Solution to the cosmic ray anisotropy problem.
Solution to the cosmic ray anisotropy problem.
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DOI：
10.1103/physrevlett.114.021101
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发表时间：
2014-08
影响因子：
8.6
通讯作者：
P. Mertsch;S. Funk
中科院分区：
文献类型：
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作者：
P. Mertsch;S. Funk
在标准的宇宙线传输扩散图中，宇宙线密度的梯度在其到达方向上引起典型的小的偶极各向异性。这被广泛宣传为寻找附近来源的工具。然而，在TeV和PeV能量下，预测的偶极振幅比实测值高出近两个数量级。在这里，我们批判性地检验这一预测的有效性，该预测基于湍流磁场系综上的平均。我们主要关注(1)在特定随机实现中偶极子与系综平均数的偏差，以及(2)规则磁场和CR梯度之间不对准的可能性。我们发现，如果场方向和梯度方向接近∼90°，偶极子振幅就会受到相当大的抑制，并且可以与观测结果相一致，这揭示了一个长期存在的问题。此外，我们还发现偶极子方向通常与梯度方向不重合，从而阻碍了对附近震源的搜索。
In the standard diffusive picture for transport of cosmic rays (CRs), a gradient in the CR density induces a typically small, dipolar anisotropy in their arrival directions. This is being widely advertised as a tool for finding nearby sources. However, the predicted dipole amplitude at TeV and PeV energies exceeds the measured one by almost 2 orders of magnitude. Here, we critically examine the validity of this prediction, which is based on averaging over an ensemble of turbulent magnetic fields. We focus on (1) the deviations of the dipole in a particular random realization from the ensemble average, and (2) the possibility of a misalignment between the regular magnetic field and the CR gradient. We find that if the field direction and the gradient direction are close to ∼90°, the dipole amplitude is considerably suppressed and can be reconciled with observations, which sheds light on a long-standing problem. Furthermore, we show that the dipole direction in general does not coincide with the gradient direction, thus hampering the search for nearby sources.