shown in dashed lines for the cloud-to-ground model and with electric field of $+2000\mathrm{V}/\mathrm{cm}$ ($200\mathrm{kV}/\mathrm{m}$ or 0.4 GV/2 km) effect on ($e^{\pm },{\mu }^{\pm },\gamma$) shown in thick solid lines and—2000 V/cm effect on ($e^{\pm },{\mu }^{\pm },\gamma$) shown in thin solid lines. Detector response is not included in this distribution.

Figure 5

Left: $(\mathrm{\Delta }N/N)\%$ vs $\mathrm{\Delta }V$, including statistical error, for a uniform electric field layer inside the cloud (IntraCloud model) using the three low energy model GHEISHA, FLUKA, and URQMD. The model uses a uniform electric field 2 km inside the thundercloud that is located 2 km above ground level. Right: $(\mathrm{\Delta }N/N)\%$ vs $\mathrm{\Delta }V$, including statistical error, for a uniform electric field layer between the cloud and ground (Cloud-to-Ground model) using the three low-energy model GHEISHA, FLUKA, and URQMD. In this model the thundercloud base is 2 km in height from the detector.

Figure 6

Left: Time evolution of the intensity variation of the secondary low-energy cosmic-ray counting rate change ($\frac{N_c-N_p}{N_p}$) or ($\mathrm{\Delta }N/N$) on the 09/27/2014 thunderstorm shown in Fig. 2. Right: NLDN events peak current (kA) vs. time of the day in UTC. The blue line denotes the starting time for each frame on the left hand side. The black and gray cross marks are the intracloud and cloud-to-ground lightning sources detected by the NLDN for each frame.

Figure 7

Left: Time evolution of the intensity variation of the secondary low-energy cosmic-ray counting rate change ($\frac{N_c-N_p}{N_p}$) or ($\mathrm{\Delta }N/N$) on the 09/27/2014 thunderstorm shown in Fig. 2. Right: Temperature variation at 1400 m ($T_c-T_p$) or ($\mathrm{\Delta }T$) for the same frames. $T_c$ is the temperature in the current frame and $T_p$ is the temperature in the previous frame. The starting time is denoted on each frame. The black and gray crosses marks are the intracloud and cloud-to-ground lightning sources detected by the NLDN for each frame.

Figure 8

An illustration of the models used in the simulation in this work is to quantify the electric field inside thunderstorms resulting in the observed variations in the EAS by the TASD detector. left: The model using a uniform electric field 2 km inside the thundercloud (Intra-Cloud model) that is located 2 km above ground level. right: The model using a uniform electric field 2 km above ground level (Cloud-to-Ground model). The gray arrow represents the direction of the positive electric field following CORSIKA’s definition, where positive electric field direction is pointing upwards.

Figure 9

Top: Time evolution of the intensity variation of the radar images for the 09/27/2014 thunderstorm from 07:25–08:55 including the Telescope Array location marked in red. Bottom: Time evolution of the intensity variation of the radar images for the 09/27/2014 thunderstorm from 18:25–19:55 including the Telescope Array location marked in red.