The nonlinear evolution of field line resonances in the Earth's magnetosphere
The nonlinear evolution of field line resonances in the Earth's magnetosphere
复制标题
地球磁层中场线共振的非线性演化
DOI：
10.1029/92ja01606
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发表时间：
1993
影响因子：
--
通讯作者：
P. Frycz
中科院分区：
文献类型：
--
作者：
R. Rankin;B. Harrold;J. Samson;P. Frycz
磁流体力学，地球磁层中的场线共振可以有非常大的速度切变和场向电流。对高纬度共振的极光雷达测量表明，与E和F区共振有关的速度往往大大大于1公里/S，频率在1至4兆赫之间。假设这些共振在基模频率上振荡，并将这些速度场沿磁力线映射到赤道平面，结果表明，在径向距离小于2000公里的范围内，赤道平面内的速度切变约为200公里/S(速度起伏的幅度为100公里/S)。利用三维磁流体力学计算机模拟程序，我们证明了共振是通过赤道平面附近开尔文-亥姆霍兹不稳定性的发展而演变的。在这个框架内，不稳定性发生在偶极磁力线上，由于在极地电离层必须满足的边界条件，共振形成了驻切Alfven波场。我们发现Kelvin-Helmholtz不稳定性的非线性演化导致涡度从赤道平面传播到极地电离层，而涡度最终导致共振的耗散。这发生在与共振相关的剪切阿尔芬场的四分之一波周期内。
Magnetohydrodynamic, field line resonances in the Earth's magnetosphere can have very large velocity shears and field-aligned currents. Auroral radar measurements of high-latitude resonances indicate that the velocities associated with the resonances in the E and F regions are often substantially greater than 1 km/s, and that the frequencies are in the interval from 1 to 4 mHz. Assuming that these resonances are oscillating at the fundamental mode frequency, and mapping these velocity fields along magnetic field lines to the equatorial plane shows that the velocity shears in the equatorial plane are of the order of 200 km/s over a radial distance of less than 2000 km (the amplitude of the velocity fluctuations is 100 km/s). Using a three-dimensional magnetohydrodynamic computer simulation code, we show that the resonances evolve through the development of Kelvin-Helmholtz instabilities near the equatorial plane. Within this framework, the instability is taking place on dipole magnetic field lines, and the resonances form a standing shear Alfven wave field due to the boundary conditions which must be satisfied at the polar ionospheres. We find that the nonlinear evolution of the Kelvin-Helmholtz instability leads to the propagation of vorticity from the equatorial plane to the polar ionosphere and that the vorticity leads ultimately to the dissipation of the resonance. This occurs within a quarter wave period of the shear Alfven field associated with the resonances.