Detection of High-Energy Gamma Rays from Winter Thunderclouds

A report is made on a comprehensive observation of a burstlike $\gamma$-ray emission from thunderclouds on the Sea of Japan, during strong thunderstorms on 6 January 2007. The detected emission, lasting for $\sim 40\sec$, preceded cloud-to-ground lightning discharges. The burst spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung photons originating from relativistic electrons. This ground-based observation provides the first clear evidence that strong electric fields in thunderclouds can continuously accelerate electrons beyond 10 MeV prior to lightning discharges.

Figure 1

Schematic views of the two main sensors and the plastic scintillator of detector-$A$—photomultiplier (PMT). Numbers represent scales in millimeter.

Figure 2

Count rates per 20 sec of 4 inorganic scintillators between 21:10 and 22:10 UT on 6 January 2007. All abscissa represent universal time, and all errors are statistical $1\sigma$. Panels (a) and (b) show the count rates in $>40\mathrm{keV}$ data from the BGO and NaI scintillators of detector-$A$, respectively. Panels (c) and (d) give $>40\mathrm{keV}$ and $>600\mathrm{keV}$ data from the NaI and CsI counters of detector-$B$, respectively. The gaps in panels (a) and (b) are due to regular interruption of data acquisition of detector-$A$ every hour.

Figure 3

Detailed count-rate histories of the radiation sensors, and the light and electric field monitors, between 21:40 and 21:50 UT. All abscissa are universal time. All bins except those in panels (c) and (d) have $1\sigma$ statistical errors. (a) Count histories per 6 sec in $>3\mathrm{MeV}$ energies from the main NaI sensors of detector-$A$, without (black) and with (red) the BGO anticoincidence. (b) 3–10 MeV counts every 6 sec from the NaI (black) and CsI (red) scintillators of detector-$B$. (c)1-sec optical data variations. (d) 1-sec electric field data variations. (e) $>1\mathrm{MeV}$ counts every 6 sec from the plastic scintillator of detector-$A$.

Figure 4

Background-subtracted photon energy spectra accumulated over the 36 sec. All horizontal and vertical axes show the photon energy and the counts per each energy bin, respectively. Error bars are statistical $1\sigma$. (a) Spectra from detector-$A$. Black and red circles show data of the main NaI counters (summed over the two units), without and with the BGO anticoincidence. (b) Spectra from detector-$B$. Black and red squares are from NaI and CsI counters, respectively. Black and red dashed lines show predictions of the best-fit incident power-law model, determined over the energy range where the dashed lines are drawn.