My current research interests are the numerical modelings on astrophysical objects such as accretion disks and astrophysical jets in the vicinity of neutron stars and black holes, and more recently on the most familiar star, the Sun. I have trained and experienced extensively on (radiation) magneto-hydrodynamic (MHD) simulations by using the state-of-the-art high performance computing resources for more than ten years.

About my research career:

My first modeling was an astrophysical jet emerging from a magnetized accretion disk around a neutron star and/or a black hole. In both cases, I found that a helical magnetic field structure, the so-called magnetic tower, drives a semi-relativistic jet along the rotation axis of the accretion disk (see Fig. 1). These results were published in ApJ and got a PhD under the supervision by Prof. Ryoji Matsumoto and Prof. Shin Mineshige. The magnetic tower can be the unified formation mechanism of astrophysical jets.

Fig. 1: Formation of a magnetic tower in a system of (a) a neutron star and (b) a black hole. Numerical computations were carried out by using Fujitsu VPP300 and VPP5000. AVS/Express on SuSE Linux was used for 3D visualization.

After my PhD, I had explored a relatively new research field “disk-seismology,” which uses oscillations of the accretion disk to investigate the effect of strong gravity in close proximity of black hole, similar to Helioseismology which was proven to investigate the internal structure of the Sun. By using three-dimensional MHD modeling of an accretion flow around a black hole, I found a pair of quasi-periodic oscillations (QPOs) of mass accretion rate which is most likely produced by a Lindblad type resonance in the accretion disk (see Fig. 2). This result suggests that multiple QPOs can be used as a proxy for measuring the mass and the angular momentum of black hole.

Fig. 2: (a) 3D MHD numerical modeling of black hole accretion flows. (b) Power spectrum of mass accretion rate toward the black hole. Resonant oscillations that are discovered by our numerical experiments are shown in a red arrow. Numerical computations were carried out by using Fujitsu VPP5000. IDL was used for analysis. AVS/Express on SuSE Linux was used for 3D visualization.

With the help of Dr. Makoto Miyoshi, a radio astronomer at the National Astronomical Observatory of Japan (NAOJ), I discovered multiple QPOs in millimeter radio emissions from the Galactic Center, Sagittarius A*. Based on my model of multiple QPOs, the observed frequencies can be interpreted as overtones of the resonant oscillations of the accretion disk around a supermassive black hole with a moderate spin parameter. This result contradict with the standard model because such a supermassive black hole could accumulate huge amount of angular momentum via accretion process and therefore it should have an extreme spin parameter. Our result suggested that the standard model must be revised so that the effect of Blandford-Znajek mechanism (Blandford & Znajek 1977) takes into account in order to extract the angular momentum of black hole during its growth phase.

In the meantime, I had decided to expand my knowledge and experiences through attending KITP workshops “Physics Outflows and Accretion Disks” in 2005. During the workshop, I discussed with Prof. Henk Spruit and Prof. Omar Blae about the issues on radiation MHD in accretion disks. Prof. Blae was the person who encouraged to investigate radiation processes of accretion disk when I participated the EURO summer school 2002 at Les Houches in France. Since then solving radiation transfer problems on MHD modeling have always been under my consideration because it is inevitable not only to change the structure of accretion disks but also to drive outflows and jets which expel surplus angular momentum.

When I had been a postdoctoral fellow at University of Tsukuba since 2005, I developed radiative transfer code including inverse-Compton scattering process in order to elucidate MHD models by comparing the observations over a broad-frequency range (see Fig. 3). At the same time, I contributed for developing a radiation MHD (RMHD) code which was aimed for evaluating the structure change of accretion disk and the acceleration of jets/outflows as a result of radiation force in a super-critical accretion flow around a black hole. Our RMHD paper was awarded as the PASJ excellent paper in 2012.

Fig. 3: (a) Synthetic Broad-band spectrum of accretion flow onto the Galactic centre modeled by using 3D MHD simulations. Error bars show time variation. (b) Synthetic images at the different frequency bands. Size of black hole shadow is shown as black dotted circles. Numerical computation were carried out by using Cray XT4 at NAOJ.

The solar atmosphere had draw my attention when Prof. Saku Tsuneta gave a lecture on Hinode observations and synthetic modelings based on state-of-the-art RMHD simulations. Since then, I have thought of applying their technique for modeling accretion disks in the future. This is the reason why I was pleased to join the Hinode project. With the help of experts on modeling the solar atmosphere, Prof. Oskar Steiner and Prof. Viggo Hansteen, who were visiting professors of NAOJ in 2009, I have learned useful techniques implemented in the state-of-the-art solar atmospheric modelings.

Our first paper of the solar atmospheric modeling was the discovery of spontaneous generation of compressive magneto-acoustic waves inside the flux tube by using two-dimensional RMHD simulations with the CO5BOLD code (Freytag et al. 2012). This new process resembles a turbulent pumping in the flux tube (Parker 1974) and therefore we called it a magnetic pumping. It sustains a persistent photospheric oscillation in the flux tube, however, the chromospheric and coronal responses to magnetic pumping were yet to be understood.

The second paper is to investigate the propagation of compressive magneto-acoustic waves across the transition region between the chromosphere and the corona as well as the consequences of the energy transport via shock waves into the upper atmosphere. We performed RMHD simulations with the Bifrost code (Gudiksen et al. 2011). We found that spontaneous generation of shock waves as a result of the magnetic pumping plays an important role for providing sufficient energy in order to maintain the chromosphere (See Fig. 4).

Figure 4: (a) A snapshot of temperature (color contour), velocity (arrows), and representative magnetic field lines (black solid curves). (b) Close-up of the lower part of the flux tube. Our result is almost identical to the previous paper by using the CO5BOLD code. Numerical computation were carried out by using Mac Pro (Two 2.8GHz Quad-core Intel Xeon Processors, RAM32GB).

I am currently interested in the generation of the other types of MHD wave such as a kink wave and an Alfvén wave, because they are responsible for driving magnetic tornadoes (Wedemeyer-Böhm et al. 2012). To examine the dominant process to generate MHD waves in more realistic magnetic field configuration, I have been investigating three-dimensional high resolution RMHD simulations of the solar atmosphere.

Figure 5: Solar tornado

Over the past ten years, my research area has been gradually expanded from accretion disks to solar atmosphere, but an underlined theme of my research has been focused on waves and oscillations in astrophysical plasma. I will continue to explore modeling of waves and oscillations in the solar atmosphere. Furthermore, I would like to apply advanced solar RMHD technique to elucidate waves and oscillations of an accretion disk around a black hole in the near future.