Turbulent dynamo in a collisionless plasma
Turbulent dynamo in a collisionless plasma
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DOI：
10.1073/pnas.1525194113
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发表时间：
2015-12
期刊：
影响因子：
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通讯作者：
F. Rincon;F. Califano;A. Schekochihin;F. Valentini
中科院分区：
文献类型：
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作者：
F. Rincon;F. Califano;A. Schekochihin;F. Valentini
意义虽然磁场放大的发电机效应转换动能流能转化为磁能早已显示在传统的磁流体力学流体，是否类似的效果是可能的，在更动态复杂的弱碰撞等离子体，如那些遇到的天体在河外尺度，是未知的。我们提出了第一个，据我们所知，确凿的数值证据和动力学图像的磁场放大的混沌运动在无碰撞等离子体。结果表明，这样的等离子发电机可能是一个可实现的物理效应在“实验室天体物理学”的实验和支持的想法，湍流发电机可能显着有助于磁化弱碰撞高能量密度天体物理等离子体，如星系团内介质。磁场遍布整个宇宙，影响着从宇宙到行星尺度的天体物理系统的形成和演化。银河系外磁场在宇宙时间内的产生和动力学放大（在附近的星系团中报道达到微高斯水平，与等离子体运动的动能接近均分，并且在至少数十千秒差距的尺度上）是主要的难题，在很大程度上不受观测的约束。在这种情况下，经常会引用发电机效应将动能转化为磁能;然而，河外等离子体是弱碰撞的（与磁流体动力学流体相反），并且在这种等离子体中是否可能通过有效的湍流发电机不稳定性来实现磁场增长和维持尚不确定。完全动力学数值模拟的弗拉索夫方程在一个6D相空间需要回答这个问题，直到最近，仍然超出了计算能力。在这里，我们表明，通过这样的模拟，磁场放大发电机不稳定性发生在一个随机驱动的，非相对论性的亚音速流最初未磁化的无碰撞等离子体。我们还发现，发电机自加速，并成为纠缠与动力学不稳定性的磁化强度增加。结果表明，这样的等离子体发电机可能在实验室实验中实现，支持的想法，即集群内介质湍流可能显着有助于放大集群磁场近均分水平的时间尺度短于哈勃时间，并强调多尺度动力学物理学在高能天体物理等离子体的关键作用。
Significance Although magnetic field amplification by a dynamo effect converting kinetic flow energy into magnetic energy has long been shown in conventional magnetohydrodynamic fluids, whether a similar effect is possible in more dynamically complex weakly collisional plasmas, such as those encountered in astrophysical objects on extragalactic scales, is not known. We present the first, to our knowledge, conclusive numerical evidence and dynamical picture of magnetic field amplification by chaotic motions in a collisionless plasma. The results suggest that such a plasma dynamo may be a realizable physical effect in “laboratory astrophysics” experiments and support the idea that turbulent dynamos may significantly contribute to the magnetization of weakly collisional high-energy density astrophysical plasmas, such as the intracluster medium of galaxy clusters. Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas.