Statistical study of “trunk-like” heavy ion structures in the inner magnetosphere

Abstract

We present a statistical study of “trunk-like” structures observed in He⁺ and O⁺ in the inner magnetosphere. The main characteristic of these structures is that the energy of the peak flux decreases earthward. Using observations from the Helium Oxygen Proton Electron (HOPE) instrument onboard Van Allen Probe A, we identify the trunks observed from November 2012 to June 2019 and obtain the universal time, <italic>L</italic> shell, magnetic local time (MLT), and energy information of each trunk’s root and tip. We then investigate the behavior of trunks in terms of their frequency of occurrence, temporal evolution, spatial and energy distribution, and trunk dependence on different geomagnetic indices. We find that (1) the trunks are always located at <italic>L</italic> = 1.5−4.0 and have a preferential location mainly concentrated at MLT = 18−24, (2) the sector within MLT = 14−16 is a forbidden zone without trunk roots, and (3) the energy of He⁺ trunks is the largest near dusk and gradually decreases in the counterclockwise direction, whereas the energy of O⁺ trunks is relatively evenly distributed with MLT and <italic>L</italic>. The differences between He⁺ and O⁺ trunks are probably due to the different charge exchange and Coulomb collision lifetime. The dependence on different geomagnetic indices indicates that the trunk structures occur more frequently during relatively quiet periods.