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₯Cι€vY}V[gΙ¨―ιαGlM[CIΜ«ΏF GEOTAILq―ϊAf[^πΝ
*Φ Ψήq[1], ½΄ ΉΆ[2], ―μ ^O[1]
ΰV qv[1], όδ T[3], μ ξ[3]
εw[1]
§³εw[2]
FΘw€[3]
Properties of cold ion population in the mid-night plasma sheet observed by GEOTAIL during solar umbra
*Kanako Seki[1]
,Masafumi Hirahara [2],Masahiro Hoshino [1]
Toshio Terasawa [1],Toshifumi Mukai [3]
Hajime Hayakawa [3]
University of Tokyo[1]
Rikkyo University[2]
Institute of Space and Astronautical Science[3]
@A positive spacecraft potential due to photoelectrons in sunlight
makes it difficult to observe ions with energies below a few
tens of eV in the geomagnetosphere. Using the GEOTAIL spacecraft
data during solar umbrae when the photoelectron supply is stopped
and spacecraft potential is expected to become negative, we investigate
existence of cold ion population in the mid-night plasma sheet.
About a quater of umbra events indicates the existence of cold
ions, which is not expected from a well-accepted view that outflowing
ions from polar ionosphere get energized in the PSBL and the
plasma sheet. Possible supply mechanisms of these cold ions will
be also discussed.
@It is well known that a large amount of cold ions typically below a few tens of eV are flowing out from the polar ionosphere as the polar wind. However, it is difficult to investigate the fate of the cold ions in the magnetosphere observationally, since spacecraft in the magnetotail usually become positively charged up to a few tens of eV due to photoelectrons without some potential control methods, and we will miss the ions below the spacecraft potential even if they exist. Trajectory tracings of ions outflowing from the polar ionosphere in empirical magnetospheric models have predicted that these ions get energized upon the entry into the plasma sheet where the gyroradius of ions become comparable to the field line curvature. Thus it is generally considered that cold ionospheric ions can no longer be cold in the night-side plasma sheet and most of them contribute to the hot plasma sheet ions.
@On the contrary, the GEOTAIL spacecraft sometimes observed extremely cold ions in the near-Earth plasma sheet (Xgsm=-8 to -18 Re), when it entered in the solar umbra. In the solar umbra, termination of photoelectron supply makes spacecraft negatively charged and ambient cold ions become detectable attracted by the negative spacecraft potential. Among all solar umbra events in 1995-1998, about a quarter of the events showed this kind of cold ion acceleration. In this study, we will investigate properties of these cold ions in the mid-night plasma sheet in order to understand why these ions can exist in the plasma sheet without getting energized.