Simultaneous appearance of energy‐dispersed ion signatures of ionospheric and magnetotail origins in the near‐Earth plasma sheet

The present paper reports on properties of multiple energy‐dispersed ion signatures observed by Geotail in the near‐Earth plasma sheet. The dispersed ion signatures are widely observed in the nightside tail region, and they occur not only in disturbed magnetospheric conditions but also in quiet conditions. According to the statistical analysis, the dispersed ion signatures are accompanied by earthward bulk flows, magnetic fluctuations, dipolarizations, and electron pressure anisotropy. It is noted that the dispersed ions have a wide pitch‐angle distribution, which suggests that the ions come from the magnetotail in which the magnetic field is much weaker than that in the observation point. Our most remarkable finding is that the dispersed ions of the magnetotail origin are sometimes (∼40% of the total) accompanied by highly collimated dispersed ions, probably of the ionospheric origin. On the basis of results from the correlative analysis of the two types, we propose a model of tail configuration changes which explains the observed signatures. This model indicates two suggestions: The region of ion acceleration above the polar ionosphere may be mapped to the middle/distant tail regions, and the ion acceleration is triggered by the tail activities probably through propagation of Alfvénic disturbances from the tail to the ionosphere.