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Geotail observations of plasma sheet ion composition over 16 years: On variations of average plasma ion mass and O + triggering substorm model
Author(s) -
Nosé M.,
Ieda A.,
Christon S. P.
Publication year - 2009
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2009ja014203
Subject(s) - plasma sheet , substorm , ion , plasma , flux (metallurgy) , solar minimum , solar cycle , solar maximum , physics , solar cycle 22 , atomic physics , solar wind , materials science , magnetosphere , nuclear physics , metallurgy , quantum mechanics
We examined long‐term variations of ion composition in the plasma sheet, using energetic (9.4–212.1 keV/ e ) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. EPIC/STICS observations are available from 17 October 1992 for more than 16 years, covering the declining phase of solar cycle 22, all of solar cycle 23, and the early phase of solar cycle 24. This unprecedented long‐term data set revealed that (1) the He + /H + and O + /H + flux ratios in the plasma sheet were dependent on the F 10.7 index; (2) the F 10.7 index dependence is stronger for O + /H + than He + /H + ; (3) the O + /H + flux ratio is also weakly correlated with the Σ Kp index; and (4) the He 2+ /H + flux ratio in the plasma sheet appeared to show no long‐term trend. From these results, we derived empirical equations related to plasma sheet ion composition and the F 10.7 index and estimated that the average plasma ion mass changes from ∼1.1 amu during solar minimum to ∼2.8 amu during solar maximum. In such a case, the Alfvén velocity during solar maximum decreases to ∼60% of the solar minimum value. Thus, physical processes in the plasma sheet are considered to be much different between solar minimum and solar maximum. We also compared long‐term variation of the plasma sheet ion composition with that of the substorm occurrence rate, which is evaluated by the number of Pi2 pulsations. No correlation or negative correlation was found between them. This result contradicts the O + triggering substorm model, in which heavy ions in the plasma sheet increase the growth rate of the linear ion tearing mode and play an important role in localization and initiation of substorms. In contrast, O + ions in the plasma sheet may prevent occurrence of substorms.

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