Reflection-driven magnetohydrodynamic turbulence in the solar atmosphere and solar wind
Reflection-driven magnetohydrodynamic turbulence in the solar atmosphere and solar wind
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DOI：
10.1017/s0022377819000540
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发表时间：
2019-08-01
影响因子：
2.5
通讯作者：
Perez, Jean C.
中科院分区：
文献类型：
--
作者：
Chandran, Benjamin D. G.;Perez, Jean C.
我们提出了三维直接数值模拟和反射驱动的磁流体动力学（MHD）在太阳风湍流的分析模型。我们的模拟描述了横向，非压缩MHD波动内的一个狭窄的磁通量管，从光球，通过色球层和日冕，并在21个太阳半径（R-圆点）的日心距离r。我们通过施加一个随机演变的光球速度场，将向外传播的“z（+）波动”发射到模拟域中。当这些波动远离太阳传播时，它们经历部分反射，产生向内传播的“z（-）波动”。反向传播的波动随后相互作用，导致波动能量级联到小尺度并消散。我们的分析模型采用动态对齐，允许强或弱湍流非线性相互作用，并将z（+）波动分为两个群体具有不同的特征径向相关长度。在我们的模拟中，z（+）和z（-）波动的惯性范围功率谱在r > 10 R（圆点）处朝着k（垂直于）（-3/2）标度演化，其中k（垂直于）是垂直于背景磁场的波矢量分量。在我们的两个模拟中，z（+）功率谱在日冕底部和r之间平坦得多，r类似于或等于4 R（圆点）。我们认为这些光谱标度是由以下原因引起的：（i）上层色球层的高通滤波，（ii）惯性范围z（-）涨落在参考系中的反常相干性随z（+）涨落向外传播;以及（iii）在r = r（m）处阿尔芬速度的径向导数的符号的变化类似于或等于1.7R（圆点），这破坏了r = r（m）和r类似于或等于2 r（m）之间的这种异常相干性。在r > 1.3R（圆点）时，我们模拟中的湍流加热率与先前开发的太阳风模型中的湍流加热率相当，该模型符合许多观测约束，与MHD湍流占快速太阳风加热的大部分的假设一致。
We present three-dimensional direct numerical simulations and an analytic model of reflection-driven magnetohydrodynamic (MHD) turbulence in the solar wind. Our simulations describe transverse, non-compressive MHD fluctuations within a narrow magnetic flux tube that extends from the photosphere, through the chromosphere and corona and out to a heliocentric distance r of 21 solar radii (R-circle dot). We launch outward-propagating 'z(+) fluctuations' into the simulation domain by imposing a randomly evolving photospheric velocity field. As these fluctuations propagate away from the Sun, they undergo partial reflection, producing inward-propagating 'z(-) fluctuations'. Counter-propagating fluctuations subsequently interact, causing fluctuation energy to cascade to small scales and dissipate. Our analytic model incorporates dynamic alignment, allows for strongly or weakly turbulent nonlinear interactions and divides the z(+) fluctuations into two populations with different characteristic radial correlation lengths. The inertial-range power spectra of z(+) and z(-) fluctuations in our simulations evolve toward a k(perpendicular to)(-3/2) scaling at r > 10R(circle dot), where k(perpendicular to) is the wave-vector component perpendicular to the background magnetic field. In two of our simulations, the z(+) power spectra are much flatter between the coronal base and r similar or equal to 4R(circle dot). We argue that these spectral scalings are caused by: (i) high-pass filtering in the upper chromosphere; (ii) the anomalous coherence of inertial-range z(-) fluctuations in a reference frame propagating outwards with the z(+) fluctuations; and (iii) the change in the sign of the radial derivative of the Alfven speed at r = r(m) similar or equal to 1.7R(circle dot), which disrupts this anomalous coherence between r = r(m) and r similar or equal to 2r(m). At r > 1.3R(circle dot), the turbulent heating rate in our simulations is comparable to the turbulent heating rate in a previously developed solar-wind model that agreed with a number of observational constraints, consistent with the hypothesis that MHD turbulence accounts for much of the heating of the fast solar wind.