Perpendicular transport in shock acceleration
Perpendicular transport in shock acceleration
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DOI：
10.1029/96ja00394
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发表时间：
1996-05
影响因子：
--
通讯作者：
J. Giacalone;J. Jokipii
中科院分区：
文献类型：
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作者：
J. Giacalone;J. Jokipii
我们考虑带电粒子在无碰撞冲击中加速的基础，其中平均磁场垂直于冲击法线方向。我们的冲击模型，这是一个数值积分的轨迹合奏的测试粒子在指定的冲击场，包括一个各向同性和均匀的，Kolmogorov样的，波动的磁场叠加在背景场。场是完全三维的，使得横向扩散是可能的。此外，散射是粒子与场相互作用的自然结果，并且不以特别的方式处理。我们考虑两个明显不同的制度的波动分量的字段：（1）在其中的场线混合（或场线随机游走），由于大规模的波动的随机分量的字段，占主导地位的垂直扩散系数和（2）在其中的大规模变化的字段被删除，垂直扩散系数较小。在第一种情况下，横向扩散系数大于第二种情况，这是由于场线混合导致场线环在几个地方与激波相交，并且当场线对流通过激波时，颗粒可以暂时被捕获。在后一种情况下，环在空间尺度上比粒子回旋半径短，粒子保持在激波附近的唯一方法是通过“共振”横向扩散。以前的数值模型和扩散理论之间的比较将被讨论，以及这些想法的重要性，天体物理学的应用，如粒子注入异常宇宙射线能量在太阳风终止冲击。从这些模拟的结果表明，简单的散射过的对流磁波动不能很容易地占非加速拾取离子的加速。
We consider the foundations of charged-particle acceleration at collisionless shocks in which the average magnetic field is perpendicular to the shock-normal direction. Our shock model, which is a numerical integration of the trajectories of an ensemble of test particles in specified shock fields, includes an isotropic and homogeneous, Kolmogorov-like, fluctuating magnetic field superimposed on a background field. The field is fully three-dimensional so that transverse diffusion is possible. Moreover, the scattering is a natural consequence of the particle interaction with the fields and is not treated in an ad hoc manner. We consider two distinctly different regimes for the fluctuating component of the field: (1) one in which the field line mixing (or field line random walk), due to large-scale fluctuations in the random component of the field, dominates the perpendicular diffusion coefficient and (2) one in which the the large-scale variations in the field are removed and the perpendicular diffusion coefficient is smaller. In the first case, the transverse diffusion coefficient is larger than in the second case owing to the field line mixing which leads to field line loops which intersect the shock in several places and particles can become temporarily trapped as the field line convects through the shock. In the latter case, the loops are on a spatial scale that is shorter than the particle gyroradii, and the only means for particles to remain near the shock is by “resonant” transverse diffusion. Comparisons between previous numerical models and the diffusive theory will be discussed as well as the importance of these ideas to astrophysical applications such as particle injection to anomalous cosmic ray energies at the solar wind termination shock. The results from these simulations suggest that simple scattering off of convected magnetic fluctuations cannot readily account for the acceleration of nonaccelerated pickup ions.