Charged dust in planetary magnetospheres: Hamiltonian dynamics and numerical simulations for highly charged grains
Charged dust in planetary magnetospheres: Hamiltonian dynamics and numerical simulations for highly charged grains
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DOI：
10.1029/94ja01231
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发表时间：
1994-09
影响因子：
--
通讯作者：
L. Schaffer;J. Burns
中科院分区：
文献类型：
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作者：
L. Schaffer;J. Burns
我们使用分析和数值计算相结合的方法来研究行星磁层中带电尘埃颗粒的动力学。我们的重点是获得有效的粒子，不一定占主导地位的引力或电磁力的结果。当我们引入两个约束条件：粒子保持在赤道平面内，磁场被视为轴对称时，对于荷质比的所有值，问题的哈密顿公式都会产生精确的结果。特别是，我们得到的平衡点的位置，稳定的周期轨道的频率，在相空间中的分离面的拓扑结构，和纵向漂移率。这些结果对于特定的应用是重要的：在土星的偶极磁场中几乎对齐的运动，以及在从母体喷射后的有限时间内在复杂磁场中任意带电粒子的轨迹。由于该模型是限制性的，我们还使用全三维运动方程的数值积分，并说明在什么条件下的约束问题产生合理的结果。我们表明，大部分的中等电荷和高电荷（旋转）粒子将总是在几百个小时内丢失到行星的大气中，通过倾斜偶极磁场运动。我们发现，晶粒必须有一个非常高的电荷质量比，以反映回环平面。因此，除了偶极倾斜非常小的土星外，尘埃环系统的可能居民是那些近似开普勒（弱电荷）颗粒或其电荷位于中间电荷区下端的颗粒。最后，我们证明了等离子体阻力的影响的回旋粒子的轨道是一个快速减少的gyroradius其次是一个缓慢的径向演变的指导中心。
We use a combination of analytical and numerical methods to investigate the dynamics of charged dust grains in planetary magnetospheres. Our emphasis is on obtaining results valid for particles that are not necessarily dominated either by gravitational or electromagnetic forces. A Hamiltonian formulation of the problem yields exact results, for all values of charge-to-mass ratio, when we introduce two constraints: particles remain in the equatorial plane and the magnetic field is taken as axially symmetric. In particular, we obtain locations of equilibrium points, the frequencies of stable periodic orbits, the topology of separatrices in phase space, and the rate of longitudinal drift. These results are significant for specific applications: motion in the nearly aligned dipolar field of Saturn, and the trajectories of arbitrarily charged particles in complex magnetic fields for limited periods of time after ejection from parent bodies. Since the model is restrictive, we also use numerical integrations of the full three-dimensional equations of motion and illustrate under what conditions the constrained problem yields reasonable results. We show that a large fraction of the intermediately charged and highly charged (gyrating) particles will always be lost to a planet’s atmosphere within a few hundred hours, for motion through tilted-dipole magnetic fields. We find that grains must have a very high charge-to-mass ratio in order to be mirrored back to the ring plane. Thus, except perhaps at Saturn where the dipole tilt is very small, the likely inhabitants of the dusty ring systems are those particles that are either nearly Keplerian (weakly charged) grains or grains whose charges place them in the lower end of the intermediate charge zone. Finally, we demonstrate the effect of plasma drag on the orbits of gyrating particles to be a rapid decrease in gyroradius followed by a slow radial evolution of the guiding center.