Zirnstein E J, Heerikhuisen J, Dayeh M A
Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA.
Department of Space Science, University of Alabama in Huntsville, Huntsville, AL 35899, USA.
Astrophys J. 2018;855(No 1). doi: 10.3847/1538-4357/aaaf6d. Epub 2018 Mar 5.
We present a new model of the () ribbon based on the secondary energetic neutral atom (ENA) mechanism, under the assumption that there is negligible pitch angle scattering of pickup ions (PUIs) outside the heliopause. Using the results of an MHD-plasma/kinetic-neutral simulation of the heliosphere, we generate PUIs in the outer heliosheath, solve their transport using guiding center theory, and compute ribbon ENA fluxes at 1 AU. We implement several aspects of the PUI dynamics, including (1) parallel motion along the local interstellar magnetic field (ISMF), (2) advective transport with the interstellar plasma, (3) the mirror force acting on PUIs propagating along the ISMF, and (4) betatron acceleration of PUIs as they are advected within an increasing magnetic field towards the heliopause. We find that ENA fluxes at 1 AU are reduced when PUIs are allowed to move along the ISMF, and ENA fluxes are reduced even more by the inclusion of the mirror force, which pushes particles away from lines-of-sight. Inclusion of advection and betatron acceleration do not result in any significant change in the ribbon. Interestingly, the mirror force reduces the ENA fluxes from the inner edge of the ribbon more than its outer edge, effectively reducing the ribbon's width by ∼6° and increasing its radius projected on the sky. This is caused by the asymmetric draping of the ISMF around the heliopause, such that ENAs from the ribbon's inner edge originate closer to the heliopause, where the mirror force is strongest.
我们基于次级高能中性原子（ENA）机制提出了一种新的（）带模型，假设在日球层顶之外拾取离子（PUIs）的俯仰角散射可忽略不计。利用日球层的磁流体动力学 - 等离子体/动力学 - 中性模拟结果，我们在外日鞘层中生成PUIs，使用引导中心理论求解它们的输运，并计算1天文单位处的带ENA通量。我们考虑了PUIs动力学的几个方面，包括（1）沿局部星际磁场（ISMF）的平行运动，（2）与星际等离子体的平流输运，（3）作用在沿ISMF传播的PUIs上的镜像力，以及（4）当PUIs在朝着日球层顶增加的磁场中被平流时的贝塔加速。我们发现，当允许PUIs沿ISMF移动时，1天文单位处的ENA通量会降低，并且由于包含了将粒子推离视线方向的镜像力，ENA通量会进一步降低。包含平流和贝塔加速不会导致带出现任何显著变化。有趣的是，镜像力使带内边缘的ENA通量比外边缘降低得更多，有效地将带的宽度减小了约6°，并增加了其在天空上投影的半径。这是由ISMF在日球层顶周围的不对称 draping 引起的，使得来自带内边缘的ENAs起源于更靠近日球层顶的地方，而那里镜像力最强。
