“…In the Jovian and Saturnian magnetosphere, the anisotropy can result from the pickup process of newly generated ions near the erupting moons (Io and Enceladus, see Kivelson et al.,
1996; Leisner et al.,
2006; Russell et al.,
2006). In the near‐Earth space, a primary source of EMIC waves is the anisotropy developed after the injection of magnetotail ions into the inner magnetosphere (Baker et al.,
1982; Birn et al.,
1998; Sergeev et al.,
2012; Yu et al.,
2021). Recognizing that perpendicular‐moving ions (with equatorial pitch angle of ∼90°) drift faster in the magnetic dipole field than bouncing ions (with equatorial pitch angle closer to
or
) (Hamlin et al.,
1961), the Earth's dipole field in the inner magnetosphere serves as a natural filter to gradually separate ions with different pitch angles, which facilitates the EMIC wave excitation as evidenced by the strong association between nightside particle injections and EMIC wave activities (Chen et al.,
2010; Jordanova et al.,
2008; Jun et al.,
2019; Remya et al.,
2018).…”