Ion composition and charge state of energetic particles in flux ropes/plasmoids

Abstract

Measurements from the energetic particles and ion composition (EPIC) instrument on Geotail are used to investigate the relative abundance of different ion species at energies ∼10 keV to 3 MeV in flux ropes or plasmoids detected at various downstream distances from ∼23 RE to ∼190 RE in the tail. In terms of charge state, we find that during encounters with these structures most of the oxygen ions are singly charged, i.e., O+ ions of ionospheric origin, while most of the helium ions are doubly charged, i.e., He++ ions of solar wind origin. Therefore the ion species from the solar wind and the ionosphere appear to be thoroughly mixed inside flux ropes or plasmoids, just like the ion composition in the tail outside these structures. This study also reveals a variety of changes in the abundance of ionospheric oxygen ions within flux ropes or plasmoids. These range from simultaneous enhancements of ionospheric oxygen ions within the structure to delayed enhancements or no significant change of ionospheric oxygen ions throughout the entire structure. The amount of ionospheric oxygen ions present in flux ropes or plasmoids increases with higher geomagnetic activity as gauged by the Kp index, but it shows no apparent trend with the downstream distance. The lack of consistent enhancement of ionospheric oxygen ions inside these structures suggests that oxygen ions may not play a significant role in their formation.