Cold dense ion flows with multiple components observed in the distant tail lobe by Geotail

Abstract

The Geotail spacecraft frequently observes cold dense ion flows (CDIFs) streaming tailward in the magnetotail lobe region (XGSM∼−10 to −210 RE). The density is often quite high (∼1/cm3), comparable to or larger than that in the plasma sheet, and the tailward speed is from 50 to 500 km/s. The CDIFs sometimes contain multiple (two or three) energy‐per‐charge components. The lowest‐ and highest‐energy components are identified as H+ and O+, respectively. The intermediate‐energy component is identified as He+. Therefore a part of the CDIFs in the distant lobes is believed to be of ionospheric origin. A possible source candidate is the upward flowing ions (UFIs) outgoing from the dayside auroral region or polar wind escaping from the polar cap and accelerated along the magnetic field direction. The high proton to O+ density ratio in the CDIFs suggests that the major component of the CDIFs originates from the solar wind and that the cold ion beams of ionospheric origin merge with the solar wind component penetrating into the magnetosphere at the mantle or the flank of the tail lobe. In some observations, the H+ ions continuously stream tailward both in the tail lobe and in the magnetosheath, and they couple with the solar wind ions through the magnetopause. Although the O+ and He+ ion flows cannot be recognized so clearly in the magnetosheath, weak O+ fluxes are sometimes present in the magnetosheath near the magnetopause. This implies that plasma of ionospheric origin may leak from the tail to the magnetosheath. The multicomponent CDIF events make it possible to discuss the sources and transport processes of the tail lobe plasma, and their behavior in or near the magnetopause provides important clues to the interactions between the lobe and the magnetosheath.