Geotail observations of plasma sheet ion composition over 16 years: On variations of average plasma ion mass and O+ triggering substorm model
Geotail observations of plasma sheet ion composition over 16 years: On variations of average plasma ion mass and O+ triggering substorm model
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DOI：
10.1029/2009ja014203
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发表时间：
2009-07
影响因子：
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通讯作者：
M. Nosé;A. Ieda;S. Christon
中科院分区：
文献类型：
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作者：
M. Nosé;A. Ieda;S. Christon
我们利用Geotail航天器上高能粒子和离子组成（EPIC）仪器的超热离子组成光谱仪（STICS）传感器获得的高能（9.4-212.1 keV/e）离子通量数据，研究了等离子体片中离子组成的长期变化。从1992年10月17日开始，EPIC/STICS观测了16年多的时间，覆盖了第22太阳周期的衰退期、第23太阳周期的全部和第24太阳周期的早期。这一前所未有的长期数据集表明：(1)等离子体层中He + /H +和O + /H +的通量比依赖于F10.7指数；(2) F10.7指数对O + /H +的依赖性强于He + /H +；(3) 0 + /H +通量比与∑Kp指数也呈弱相关；(4)等离子体层中He 2+ /H +通量比没有长期变化趋势。根据这些结果，我们推导出了与等离子体薄片离子组成和F10.7指数相关的经验方程，并估计出太阳极小期等离子体离子的平均质量从~ 1.1 amu变化到太阳极大期的~ 2.8 amu。在这种情况下，太阳极大期的阿尔芬速度降低到太阳极小期的60%。因此，等离子体层的物理过程被认为在太阳极小期和太阳极大期之间有很大的不同。我们还比较了等离子体片离子组成的长期变化与亚暴发生率的长期变化，这是通过Pi2脉冲的数量来评估的。两者之间无相关或负相关。这一结果与O +触发亚暴模型相矛盾。在该模型中，等离子体片中的重离子增加了线性离子撕裂模式的增长速度，并在亚暴的定位和引发中起着重要作用。相反，等离子体薄片中的O +离子可以防止亚暴的发生。
[1] 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 F10.7 index; (2) the F10.7 index dependence is stronger for O + /H + than He + /H + ; (3) the 0 + /H + flux ratio is also weakly correlated with the ∑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 F10.7 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 Alfven 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.