Thunderclouds generate very strong electric fields that can accelerate electrons to relativistic speeds. This ‘electron avalanche’ goes on to collide with atoms and produce an intense burst of radiation, which is sometimes followed by lightning.
Recently, Harufumi Tsuchiya at the RIKEN Advanced Science Institute and co-workers observed a long radiation burst above the mountains of Japan that they believe was associated with thunderclouds, but not with any lightning1. Their crucial breakthrough was to measure both charged particles and gamma-rays, giving important information on the origins of the burst.
Most radiation bursts only last up to a few milliseconds, but longer events lasting up to a few minutes have been observed from aircraft, on high mountains and near the coast of Japan. These longer bursts have puzzled scientists because theory suggests that an electron avalanche should last only a few microseconds at most.
Tsuchiya and co-workers have been working to understand these elusive long radiation bursts. In January 2007 they detected a long burst of gamma-rays over the Sea of Japan2. More recently, they installed a detector near Norikura Cosmic-Ray Observatory, located 2,770 meters above sea level in the mountains of Gifu prefecture, which regularly experiences thunderstorms.
The new detector contains a plastic scintillator that is very sensitive to charged particles, but not to photons, and a sodium iodide scintillator that is sensitive to both types of radiation. By combining these two signals, the researchers were able to record both charged particles and gamma rays from a burst lasting 90 seconds during thunderstorms shortly after midnight on 20 September 2008.
“Although we made our detector by ourselves, the so-called ‘anti-coincidence method’ to distinguish between particles and photons is very common in cosmic-ray measurement and satellite observations,” says Tsuchiya. Thanks to this simultaneous detection, the researchers were able to calculate that the main source of the burst was only about 90 meters away from their detector, and resulted from a 200-meter-long acceleration region in the thunderclouds.
The charged particles they recorded were probably the most energetic electrons, which survive long distances in the atmosphere. In fact, the researchers estimate that the burst produced around 300 million electrons with energies of over 20 MeV. These are higher energies than previously anticipated, and could go some way to explaining how longer radiation bursts might arise from thunderclouds (Fig. 1).
“We believe this kind of observation would be useful to better understand a fast charging mechanism of thunderclouds, and possibly lightning initiation,” says Tsuchiya.
References
1. Tsuchiya, H., Enoto, T., Torii, T., Nakazawa, K., Yuasa, T., Torii, S., Fukuyama, T., Yamaguchi, T., Kato, H., Okano, M. et al. Observation of an energetic radiation burst from mountain-top thunderclouds. Physical Review Letters 102, 255003 (2009).
2. Tsuchiya, H., Enoto, T., Yamada, T., Yuasa, T., Kawaharada, M., Kitaguchi, T., Kokubun, M., Kato, H., Okano, M., Nakamura, S. & Makishima, K. Detection of high-energy gamma rays from winter thunderclouds. Physical Review Letters 99, 165002 (2007).
The corresponding author for this highlight is based at the RIKEN Cosmic Radiation Laboratory
