Geotail observations of ionospheric origin cold ions in the plasma sheet: An escape route and hidden ion components

*Masafumi Hirahara[1] ,Kanako Seki [2],Toshifumi Mukai [3]
Dept of Physics, Rikkyo University[1]
University of Tokyo[2]
ISAS[3]

Satellite observations during these three decades have made it evident that the ionosphere is one of origin for the magnetospheric ions besides the solar wind. Plasma sheet observations by the previous satellite have shown that the energetic heavy ions of ionospheric origin could be dominant particularly at active times. It is, however, not verified that the total population of the ionospheric origin ions in the magnetosphere quantitatively matches that of the outflowing ions from the polar ionosphere. In this talk, we present two types of the Geotail observations exhibiting the ionospheric ion population with low energies in the equatorial near- Earth magnetosphere.

Satellite observations during these three decades have made it evident that the ionosphere is one of origin for the magnetospheric ions besides the solar wind. We also have presented the energetic O+ flows from the Geotail data in the lobe/mantle. The statistical studies indicate these ionospheric ion fluxes in the lobe/mantle are not large enough to explain the outflow rate of the ionospheric ions estimated from polar-orbiting satellite results. Plasma sheet observations by the previous satellite have shown that the energetic heavy ions of ionospheric origin could be dominant particularly at active times, which seems consistent with a scenario that the ionosphere could greatly supply the plasmas into the magnetosphere. It is, however, not verified that the total population of the ionospheric origin ions in the magnetosphere quantitatively matches that of the outflowing ions from the polar ionosphere. In this talk, we present two types of the Geotail observations exhibiting the ionospheric ion population with low energies in the equatorial near-Earth magnetosphere.
As first, the plasma data on November 8, 1998 show that just after a substorm onset Geotail moved from the lobe region to the plasma sheet and observed both multiple ion energy dispersions of more than 10 keV and tailward ion flows of lower than a few keV. The distribution functions of the low-energy ions consist of three components, which are probably H+, He+, and O+ from the lowest-energy components, as indicated by the ExB velocity analyses. These phenomena enables us to discuss an escape route of the ionospheric ions injected directly into the near-Earth plasma sheet without large heating/energization.
As another type of cold ion observations by Geotail, low- frequency periodic plasma bulk oscillations are often seen in the equatorial near-Earth magnetosphere. It should be noted that two or three discrete ion signatures with low energies are clearly seen during the oscillations of ion flux enhancements. Similar to the first type, the ExB velocity analyses show that the ion composition could be H+, (He+,) and O+. The periodic appearances of these discrete ion signatures are well correlated with ion densities calculated from the plasma data, which means that a significant amount of cold ions are hidden from general ion measurements in the previous missions.
We propose that one of the most important subjects in future magnetospheric missions is to precisely determine the cold ion densities of typical ion species.