Three-dimensional Hybrid Simulation Results of a Variable Magnetic Helicity Signature at Proton Kinetic Scales
Three-dimensional Hybrid Simulation Results of a Variable Magnetic Helicity Signature at Proton Kinetic Scales
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DOI：
10.3847/1538-4357/ac3bbc
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发表时间：
2022-01
期刊：
影响因子：
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通讯作者：
B. Vasquez;S. Markovskii;Charles W. Smith
中科院分区：
文献类型：
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作者：
B. Vasquez;S. Markovskii;Charles W. Smith
对粒子质子和热流体电子进行了三维混合动力学模拟。在大尺度上初始的AlfvéNic涨落与背景磁场具有高度倾斜的波矢，演化为在质子动力学尺度上耗散的湍流能量级联。伴随着质子刻度的是一个带有峰值的光谱磁螺旋度特征。在不同的大尺度交叉螺旋度、不同的初始起伏相位和波矢配置下进行了一系列模拟运行。从模拟中，通过对垂直于背景磁场的波数的贡献相加来评估所谓的总磁螺旋度峰值。然后将总和与通过模拟箱沿着航天器般的轨迹计算的折合磁螺旋度进行比较。折合波数结合了不同垂直波数的螺旋度，并与采样方向有关。因此，总和是表征湍流的较好物理量。平均而言，减少与总的比率为0.45。总磁螺旋度和折合磁螺旋度在初始涨落条件下表现出内在的变异性。这种可变性有助于在观测到的太阳风分布中发现的散射，降低了磁螺旋度作为交叉螺旋度的函数。
Three-dimensional hybrid kinetic simulations are conducted with particle protons and warm fluid electrons. Alfvénic fluctuations initialized at large scales and with wavevectors that are highly oblique with respect to the background magnetic field evolve into a turbulent energy cascade that dissipates at proton kinetic scales. Accompanying the proton scales is a spectral magnetic helicity signature with a peak in magnitude. A series of simulation runs are made with different large-scale cross helicity and different initial fluctuation phases and wavevector configurations. From the simulations a so-called total magnetic helicity peak is evaluated by summing contributions at a wavenumber perpendicular to the background magnetic field. The total is then compared with the reduced magnetic helicity calculated along spacecraft-like trajectories through the simulation box. The reduced combines the helicity from different perpendicular wavenumbers and depends on the sampling direction. The total is then the better physical quantity to characterize the turbulence. On average the ratio of reduced to total is 0.45. The total magnetic helicity and the reduced magnetic helicity show intrinsic variability based on initial fluctuation conditions. This variability can contribute to the scatter found in the observed distribution of solar wind reduced magnetic helicity as a function of cross helicity.