Early exploration of Mercury, based on data collected by Mariner 10 during its three flybys, revealed that it was the only terrestrial planet in our solar system, other than Earth, to possess a global dipolar magnetic field (Ness et al., 1974). A subsequent mission known as MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER), sent the first spacecraft to orbit around Mercury (Solomon et al., 2007). It confirmed the dipolar field and found it was similar to Earth's in that the magnetic field lines of Mercury are divergent near the south pole and convergent toward the north pole; Mercury's dipole moment, however, is only about 195 ± 10 nT R M 3 (R M = 2,440 km is the radius of Mercury)-much weaker than Earth's (4/10000 of Earth's dipole moment)-and Mercury's dipole center is shifted northward by about 484 ± 11 km (0.2 R M ) (Anderson, Johnson et al., 2011). Further, Mercury has no atmosphere but possesses a tenuous surface-bounded exosphere. As the closest planet to the Sun, Mercury encounters a much stronger impingement of solar wind, whose density and dynamic pressure are an order of magnitude higher than those at Earth. In comparison to Earth, the result is a much smaller, weaker and more dynamic magnetosphere (