The evolution of solar wind strahl with heliospheric distance
The evolution of solar wind strahl with heliospheric distance
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DOI：
10.1002/2016ja023656
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发表时间：
2017-04
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影响因子：
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通讯作者：
G. Graham;I. J. Rae;Christopher J. Owen;Andrew P. Walsh;C. Arridge;L. Gilbert;G. Lewis;G. H. Jones;Colin Forsyth;A. J. Coates;J. H. Waite
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作者：
G. Graham;I. J. Rae;Christopher J. Owen;Andrew P. Walsh;C. Arridge;L. Gilbert;G. Lewis;G. H. Jones;Colin Forsyth;A. J. Coates;J. H. Waite
超热电子的场对准束，被称为“strahl”，是太阳风等离子体中经常观察到的成分。然而，strahl电子布居的形成和行星际演化还有待完全理解。当strahl电子远离太阳时，它们会移动到磁场强度降低的区域，因此会受到绝热聚焦的影响。然而，在1 Au处观察到的strahl俯仰角分布的宽度比预期的要宽得多。先前的研究已经发现，观测到的平均strahl俯仰角宽度实际上随着日心径向距离的增加而增加。这意味着，strahl电子必须受到某种形式的俯仰角散射过程或过程，其中的细节仍然难以捉摸。在本文中，我们使用卡西尼电子测量检查strahl光束跨越的距离范围约为8 Au，从它的地球飞越在1999年，直到2004年插入到土星周围的轨道。我们发现，在一般情况下，有一个相对恒定的速率加宽的strahl螺距角分布之间的距离为1.1和5.5 Au。我们从这个距离以外的观测结果表明，strahl星群很可能是完全分散的，大概是形成晕的一部分。我们发现在不同的径向距离的多个能量依赖性，这意味着有多个strahl散射机制的操作。
Field‐aligned beams of suprathermal electrons, known as “strahl,” are a frequently observed constituent of solar wind plasma. However, the formation and interplanetary evolution of the strahl electron populations has yet to be fully understood. As strahl electrons travel away from the Sun, they move into regions of decreasing magnetic field strength and thus are subject to adiabatic focusing. However, the widths of strahl pitch angle distributions observed at 1 AU are significantly broader than expected. Previous investigations have found that the average observed strahl pitch angle width actually increases with heliocentric radial distance. This implies that strahl electrons must be subjected to some form of pitch angle scattering process or processes, details of which as of yet remain elusive. In this paper, we use Cassini electron measurements to examine strahl beams across a distance range of approximately 8 AU, from its Earth Flyby in 1999 until its insertion into orbit around Saturn in 2004. We find that, in general, there is a relatively constant rate of broadening of strahl pitch angle distributions with distance between ∼1 and 5.5 AU. Our results from beyond this distance indicate that the strahl population is likely to be completely scattered, presumably to form part of the halo. We find multiple energy dependences at different radial distances implying that there are multiple strahl scattering mechanisms in operation.