Observation of an Energetic Radiation Burst from Mountain-Top Thunderclouds

During thunderstorms on 20 September 2008, a simultaneous detection of $\gamma$ rays and electrons was made at a mountain observatory in Japan located 2770 m above sea level. Both emissions, lasting 90 sec, were associated with thunderclouds rather than lightning. The photon spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung $\gamma$ rays arriving from a source which is 60–130 m in distance at 90% confidence level. The observed electrons are likely to be dominated by a primary population escaping from an acceleration region in the clouds.

Figure 1

Count rates per 30 sec of $>10\mathrm{keV}$ NaI (top) and $>500\mathrm{keV}$ plastic (bottom) scintillators between 15:15 and 16:15 UT on 20 September 2008. The horizontal axis represents universal time, and all errors are statistical $1\sigma$. Each solid curve shows estimated background level.

Figure 2

Detailed count histories of the NaI counter with anticoincidence, and the electric field and light sensors, between 15:40 and 15:52 UT. All abscissa are universal time. Panels  (a) and (b) show count histories per 7 sec of the NaI counter in 0.01–3 MeV and 3–12 MeV energies, respectively, with $1\sigma$ statistical errors. (c) One-second electric field data variations. (d) One-second optical data outputs.

Figure 3

(a) The background-subtracted photon energy spectra (black points) obtained from the NaI signals with anticoincidence. The horizontal and vertical axes represent the photon energy and the counts per energy bin, respectively. Error bars are statistical $1\sigma$. Each curve shows prediction of an incident power-law model. Air mass in $\mathrm{g}{\mathrm{cm}}^{-2}$ corresponding to the assumed source distance is shown in parentheses. (b) The ${\chi }^2$ value of the model fit to the data, plotted against the assumed source distance. Horizontal lines from bottom to top represent 68%, 90%, and 99% confidence levels.