[P-EM08] 宇宙天気・宇宙気候

[PEM08-14] How do auroral substorms depend on Earth's dipole magnetic moment?

*海老原 祐輔¹、田中 高史² (1.京都大学生存圏研究所、2.九州大学国際宇宙天気科学・教育センター)

キーワード：オーロラ・サブストーム、地球磁場強度、MHDシミュレーション

The near-Earth space environment is suggested to depend on Earth's dipole magnetic moment M. The dependence on M is not straightforward since the solar wind-magnetosphere-ionosphere coupling system is complicated. Here, we show how the change in M affects the development of an auroral substorm by using global magnetohydrodynamics (MHD) simulation. We artificially increased M by a factor of 1.5 (corresponding to the Earth about 630 BC) and decreased M by a factor of 1.5 (corresponding to the Earth about 2850 AD) in the MHD simulation. The ionospheric conductivity decreases with increasing M, in accordance with the aid of empirical relations. An auroral substorm took place regardless of M, but its development depends largely on M. The major results can be summarized as follows. In the future (when M decreases monotonically), (1) the expansion onset of the substorm takes place later, (2) the auroral electrojet develops slowly, (3) the amount of energy coming into the magnetosphere decreases whereas the maximum auroral electrojet increases, and (4) the bright auroral region expands more equatorward. The first two consequences are probably associated with the slow magnetospheric convection. The third consequence is associated with the dependence the ionospheric conductivity on M. The fourth consequence is related to the weakness of the magnetic pressure force that impedes earthward penetration of hot plasma. It is shown that the evolution of substorms depends largely on the value of Earth's dipole moment.