From Jets to Superflares: Extraordinary Activity of Magnetized Plasmas in the Universe

The development of astronomical observations of various objects in 20th century has revealed that the universe is full of explosions (flares or bursts) and plasma outflows such as high-speed jets. Why is our universe filled with such extraordinary activity? When I started astrophysical research in 1977, I was fascinated with a puzzle why the nuclei of distant galaxies produce relativistic jets, collimated supersonic outflows. Soon after I learned observations of astrophysical jets, I learned solar observations, which show the importance of magnetic field in the production of flares and jets, though detailed physics is still not understood well at that time. I hypothesized that the jets may be accelerated by magnetic force both on the Sun and galaxies: in the case of the galaxies, magnetic field may be twisted by the rotation of accretion disk plasma, whereas on the Sun magnetic field can be twisted in the solar convection zone. During the untwisting process of a twisted magnetic flux tube, the jet may be accelerated. Then I started magnetohydrodynamic (MHD) numerical simulations of both solar and astrophysical jets. Fortunately, I succeeded in reproducing astrophysical jets from magnetized accretion disks using time dependent MHD simulations for the first time (1985, 1986). I was also lucky since I became a member of space solar observation missions Yohkoh (1991-2000) and Hinode (2007-present), and discovered X-ray jets in the corona (1992), as well as chromospheric anemone jets (2007). Both phenomena were successfully explained by the magnetic reconnection model. From observations of flares and jets on the Sun, I realized the importance of plasmoid ejections in magnetic reconnection (1995), and proposed the unified model of flares and jets on the basis of the plasmoid-induced reconnection and fractal reconnection (2001). More recently, as an extension of solar flare studies, I was fortunate enough to discover superflares on solar type stars with young colleague (2012), which may be important for the existence or survivability of human beings and life on the Earth and exoplanets. In conclusion, through these studies, I learned the reason why our universe is filled with extraordinary activity is that magnetized plasmas are so active and dynamic.