Periodic emergence of multicomposition cold ions modulated by geomagnetic field line oscillations in the near‐Earth magnetosphere

Abstract

In the equatorial magnetosphere during perigee passing, the Geotail spacecraft sometimes observes periodic enhancements of cold ion flux embedded in the energetic ion component. In particular, the dawnside and duskside regions, the periodic signatures indicate not only oscillating bulk motions of both energetic and cold ion components in the inner magnetosphere but also the existence of multiple ion components at thermal energies, namely, singly‐charged hydrogen, helium, and oxygen of ionospheric origin. Although these cold components would usually be invisible due to the low energy limit of the instrument and the positive potential of the spacecraft, the excitation of Pc 5 pulsations often causes the periodic emergence of the hidden cold ions after their escape from the ionosphere without further significant energization. The distinctive multicomponent ion signatures can be seen frequently from 1999 to 2002, near solar maximum, but not earlier in the Geotail mission, in 1995–1998 in the early rising phase of the solar cycle. It is probable that outflow processes of the ionospheric ions are active at times of high solar activity. A significant fraction of the near‐Earth magnetospheric ion population is dominated by ionospheric ions, as shown in the density calculations for the separate ion species.