Two types of energy‐dispersed ion structures at the plasma sheet boundary
Authors: Sauvaud J.‐A. , Kovrazhkin R. A. 
Published: December 2004

We study two main types of ion energy dispersions observed in the energy range ∼1 to 14 keV on board the Interball‐Auroral (IA) satellite at altitudes 2–3 RE at the poleward boundary of the plasma sheet. The first type of structure is named velocity dispersed ion structures (VDIS). It is known that VDIS represent a global proton structure with a latitudinal width of ∼0.7–2.5°, where the ion overall energy increases with latitude. IA data allow to show that VDIS are made of substructures lasting for ∼1–3 min. Inside each substructure, high‐energy protons arrive first, regardless of the direction of the plasma sheet boundary crossing. A near‐continuous rise of the maximal and minimal energies of consecutive substructures with invariant latitude characterizes VDIS. The second type of dispersed structure is named time‐of‐flight dispersed ion structures (TDIS). TDIS are recurrent sporadic structures in H+ (and also O+) with a quasi‐period of ∼3 min and a duration of ∼1–3 min. The maximal energy of TDIS is rather constant and reaches ≥14 keV. During both poleward and equatorward crossings of the plasma sheet boundary, inside each TDIS, high‐energy ions arrive first. These structures are accompanied by large fluxes of upflowing H+ and O+ ions with maximal energies up to 5–10 keV. In association with TDIS, bouncing H+ clusters are observed in quasi‐dipolar magnetic field tubes, i.e., equatorward from TDIS. The electron populations generally have different properties during observations of VDIS and TDIS. The electron flux accompanying VDIS first increases smoothly and then decreases after Interball‐Auroral has passed through the proton structure. The average electron energy in the range ∼0.5–2 keV is typical for electrons from the plasma sheet boundary layer (PSBL). The electron fluxes associated with TDIS increases suddenly at the polar boundary of the auroral zone. Their average energy, reaching ∼5–8 keV, is typical for CPS. A statistical analysis shows that VDIS are observed mainly during magnetically quiet times and during the recovery phase of substorms, while sporadic and recurrent TDIS are observed during the onset and main phases of substorms and magnetic storms and, although less frequently, during substorm recovery phases. From the slope of the (velocity)−1 versus time dispersions of TDIS, we conclude that they have a sporadic source located at the outer boundary of the central plasma sheet, at distances from 8 to 40 RE in the equatorial plane. The disappearance of the PSBL associated with TDIS can be tentatively linked to a reconfiguration of the magnetotail, which disconnects from the Earth the field lines forming the “quiet” PSBL. We show that VDIS consist of ion beams ejected from an extended current sheet at different distances. These ion beams could be formed in the neutral sheet at distance ranging from ∼30 RE to ∼100 RE from the Earth. Inside each substructure the time‐of‐flight dispersion of ions generally dominate over any latitudinal dispersion induced by a dawn‐dusk electric field. These two main types of energy‐dispersed ion structures reflect probably two main states of the magnetotail, quiet and active. Finally, it must be stressed that only ∼49% (246 over 501) of the Interball‐Auroral auroral zone‐polar cap boundary crossings can be described as VDIS or TDIS. On the other 51% of the crossings of the plasma sheet boundary, no well‐defined ion dispersed structures were observed.