Alpha particle thermodynamics in the inner heliosphere fast solar wind
Alpha particle thermodynamics in the inner heliosphere fast solar wind
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DOI：
10.1051/0004-6361/201834900
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发表时间：
2018-12
影响因子：
6.5
通讯作者：
D. Stansby;D. Perrone;L. Matteini;T. Horbury;C. Salem
中科院分区：
文献类型：
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作者：
D. Stansby;D. Perrone;L. Matteini;T. Horbury;C. Salem
上下文日冕和太阳风中的等离子体过程可以通过研究不同离子种类的热力学性质来探测。然而，大多数对太阳风中正离子的原位观测都是在1 Au处进行的，在那里关于它们的太阳源特性的信息可能已经被不可逆地抹去了。目标。在这项研究中，我们的目标是使用在0.3 Au和1 Au的日心距离的α粒子的属性，研究等离子体过程发生在观察点，并推断发生在0.3 Au内的过程，通过比较我们的结果，以前的等离子体遥感观测更接近太阳。方法.我们重新处理了原始的Helios正离子分布函数，分离出α粒子群，并计算了α粒子数密度、速度和磁场垂直和平行温度。然后，我们研究了在0.3 Au和1 Au之间观测到的快速太阳风中α粒子温度的径向变化。结果在0.3 Au和1 Au AU之间的α粒子在磁场垂直方向上被加热并且在磁场平行方向上被冷却。α粒子演化受α不稳定性阈值的限制，这为解释所观察到的平行冷却和垂直加热提供了一种可能的机制。我们的观测结果表明，在靠近太阳的地方，α粒子在垂直方向上受热，而在0.3 Au处观察到的大磁场平行温度可能是由于0.3 Au内部的双重绝热膨胀和α粒子减速的综合效应。
Context. Plasma processes occurring in the corona and solar wind can be probed by studying the thermodynamic properties of different ion species. However, most in situ observations of positive ions in the solar wind are taken at 1 AU, where information on their solar source properties may have been irreversibly erased. Aims. In this study we aim to use the properties of alpha particles at heliocentric distances between 0.3 AU and 1 AU to study plasma processes occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previous remote sensing observations of the plasma closer to the Sun. Methods. We reprocessed the original Helios positive ion distribution functions, isolated the alpha particle population, and computed the alpha particle number density, velocity, and magnetic field perpendicular and parallel temperatures. We then investigated the radial variation of alpha particle temperatures in fast solar wind observed between 0.3 AU and 1 AU. Results. Between 0.3 AU and 1 AU alpha particles are heated in the magnetic field perpendicular direction and cooled in the magnetic field parallel direction. Alpha particle evolution is bounded by the alpha firehose instability threshold, which provides one possible mechanism to explain the observed parallel cooling and perpendicular heating. Closer to the Sun our observations suggest that the alpha particles undergo heating in the perpendicular direction, whilst the large magnetic field parallel temperatures observed at 0.3 AU may be due to the combined effect of double adiabatic expansion and alpha particle deceleration inside 0.3 AU.