Statistical Study Of The "elephant Trunk" Heavy Ion Structure In The Inner Magnetosphere

Charged particles with different energies trapped by the quasi-dipole magnetic field form the relativistic radiation belt,energetic ring current,and cold plasmasphere in the inner magnetosphere.The interaction between these particles and the time-varying background electromagnetic fields makes the inner magnetosphere a highly dynamic space environment.As the main source of the inner magnetospheric particles,plasma sheet particles are injected into the inner magnetosphere during magnetic storm or substorm.Several satellite missions have observed distinct features of plasma sheet ions entering the inner magnetosphere.One of these features appears in the form of energy spectrum features such as“nose-like”structures,“wedge-like”structures,ion spectral gaps,“finger-like”structures,and“trunk-like”structures.The study of the energy spectrum is very important because they are a significant sign of new particles'injection into the inner magnetosphere from the magnetotail plasma sheet.Different spectrums in the inner magnetosphere are attributed to the single or combined effects along the drift paths.The investigations of the formation mechanism of ion spectral structures are helpful to understand the transport,acceleration,and loss of ions from the plasma sheet.We present a statistical study of“trunk-like”structures observed in He~+and O~+in the inner magnetosphere.The main characteristic of this structure is that the energy of the peak flux decreases Earthward.We investigate the behavior of trunks in terms of occurrence frequency,temporal evolution,spatial and energy distribution,as well as dependence on different geomagnetic indices,and analyze the formation mechanism of the trunk structures.The main work and results of this paper are as follows:1.Analysis of typical case.We study in detail the characteristics of the typical“trunk”structure on February 17,2018.We use the geomagnetic indices to analyze the variation of geomagnetic activity before the formation of this trunk structure.No magnetic storm occurred within 24 hours before the trunk structure,but three consecutive substorms appeared,and the trunk appeared at the end of the substorm recovery phase.2.A database of trunk structures is created.Using observations from the HOPE instrument onboard Van Allen Probe A,we obtain 862 He~+trunk spectral structures and 202 O~+trunk spectral structures observed from November 2012 to June 2019 and determine the UT,L,MLT,and energy of each trunk's root and tip.The failure of H~+trunk observation is probably due to the short charge exchange lifetime along the drift paths.3.Investigation of the spatial distribution and energy dependence of trunk structures.We find that(1)the trunk structures are always located in the range of L?1.5-4.0 and have a preferential location mainly concentrated in the range of MLT?18-24.The MLT?14-16 sector is a forbidden zone without trunk roots,and there is a similar sector(MLT?3-5)on the nightside with a few roots.Trunk ions drift from the nightside along the open trajectory to the dayside and enter the closed trajectory region with very small azimuthal drift velocities.Because the velocity becomes smaller,trunk ions take a longer drift time to reach the afternoon sector and are more easily to be depleted due to charge exchange and Coulomb collision.This is probably the reason why the trunk structures have a preferential location in the nightside and the forbidden region exists in the afternoon sector.(2)The energy of the He~+trunks is the largest near dusk and gradually decreases in the counterclockwise direction,while the energy of O~+trunks is relatively evenly distributed with MLT and L.Ions with relatively low energies dominated by the E×B drift will drift eastward.Ions with high enough energies dominated by the gradient-curvature drift will drift westward.Therefore,high-energy trunk ions are more likely to exist on the dusk-side than on the dawn-side.It is difficult for low-energy trunk ions to reach dusk through the dawn-side and the dayside.As a result,many high-energy He~+trunks are formed near dusk.Since high-energy trunk ions drift very slowly on the closed trajectory and O~+is easier to be lost than He~+,therefore there is no high-energy O~+trunk gathering near dusk.4.Investigation of the geomagnetic condition dependence of trunk structures.Three geomagnetic indices are used to analyze the geomagnetic activity dependence of trunks.We find that strong geomagnetic activity will inhibit the formation of trunks and a relatively quiet space environment is more conducive to the occurrences of trunks.However,more trunks on the dayside with higher geomagnetic disturbance and substorm intensity indicates that the trunks in the nightside are mainly inhibited.This is reasonable since a stronger convective electric field makes the closed orbit region smaller,and it is more difficult for trunk ions to reach the nightside through the dayside.The number of trunks on the nightside decreases,and the occurrence rate on the dayside increases naturally.