Magnetic fluctuations embedded in dipolarization inside geosynchronous orbit and their associated selective acceleration of O+ ions
Magnetic fluctuations embedded in dipolarization inside geosynchronous orbit and their associated selective acceleration of O+ ions
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DOI：
10.1002/2014ja019806
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发表时间：
2014-06
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通讯作者：
M. Nosé;K. Takahashi;K. Keika;L. Kistler;K. Koga;H. Koshiishi;H. Matsumoto;M. Shoji;Y. Miyashit
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作者：
M. Nosé;K. Takahashi;K. Keika;L. Kistler;K. Koga;H. Koshiishi;H. Matsumoto;M. Shoji;Y. Miyashit
我们使用工程试验卫星VIII和主动磁层粒子示踪剂探测器/CCE卫星研究了内磁层（地心距离≤6.6 RE）中偶极中嵌入的磁波动及其相关的离子通量变化。我们选取了7个在磁暴主相期间发生的Dst指数最小值小于-40 nT的偶极化事件。结果表明：（1）所有的双极化事件都伴随着强的磁涨落，其主频率接近于局域O+离子的回旋频率;（2）磁涨落在与局域磁场近似平行的分量中出现显著的振幅;（3）仅在1-10 keV能量范围内观察到O+离子的强通量增强。就磁涨落的频率和主导分量而言，它们被认为是由最近用线性理论确定的漂移驱动的电磁离子回旋（EMIC）不稳定性激发的。我们在由漂移驱动的EMIC不稳定性的线性色散关系建模的电磁场中对H+和O+离子进行粒子追踪。结果表明，O+离子被加速到0.5 ~ 5 keV能量范围后，谱形发生了明显的变化，而H+离子的谱形没有明显变化，与观测结果一致。因此，我们认为，与偶极相关的电磁涨落可以加速O+离子的本地和nonachicatically在内磁层。O+离子的这种选择性加速可能在增强暴时环电流中的O+能量密度中起作用。
We study magnetic fluctuations embedded in dipolarizations in the inner magnetosphere (a geocentric distance of ≤6.6 RE) and their associated ion flux changes, using the Engineering Test Satellite VIII and Active Magnetospheric Particle Tracer Explorers/CCE satellites. We select seven events of dipolarization that occur during the main phase of magnetic storms having a minimum value of the Dst index less than −40 nT. It is found that (1) all of the dipolarization events are accompanied by strong magnetic fluctuations with the major frequency close to the local O+ gyrofrequency; (2) the magnetic fluctuations appear with significant amplitude in the component nearly parallel to the local magnetic field; (3) the strong flux enhancement is seen in the energy range of 1–10 keV only for O+ ions. In terms of frequency and dominant components of the magnetic fluctuations, they are considered to be excited by the drift‐driven electromagnetic ion cyclotron (EMIC) instability that is recently identified with the linear theory. We perform particle tracing for H+ and O+ ions in the electromagnetic fields modeled by the linear dispersion relation of the drift‐driven EMIC instability. Results show that the O+ ions are accelerated to the energy range of 0.5–5 keV and undergo a significant modification of the spectral shape, while the H+ ions have no clear change of spectral shape, being consistent with the observations. We therefore suggest that the electromagnetic fluctuations associated with the dipolarizations can accelerate O+ ions locally and nonadiabatically in the inner magnetosphere. This selective acceleration of O+ ions may play a role in enhancing the O+ energy density in the storm time ring current.