A particle simulation of the westward‐traveling surge
A particle simulation of the westward‐traveling surge
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西行涌流的粒子模拟
DOI：
10.1029/93ja00641
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发表时间：
1993
影响因子：
--
通讯作者：
C. Meng
中科院分区：
文献类型：
--
作者：
Takashi Yamamoto;K. Makita;C. Meng
数值模拟结果表明，在开放场线和闭合场线交界处出现向西的等离子体流，可以将西移涌浪的演化与极光形变联系起来。假设西向流是由磁场重联区域的非绝热质子加速通过增强西向感应电场产生的。利用二维静电粒子程序进行了数值模拟，该程序能够研究垂直于地磁场的平面内的等离子体动力学。本文提出的西移涌浪模型假定在与高纬度极光电离层磁连接的等离子体片边界层中有如下背景状态(1)和(2)。(1)势垒(Δϕ<；−10kV)在方位向均匀分布，形成大规模的倒V结构。(2)在大尺度势井中存在大量方位向排列的致密等离子体片(称为弧片)。这种电弧片中的电子可以被分布在低海拔的场向电场加速，因此它们可以在电离层高度产生一些纵向拉长的离散弧线。最初，一股向西的等离子体流出现在最后一个闭合的磁壳上(接近当地午夜)，在那里有许多弧片。在垂直于地磁场的平面上，数值跟踪离散弧线的时间和空间演化。模拟结果表明，一些离散的弧线可以在几分钟内卷曲，形成浪涌结构。结果表明，激波水头以∼1公里/S的速度向西移动，离散弧的卷积是由于加速质子携带的空间正电荷的局部积累所致。这种正电荷在磁层中的积累不是很强，因此极光电子仍然可以通过势垒产生的场向势降向电离层加速。场向势降的减小和重联区电子加速的共同作用可能会导致在浪涌中观察到的沉淀电子的平坦和硬微分能谱的产生。数值模拟的涌浪与观测到的涌浪在动力学上的显著相似性强烈地表明，西行涌浪是由开放场线和闭合场线交界处的局部西向等离子体流驱动的。(最近，来自EXOS-D卫星的高空(∼10,000公里)观测(T.Yamamoto等人，手稿正在准备中，1993年)首次揭示了一种等离子体喷流的存在，就在一次激增的极地。)通过假设粒子在重新连接过程中加速的更大范围的扩散(在纬度上)，也可以再现极光膨胀。
By numerical simulations, it is shown that the evolution of a westward-traveling surge can be identified with the aurora deformation due to the appearance of a westward plasma flow at the boundary between the open and closed field lines. It is assumed that the westward flow is generated from nonadiabatic proton acceleration in the region of magnetic field reconnection by enhancement of the westward inductive electric field. The numerical simulation is performed by using the two-dimensional electrostatic particle code, which is capable of studying plasma dynamics in a plane perpendicular to the geomagnetic field. The present model for the westward-traveling surge assumes the following background state ((1) and (2)) in the plasma sheet boundary layer magnetically connected to the high-latitude auroral ionosphere. (1) A well potential (Δϕ < −10 kV) is distributed uniformly in the azimuthal direction, being responsible for a large-scale inverted-V structure. (2) A number of azimuthally aligned sheets of dense plasmas (which are called arc sheets) exist in the large-scale potential well. The electrons in such arc sheets can be accelerated by field-aligned electric fields distributed at low altitudes, and they can thus produce a number of discrete arcs longitudinally elongating at the ionospheric altitude. Initially, a westward plasma flow appears (near local midnight) at the last closed magnetic shell, inside which a number of the arc sheets are located. The temporal and spatial evolution of the discrete arcs are numerically followed in a plane perpendicular to the geomagnetic field. The simulation results show that some discrete arcs can convolute in a few minutes and form the surge structure. The surge head is found to typically move westward with a speed of ∼ 1 km/s. Convolution of discrete arcs is due to local accumulation of positive space charges carried by the accelerated protons. Such positive charge accumulation in the magnetosphere is not strong so that the auroral electrons can still be accelerated toward the ionosphere by the field-aligned potential drop resulting from the potential well. Both effects of the diminished field-aligned potential drop and electron acceleration in the reconnection region may lead to production of the flat and hard differential energy spectrum of precipitating electrons observed inside a surge. A remarkable similarity in dynamics between the numerically simulated surges and the observed ones strongly suggests that the westward-traveling surge is driven by a local westward plasma flow at the boundary between the open and closed field lines. (Recently, the high-altitude (∼ 10,000 km) observations from the EXOS-D satellite (T. Yamamoto et al., manuscript in preparation, 1993) have first revealed the existence of a plasma jet just poleward of a surge.) An expanding auroral bulge can also be reproduced by assuming wider (in latitude) spreading of the particles accelerated in the reconnection process.