Although the global structure and the dynamics of the Earth's magnetosphere interacting with the interplanetary magnetic field (IMF) has been studied so far, the triggering mechanism of the substorm onset has not been clarified completely. On the other hand, by developing a new methodology to determine the magnetic helicity flux from the data of magnetograph observation, Kusano et al. (2002) recently found that solar flares could be triggered by the magnetic helicity injection of the different sign from pre-flare state. Thus, they proposed that the annihilation of helicity may cause flares (Kusano et al., 2003). If the annihilation of helicity could work also as substorm onset mechanism, turning of the dawn-dusk component of the IMF may correlate with substorm onset, because it is prospected that the sign of the helicity injection from IMF to the magnetosphere is determined by the dawn-dusk field. In fact, the correlation between the dawn-dusk turning of the IMF and the substorm onset was observed by Bae et al. (2001). However, for the study of the interaction of the Earth's magnetosphere with the IMF, the view point the magnetic helicity injection has not been introduced so far. Therefore, particularly in this paper, the magnetic helicity injection process into the Earth's magnetosphere is investigated in detail by using the global MHD simulation. The results clearly show that the magnetic helicity is injected deeply into the magnetosphere through magnetic reconnection with the dawn-dusk component of the IMF. Interestingly, the sign of the magnetic helicity at the nightside is different from the sign at the dayside. This suggests that the IMF, which is zero helicity in our model, plays a role to transfer the magnetic helicity from the dayside to the nightside. We also discuss the applicability of 'annihilation of helicity model' to the substorm onset model.

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