Combining Cherenkov and scintillation detector observations with simulations to deduce the nature of high-energy radiation excesses during thunderstorms

We present co-observations of three strong count-rate enhancements associated with thunderstorms observed over 17 April 2015 to 23 September 2015 by the High Altitude Water Cherenkov (HAWC) array, and a suite of small scintillation detectors comprising the Gamma-ray Observations During Overhead Thunderstorms (GODOT) instrument. Because the HAWC array is most sensitive to ionizing radiation at high energies ($>100\mathrm{MeV}$), and the small scintillation detectors are most sensitive to ionizing radiation at low energies ($3–20\mathrm{MeV}$), we investigate using the ratio of these detector counting rates variations to infer spectral characteristics of these enhancements, and understand the physics behind these thunderstorm accelerator mechanisms. We consider two extreme mechanisms that can produce these enhancements: a point source of relativistic runaway electron avalanches (RREA), and modification of the background cosmic-ray spectrum (MOS) from an electric field profile that is everywhere below the RREA threshold. We simulate the responses of HAWC and the GODOT $12.7\mathrm{cm}\times \oslash 12.7\mathrm{cm}$ NaI(Tl) scintillator and show that their ratio can discern between the two models, and that the observed thunderstorm rate enhancements are incompatible with the spectra from a point source of RREA, but consistent with our model of MOS for thunderstorm potentials within the range of $-250$ to 250 MV.

Figure 1

The 300 HAWC WCDs indicated by circles and the relative location of the GODOT detector (red star) during 2015, April 17th to September 21st.

Figure 2

Thunderstorm ground enhancements co-observed by (a) HAWC and the three GODOT detectors, (b) $5\times 5$ NaI, (c) $5\times 5$ plastic (LgPl), (d) $1\times 1$ plastic (SmPl) for the dates of 2015 May 15, (left column), July 15th at 21:10 UTC (middle column), and July 15th at 22:40 UTC (right column). For each detector (except SmPl) the background rate is shown (dashed line) as well as the $2\sigma$ tolerance interval (fine dashed). The shaded regions show excess counts (background subtracted total) attributed to the thunderstorm variation, with the total number of counts in the shaded region indicated in red. Blue vertical lines in the HAWC data show nearby lightning flashes registered by WWLLN.

Figure 3

HAWC and the GODOT NaI detector effective area for gammas, electrons, positrons and muons as a function of kinetic energy. HAWCs effective areas are indicated by the black lines, the GODOT NaI effective areas are indicated by the thick red lines. The NaI effective areas have been multiplied by $10^6$.

Figure 4

Simulated ratio of detector variations for MOS and RREA. (a) Simulated ratio of HAWC and GODOT NaI detector variations for MOS, as a function of the sea level equivalent electric field, $E_0$, that parametrizes the electric field profile, as well as the total MOS potential, $U_{\mathrm{MOS}}$. (b) Simulated ratio of HAWC and GODOT detector variations for a point source of RREA, as a function of zenith viewing angle to the source, for various source altitudes. The range of observed detector variations from each time bin in the shaded regions in Fig. 2, $(\mathrm{\Delta }R/R_0{)}_{\mathrm{HAWC},\mathrm{obs}}/(\mathrm{\Delta }R/R_0{)}_{\mathrm{GODOT},\mathrm{obs}}$, for TGE 150503 and TGE 150715a, b are indicated by red and blue band respectively. The error bars indicate uncertainties from Poisson statistics (see Supplemental Material [32]).

Figure 5

Top: Raw and Processed HAWC Scaler rate data. (Red) combined count rate from 1044 “good” HAWC PMTs, showing a large FRED event near 2218 UTC. (Black) 67 FRED-free PMTs used for this event. FRED-free indicates no FRED event contamination within 30 minutes of our time interval of interest (vertical lines). (Blue) Combined 67 FRED-free PMT rates scaled up to equivalent 1044 PMT rate, smoothed with 2.5 second sliding median filter. Bottom: Raw GODOT NaI time-tagged event data. The 7-minute interval between the vertical lines corresponds to the data in Figs. 2 and 2. TGE 150503 seen by HAWC is indicated within the green circle.