Role of IMF Bx in the solar wind-magnetosphere-ionosphere coupling

As far as the role of the interplanetary magnetic field (IMF) in the solar wind‐magnetosphere‐ionosphere (SMI) coupling is concerned, the role of the IMF Bx has more or less been ignored. Recent studies have shown that the IMF Bx plays an important role in the geometry of the bow shock under low Alfvén Mach numbers. Using global MHD simulations, this paper presents a further examination of the effects of the IMF Bx on the geometry of the magnetopause, the ionospheric transpolar potential, and the magnetopause reconnection rate, which quantify the SMI coupling process, under low Alfvén Mach numbers. The role of the IMF Bx manifests itself in three aspects: (1) the magnetopause shifts toward either north or south, depending on whether the Bx · Bz is negative or positive, whereas the bow shock expands in the opposite direction; (2) during southward IMF, the magnetic merging line shifts northward (southward) on the day side and southward (northward) on the night side for Bx > 0 (Bx < 0); (3) both the ionospheric transpolar potential and the magnetopause reconnection rate decrease with increasing Bx, and the relative reduction may reach as high as 20% under extreme cases. The physical mechanism for this reduction is attributed to the change in the width of the magnetosheath, which is sensitive to the variation of Bx under low Alfvén Mach numbers.