Tamao travel time of sudden impulses and its relationship to ionospheric convection vortices
Tamao travel time of sudden impulses and its relationship to ionospheric convection vortices
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DOI：
10.1029/2005ja011578
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发表时间：
2006-08
影响因子：
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通讯作者：
P. Chi;Dong‐Hun Lee;C. Russell
中科院分区：
文献类型：
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作者：
P. Chi;Dong‐Hun Lee;C. Russell
[1]我们用MHD数值模拟研究了突然脉冲(SI)在三维磁层中的传播，以评估Tamao旅行时近似的精度和初始脉冲(PI)的动力学性质。结果表明，SI信号的传播受非均匀Alfven速度剖面的影响很大，在等离子体层外的高Alfven速度区可以使信号在最短的时间内到达电离层高度。在电离层高度，由波幅的第一个峰值定义的PI的模拟到达时间在与外磁层相连的不变纬度范围内与Tamao的预测非常一致。然而，到达等离子体层内固定纬度的时间比Tamao的预测略早，因为脉冲在到达等离子体顶层时偏离了点源。相反，PI的起始时间对应于磁场的第一个偏转，几乎不随纬度变化，这与公认的几乎同时PI起始的观测结果一致，而不需要引用地球-电离层波导模型。我们的模拟还表明，PI到达时间的差异强烈地影响了电离层对流涡旋的演变。孪生涡旋首先出现在与等离子体顶层相连的纬度略高的地方，由于脉冲信号在较高纬度的延迟到达，它们向极地移动。由于稠密的等离子体层对信号的延迟，另一对涡旋较晚出现在等离子体层纬度。我们的结果不仅提供了对Tamao路径的定量评估，有助于旅行时间磁震学的未来发展，而且通过结合MHD波的传播特征，揭示了PI诱导的电离层对流涡旋的演化。
[1] We have examined the propagation of sudden impulses (SI) by an MHD numerical simulation in the three-dimensional magnetosphere to assess the precision of the Tamao travel time approximation and the dynamic properties of preliminary impulses (PI). The results show that the propagation of SI signals is strongly influenced by the inhomogeneous Alfven velocity profile, and the region of high Alfven velocity immediately outside the plasmasphere allows the signals to reach ionospheric altitudes in the shortest time possible. At the ionospheric altitude, the simulated arrival time of the PI, defined by the first peak in wave amplitude, is in extremely good agreement with Tamao's prediction throughout the range of invariant latitudes connected to the outer magnetosphere. The arrival time at invariant latitudes within the plasmasphere, however, is slightly earlier than Tamao's prediction because of the fact that the impulse deviates from a point source as it arrives at the plasmapause. In contrast, the onset time of the PI, which corresponds to the first deflection in the magnetic field, varies little with invariant latitude, consistent with the well-established observations of the nearly simultaneous PI onset without the need to invoke the Earth-ionosphere waveguide model. Our simulation also shows that the differentiation in PI arrival time strongly influences the evolution of the ionospheric convection vortices. The twin vortices first appear slightly above the latitude connected to the plasmapause, and they move poleward because of the late arrival of impulse signals at higher latitudes. Another pair of vortices appears later at plasmaspheric latitudes owing to the delay of signals by the dense plasmasphere. Our results not only provide quantitative assessment on the Tamao path, which can benefit future development in travel time magnetoseismology, but they also demonstrate the evolution of PI-induced ionospheric convection vortices by incorporating the propagation characteristics of MHD waves.