Radial evolution of the solar wind in pure high-speed streams: HELIOS revised observations
Radial evolution of the solar wind in pure high-speed streams: HELIOS revised observations
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DOI：
10.1093/mnras/sty3348
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发表时间：
2018-10
影响因子：
4.8
通讯作者：
D. Perrone;D. Stansby;T. Horbury;L. Matteini
中科院分区：
文献类型：
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作者：
D. Perrone;D. Stansby;T. Horbury;L. Matteini
航天器的观测表明，太阳风中的质子温度随着径向距离的下降比绝热预测的预期要慢得多。通常，以前的研究主要是通过使用体积速度作为序参数来区分不同的区域来研究太阳风等离子体的演化。相反，在这里，我们通过重新处理来自0.3到1AU之间的HELIOS卫星的粒子数据，研究了纯净和均匀的快流(即在几次太阳自转中保持相同的明确的冕洞等离子体流)的径向演化。我们已经确定了来自三个不同来源的16个未受扰动的高速冕洞等离子体的间隔，并在不同的径向距离上进行了测量。观测表明，对于所有三个流，(I)对于径向膨胀的等离子体，质子密度如预期的那样减少，而不像以前的分析发现的那样下降得更慢；(Ii)磁场偏离了Parker预测，径向和切向分量的下降速度分别比预期的要慢和快；(Iii)双绝热不变量被违反，并且观测到了熵的增加；(Iv)质子核温度的各向异性受到镜像模不稳定性的约束；(V)碰撞频率不是恒定的，而是随着等离子体离开太阳而降低。目前的工作提供了对纯快速太阳风中加热问题的洞察，适合于下一次太阳任务，特别是对于帕克太阳探测器，它使我们能够预测离太阳更近的高速太阳风环境。
Spacecraft observations have shown that the proton temperature in the solar wind falls off with radial distance more slowly than expected for an adiabatic prediction. Usually, previous studies have been focused on the evolution of the solar-wind plasma by using the bulk speed as an order parameter to discriminate different regimes. In contrast, here, we study the radial evolution of pure and homogeneous fast streams (i.e. well-defined streams of coronal-hole plasma that maintain their identity during several solar rotations) by means of re-processed particle data, from the HELIOS satellites between 0.3 and 1 AU. We have identified 16 intervals of unperturbed high-speed coronal hole plasma, from three different sources and measured at different radial distances. The observations show that, for all three streams, (i) the proton density decreases as expected for a radially expanding plasma, unlike previous analysis that found a slower decrease; (ii) the magnetic field deviates from the Parker prediction, with the radial and tangential components decreasing more slowly and quickly than expected, respectively; (iii) the double-adiabatic invariants are violated and an increase of entropy is observed; (iv) the proton-core temperature anisotropy is constrained by mirror mode instability; (v) the collisional frequency is not constant, but decreases as the plasma travels away from the Sun. The present work provides an insight into the heating problem in pure fast solar wind, fitting in the context of the next solar missions, and, especially for Parker Solar Probe, it enables us to predict the high-speed solar-wind environment much closer to the Sun.