Solar Flare Model
Solar flares are believed to be violent explosions where the huge amount of magnetic energy stored in the active region is abruptly liberated to make and accelerate extremely high-temperature plasma. We are developing a data-driven magnetohydrodynamic (MHD) simulation in order to improve our understanding and the predictability of flare onset. We have successfully simulated flare, which werewas observed on December 13, 2006, by considering the three-dimensional magnetic field structure and the motion of plasma on the solar surface. The figure represents the results of that the simulation, in which we can see that plasma erupts upward with magnetic reconnection in the complicated magnetic structure of active region NOAA 10930.
Fig.1: The numerical simulation of the large flare on December 13, 2006: Magnetic field lines (blue lines) and velocity structures (the more reddish, the faster) indicate that fast plasma flow results from magnetic reconnection (left figure) and then rises forming a shock wave in its front (right panel).
Solar Active Region Model
| Active regions on the Sun are the sources of solar flares and coronal mass ejections (CMEs) Therefore, precise modeling is necessary to achieve space weather forecasts. We have done the precise modeling of active regions based on high-resolution observational data from the Japanese solar satellite Hinode, which was launched in 2006.
The figure on the right is the result of the reconstruction of the three-dimensional magnetic structure using observational data before the onset of the flare on December 13. Each line represents a magnetic field line, and the color scale indicatesis the normal component of magnetic field on the solar surface. The white surfaces corresponds to the region of strong electric current
density, in whicih free magnetic energy to drive solar eruption might
be stored. | 
Fig.2: The magnetic structure of the active region before the large flare on December, 2006. |
Coronal mass ejection model
Coronal mass ejections (CMEs) are explosive phenomena where the ejecting magnetic structures, associated with disruption of the coronal magnetic field, such as flares, are expelled into interplanetary space interacting with the ambient field. For CME modeling, we should perform realistic MHD simulations including the effects of the magnetic and plasma conditions of ejecting structures and ambient magnetic, gravity, and velocity fields.
| The figure on the right shows the result of three dimensional MHD simulation where information obtained from the results of the AR model is adapted to the initial condition. A twisted structure can escape from the Sun interacting with the ambient field. | 
Fig.3: Results of MHD simulation of CME. The sphere shows the surface of the Sun (colors: magnetic field) and tubes displays magnetic field lines. |
Solar Wind model
The plasma blast toward interplanetary space from the sun is called a solar wind, which reaches speeds of about 400 km/s to 800 km/s near the earth. Eruptions of the corona and plasma, caused by flares and prominence eruptions, are called coronal mass ejections (CME), and this propagates in the solar wind and reaches the Earth. Through interaction with the solar wind, the propagation speed, spread angle, and amplitude of the formed shock wave can change substantially.
It is also known that geomagnetic storms often occur around the earth when the direction of the magnetic field in the solar wind (southward) is oriented opposite to that of the earth (northward). As CMEs often have a southward-oriented magnetic field, they are important causes of geomagnetic storms. How the change in the shape of the magnetic field as the CME propagates toward the Earth's magnetosphere gives rise to the southward magnetic field that reaches the earth is a problem directly related to space weather. Because realistic solar wind modeling is important, we are carrying out three dimensional MHD simulations using the observational data.
In 1958, a solar wind (about 400 km/s) was theoretically predicted by Parker, and it was confirmed by the Venus probe, Mariner 2, in 1962. However, a high-speed solar wind of about 800 km/s was discovered that blew from low temperature coronal holes. In Parker's theory, the speed of the solar wind increases with temperature, so it is a mystery that high speed winds blow from low temperature coronal holes. A new mechanism for the acceleration of the solar wind was needed, which we have been able to clarify. | 
Fig.4：Results of a three dimensional simulation of solar wind. This snap shot is of the steady state. The curve lines show magnetic field lines. The colors show open (blue) and closed (gold) field lines. Red and blue colors on the central sphere show the polarity of the magnetic field on photosphere. |
Interplanetary model
The accelerated solar wind flows outward into interplanetary space. The power to create terrestrial aurora is naturally generated in the solar wind-magnetosphere system.
| Three-dimensional MHD simulations of the large-scale solar wind structures are therefore needed to predict the time-varying solar wind parameters at the Earth's position. It is important to reconstruct or predict the realistic shape of heliospheric current sheets and/or propagation of coronal mass ejections interacting with the background solar wind to predict geomagnetic storms or radiation belt enhancement.
The figure shows propagation of the CME associated with a December 2006 X-class flare. The speed of solar wind plasma is color coded. In this simulation, the background solar wind speed map was obtained from the IPS of Nagoya University, and the background solar wind magnetic field was obtained from the Wilcox Solar Observatory. | 
Fig.5: Results of MHD simulation of CME propagation.Results of MHD simulation of CME propagation. |
Solar wind-magnetosphere interaction model
The solar wind ejected from the sun changes because of solar flares on the solar surface, and interplanetary disturbances encounters with the earth's magnetosphere. The geomagnetic field is impinged by the pressure of the solar wind on the Sun side; conversely, the shape becomes a long extended tail on the side opposite to the Sun. For southward IMF, magnetic reconnection occurs in the subsolar region and plasma sheet. The energy is stored in the magnetotail and released by instabilities to excite active aurora and substorms/storms.
Figure shows three-dimensional global MHD simulation results during a solar flare event in December 2006. When the shock wave related to CME arrived at the Earth, the earth's magnetosphere was compressed causing magnetosperic storms.
| Fig.6: Figure: Three-dimensional MHD simulation results of solar wind-magnetosphere interaction, where ionospheric convection and electric potential in the polar region and three-dimensional configuration of magnetosphere are shown |
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