Study of Magnetic Helicity Dynamics in Solar and Space Plasmas

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Magnetic helicity is a physical variable, which can quantify the total linking number of magnetic field flux. Since magnetic helicity is a conserved quantity in the ideal magnetohydrodynamic (MHD) process, and is a proxy of magnetic free energy, it is widely believed that magnetic helicity may play a role as tracer and driver of MHD activities. We have investigated the dynamics of magnetic helicity in the Sun-Earth connection system. First, we have developed the methodology to measure magnetic helicity flux across the solar surface based on the madnetogram observation, and found that the helicity injection into the solar corona is well correlated with the soft X-ray emission activity in the solar corona. Second, we have developed the new numerical tool to reconstruct the three-dimensional magnetic field in the solar corona from vector magnetogram, and applied it to the data provided by the new solar satellite observatory HINODE.. As a result, we have revealed that magnetic field in pre-flare phase indeed has a sheared structure, which is well consistent with the paring of two-ribbon appearing after the onset of flare. Third, we created the state-of-the-art numerical scheme for ideal MHD equation, called HUD scheme, and have demonstrated that the computational performance of HLLD scheme is better than any previous schemes for the accuracy, robustness, efficiency and handiness of computation. Finally, we have applied the HLLD scheme to the solar coronal simulation, which fully includes the spherical geometry, as well as to the magnetospheric simulation, in which more generic equation of state is taken into account. The solar coronal simulation has revealed that the critical magnetic helicity to drive the onset of coronal mass ejection (CME) is sensitive to the spatial structure of magnetic helicity injection.