I was interested in experimental physics for elementary particles when I was a high-school student, and entered Department of Physics, School of Science, The University of Tokyo. After I studied measurement technique for MeV photons with scintillation crystals and charged particles with silicon photo-multipliers, I got interested in observational studies of high-energy phenomena in nature, rather than controlled accelerator experiments for fundamental physics. I joined High-energy Astrophysics Laboratory at The University of Tokyo, and got my Ph.D in 2020.
Exploring high-energy phenomena in lightning and thunderclouds – High-energy Atmospheric Physics –
Coincident with lightning flashes and passages of thunderclouds, gamma-ray photons reaching tens of MeVs have been detected. Those are evidence to demonstrate that electrons are accelerated by strong electric fields in lightning and thunderclouds, and emit high-energy photons via bremsstrahlung. In other words, lightning and thunderclouds are natural particle accelerators. MeV photons are the highest-energy electromagnetic emissions among photons naturally produced inside the Earth. I lead the Gamma-Ray Observation of Winter Thunderclouds (GROWTH) experiment with Dr. Teruaki Enoto (RIKEN), Dr. Harufumi Tsuchiya (Japan Atomic Energy Agency), and Dr. Kazuhiro Nakazawa (Nagoya University). The GROWTH experiment aims at detecting high-energy phenomena during winter thunderstorms in coastal areas of the Sea of Japan.
Gamma rays coincident with lightning flashes were discovered by a gamma-ray astronomy satellite in space, and now called “terrestrial gamma-ray flashes” (TGFs). As well, ground-based experiments including our GROWTH experiments have observed such phenomena. Based on our observation in 2017, we demonstrated that a downward-oriented TGF triggered photonuclear reactions in the atmosphere, and produced neutrons and positrons. This result was published in Nature in November, 2017. The fact that a lightning discharge triggered nuclear reactions and produced isotopes attracted general audience all over the world.
In addition to lightning flashes, thunderclouds also emit gamma rays without flashes. These gamma rays are thought to last for several to several tens of minutes and to move with thunderclouds. However, we never understand when they are produced, how long they last for, and when they terminate. Also the relation between the gamma-ray emissions and lightning flashes, namely whether gamma rays from thunderclouds facilitate lightning flashes or not, is an enigma. To resolve these questions, we expanded our observation sites to Kanazawa, and started a mapping observation campaign with multiple gamma-ray detectors.
Lightning flashes are familiar phenomena to us. They have been observed by optical, sound, radio, electric-field, and radar measurements. It is insufficient for gamma-ray observations only to reveal natures of high-energy phenomena in thunderstorms. I’m collaborating with domestic and international researchers and adopting various observation techniques.
As a further large mission, I joined the Taranis mission. Taranis is a french satellite mission dedicated for lightning observations led by Centre National d’Etude Spatiale (CNES). It is expected to give us great insights into TGFs and transient luminous events. Taranis will be launched in June, 2020. I stayed in Laboratoire Astroparticule et Cosmologie at Université Paris Diderot from April to October 2018, and joined calibration and validation programs of X-ray, Gamma-ray, and Relativistic Electron detectors onboard Taranis. As a member of the Taranis team, I’ll also gain forefront sciences of lightning after the launch.