非磁化情况下通过动力学Kelvin-Helmholtz不稳定性产生大尺度磁场
Large-scale magnetic field generation via the kinetic Kelvin-Helmholtz instability in unmagnetized scenarios
摘要：
无碰撞等离子体不稳定性是天体物理学场景中产生磁场的基础，但它们的作用在不存在速度剪切的场景中得到了解决。在这项工作中，我们表明，在研究现实的天体物理场景时，必须考虑速度剪切，因为这些触发无碰撞开尔文-亥姆霍兹不稳定性（KHI）。我们提出了第一个自洽的三维（3D）的粒子在细胞（PIC）模拟的KHI在相关的条件下，非磁化的相对论外流与速度剪切，如活动星系核（AGN）和伽马射线暴（GRB）。我们展示了一个强大的大规模直流磁场的产生，它延伸到整个剪切表面，达到几十个电子集肤深度的厚度，并坚持时间尺度远长于电子的时间尺度。这种直流磁场不被MHD模型捕获，因为它是由固有的动力学效应引起的。结果表明，KHI能产生强磁场，在电子时间尺度上的均分值可达ε_B/ε_p ~ 10^-3。KHI诱导的磁场具有导致明显辐射特征的特征结构，并且可以播种湍流发电机放大过程。KHI的动力学与非热辐射模型有关，并且在存在速度剪切的情况下，也会对相对论激波的形成产生强烈影响。
英文摘要：
Collisionless plasma instabilities are fundamental in magnetic field generation in astrophysical scenarios, but their role has been addressed in scenarios where velocity shear is absent. In this work we show that velocity shears must be considered when studying realistic astrophysical scenarios, since these trigger the collisionless Kelvin-Helmholtz instability (KHI). We present the first self-consistent three-dimensional (3D) particle-in-cell (PIC) simulations of the KHI in conditions relevant for unmagnetized relativistic outflows with velocity shear, such as active galactic nuclei (AGN) and gamma-ray bursts (GRBs). We show the generation of a strong large-scale DC magnetic field, which extends over the entire shear-surface, reaching thicknesses of a few tens of electron skin depths, and persisting on time-scales much longer than the electron time scale. This DC magnetic field is not captured by MHD models since it arises from intrinsically kinetic effects. Our results indicate that the KHI can generate intense magnetic fields yielding equipartition values up to ε_B/ε_p ~ 10^-3 in the electron time-scale. The KHI-induced magnetic fields have a characteristic structure that will lead to a distinct radiation signature, and can seed the turbulent dynamo amplification process. The dynamics of the KHI are relevant for non-thermal radiation modeling and can also have a strong impact on the formation of relativistic shocks in presence of velocity shears.
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