Oxygen ion acceleration and transport in the near-Earth plasma sheet during an isolated substorm
Oxygen ion acceleration and transport in the near-Earth plasma sheet during an isolated substorm
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发表时间：
2014-04
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通讯作者：
Y. Nakayama;Y. Ebihara;Takashi Tanaka
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作者：
Y. Nakayama;Y. Ebihara;Takashi Tanaka
亚暴期间高能离子的快速增强是内磁层研究中尚未解决的问题之一。以前，有两个不同的过程被提出来解释增强。一个是来自近地等离子体片的输运，另一个是局部加速。为了测试这两个过程，我们在电场和磁场下进行了测试粒子模拟，这些电场和磁场是由Tanaka等人开发的全球MHD模拟自洽获得的。（2010，JGR）。氧离子以1分钟的间隔在叶区释放。波瓣中的分布函数被假定为漂移麦克斯韦分布。温度假定为20 eV，密度为105 cm-3，平行速度由MHD模拟给出。总共有数亿个粒子被追踪。根据刘维尔定理，每个测试粒子携带粒子的真实的数量。追踪粒子后，我们重建的6维相空间密度的氧离子，以及方向微分数通量，以便能够作出直接比较，与原位卫星观测。亚暴发生后，离子的微分通量迅速增强的能量范围从50到150 keV的径向距离R大于7的夜侧在赤道平面上。由于离子的梯度B和曲率漂移，通量增强区先向暗侧传播，再向向阳侧传播。我们还在一个固定的位置绘制了差分通量的能量-时间谱图，与CRRES卫星观测结果进行了直接比较。在7.2 Re和22.4 MLT下，离子通量在开始后约10分钟突然增强。增强首先出现在120 keV，随后随着时间的推移，较低的能量。能量-时间分散与CRRES观察到的相似[Fu et al.，2002年]。能量-时间色散的陡度取决于离子的源位置。经过一段时间后，首先出现高能离子流，然后是较低能量的离子流。这被称为漂移回波，由通过B级和曲率漂移环绕地球的离子产生。我们将更详细地讨论加速过程，预先存在的离子的作用，以及氧离子的总动能及其对环电流发展的源分布函数的依赖性。
Rapid enhancements of energetic ions during a substorm are one of the unsolved issues in the inner magnetospheric research (<7 Re). Previously, two distinct processes have been suggested to explain the enhancements. The first one is transport from the near-earth plasma sheet, and the other one is local acceleration. To test the both process, we performed test particle simulation under the electric and magnetic fields that are self-consistently obtained by the global MHD simulation developed by Tanaka et al. (2010, JGR). Oxygen ions are released in the lobe region with an interval of 1 minutes. The distribution function in the lobe is assumed to be drifting Maxwellian. The temperature is assumed to be 20 eV, the density is 105 cm-3, and the parallel velocity is given by the MHD simulation. In total, a few hundreds of millions of particles are traced. Each test particle carries the real number of particles in accordance with the Liouville theorem. After tracing particles, we reconstruct 6-dimensional phase space density of the oxygen ions, as well as the directional differential number flux so as to be able to make a direct comparison with in-situ satellite observations. Just after a substorm onset, the differential flux of the ions is rapidly enhanced in the energy range from 50 to 150 keV at radial distance R greater than 7 on the nightside in the equatorial plane. The region of the enhanced flux propagates duskward, then to dayside because of grad-B and curvature drift of the ions. We also plotted energy versus time spectrograms of the differential flux at a fixed position to make a direct comparison with the CRRES satellite observation. At 7.2 Re and at 22.4 MLT, the ion flux is suddenly enhanced about 10 minutes after the onset. The enhancement appears first at 120 keV, followed by lower energy as time proceeds. The energy-time dispersion is similar to that observed by CRRES [Fu et al., 2002]. The steepness of the energy-time dispersion depends on the source location of the ions. After a while, a high energy ion flux appears first, followed by that at lower energies. This is a called a drift echo, arising from the ions that encircled the Earth by the grad-B and curvature drift. We will discuss the acceleration processes in more detail, the role of pre-existing ions, and the total kinetic energy of the oxygen ions and its dependence on the source distribution function in terms of the ring current development.