Outflowing ionospheric ions observed by Geotail and Akebono and their transport in the near-Earth and mid-tail magnetosphere

Abstract

Several types of discrete ion signatures have been found in different regions of the Earth's magnetosphere. The discovery of ionospheric-origin ion flows in the distant magnetotail by Geotail has been one of the most remarkable findings by ISTP satellites. In an event study, flow velocity and density variations are compared with geomagnetic field data and auroral indices AU, AL, and AE. Following a sudden increase of the geomagnetic fields observed on the ground and by Geotail, clear double-component ion flows consisting of H + and O + were detected in the southern lobe/mantle region. The auroral indices were well correlated with the tailward velocities of the multi-composition ion flows after the sudden compression of the magnetosphere was recorded by magnetometers at ground stations and Geotail and the geomagnetic activity was enhanced. Another recent finding of Geotail has been distinctly energy-dispersed ion signatures in the equatorial plasma sheet and the dayside magnetosphere near the magnetosphere-magnetosheath boundary. Similar ion dispersions have been reported from satellite observations at low altitudes. The ion energies ranged from a few tens of eV/ q to ∼ 10 keV/ q, and pitch angle distributions showed the properties of bidirectional field-aligned cold flows which are multiple at different energies. We propose that the origin of the ion beams observed near the nightside and dayside equatorial regions is to be found in upward flowing ion beams or conics generated at low altitudes. These could contribute 30–60 percent of the ion population in the dayside magnetosphere. Although some time-dependent effects might be involved, the primary process causing energy-dispersed signatures would discriminate ion energies spatially, for example, the velocity filter effect due to the E×B . On the basis of these observations by Geotail and Akebono, we discuss the process of the acceleration and transport of the ionospheric ions supplied to the magnetosphere.